BASIC PHOSPHATIC SLAGS.
69. History and Manufacture.—The basic process for the manufacture of Bessemer steel is known in Europe as the Thomas or Thomas and Gilchrist process, and the slags rich in phosphate, one of the waste products of the process, are known by the same name. In this country all the phosphatic slags which have been made in the manufacture of steel have been obtained working under the patents of Reese, and, when prepared for the market, are known as odorless phosphate. The only place where these slags have been made in this country is Pottstown, Pennsylvania. In Europe they are extensively manufactured, in England, France, and Germany, and their use for agricultural purposes has increased until it is quite equal to that of superphosphates.
The quantity of basic slag manufactured in Germany in 1893 was 750,000 tons; in England 160,000; in France 115,000, making the total production of central Europe about 1,000,000, a quantity sufficient to fertilize nearly 5,000,000 acres.
70. Process of Manufacture.—The principle of the process depends upon the arrangement of the furnaces, by means of which the phosphoric acid in the pig iron is caused to combine with the lime which is used as a flux in the converters. A general outline of the process is as follows:
The pigs, which contain from two to four per cent of phosphorus, are melted and introduced into a Bessemer converter lined with dolomite powder cemented with coal-tar, into which has previously been placed a certain quantity of freshly-burned lime. For an average content of three per cent of phosphorus in the pig iron, from fifteen to twenty pounds of lime are used for each 100 pounds of pig iron. As soon as the melted pig iron has been introduced into the converter, the air-blast is started, the converter placed in an upright position, and the purification of the mass begins. The manganese in the iron is converted into oxid, the silicon into silica, the carbon into carbon dioxid and oxid, and the phosphorus into phosphoric acid.
By reason of the oxidation processes, the whole mass suffers a rise of temperature amounting in all to about 700° above the temperature of the melted iron. At this temperature the lime which has been added, melts, and, in this melted state, combines with the phosphoric acid, and the liquid mass floats upon the top of the metallic portion, which has, by this process, been converted into steel.
As soon as the process, which occupies only about fifteen minutes, is completed, the fused slag is poured off into molds, allowed to cool, broken up, and ground to a fine powder. For each five tons of steel which are made in this way, about one ton of basic slag is produced.
In another process, in order to make a slag richer in phosphoric acid, a lime is employed which contains a considerable percentage of phosphate. Although the slag thus produced is richer in phosphoric acid, it is doubtful whether it is any more available for plant growth than that made in the usual way with lime free from phosphoric acid. In other words, when a basic slag is made with a lime free from phosphoric acid, nearly the whole of the phosphoric acid is combined as tetrabasic calcium phosphate. On the other hand, when the lime employed contains some of the ordinary mineral phosphate the basic slag produced becomes a mixture of this mineral phosphate with the tetracalcium salt. The mineral phosphate is probably not rendered any more available than it was before.
It is easily seen from the above outline of the process of manufacture, that basic slags can have a very widely divergent composition. When made from pig iron poor in phosphorus, the slag will have a large excess of uncombined lime and consequently the content of phosphoric acid will be low. When made from pigs rich in phosphorus there may be a comparative deficiency of lime, and in this case the content of phosphoric acid would be unusually high.
It is found also that the content of iron in the slag varies widely. In general, the greater the content of iron the harder the slag and the more difficult to grind. If the pig iron contain sulfur, as is often the case, this sulfur is found also in the slag in combination with the lime, either as a sulfid or sulfate.
No certain formula can therefore be assigned to basic slags and the availability of each one must be judged by its chemical composition.
71. Composition of Basic Slag.—The slags produced by the method above outlined may be amorphous or crystalline. When large masses are slowly cooled the interior often discloses a crystalline composition. In some samples analyzed in this laboratory the crystals were found to be of two forms; viz., acicular and tabular.[62] They had the following composition:
Calculated Per Cents as
| CaO. | Fe₂O₃. | Al₂O₃. | MgO. | V₂O₂ . | P₂O₅. | SiO₂. | |
|---|---|---|---|---|---|---|---|
| Acicular crystals | 42.69 | 20.98 | 3.71 | 0.49 | 0.18 | 27.06 | 4.96 |
| Tabular crystals | 53.61 | 9.64 | 0.91 | 0.08 | —— | 33.92 | 1.75 |
These data show that the two sets of crystals belong to two distinct mineral forms. The presence of vanadium in one of the samples is worthy of remark, and leads to the suggestion that in the slags made of phosphoriferous pigs may be found any of the rare metals which may exist in the ores from which the pigs were made. The amorphous portions may have a widely varying composition and consequent content of phosphoric acid. In all good slags, however, whether in crystalline form or as amorphous powder, the lime and phosphoric acid will be found combined as tetracalcium phosphate (Ca₄P₂O₉).
72. Molecular Structure of Tetracalcium Phosphate.—Several theories have been advanced in respect of the atomic arrangement of the elements contained in a molecule of tetracalcium phosphate. It must be confessed that so little is known concerning the reactions of this body as to make theories of its constitution largely visionary. But the existence in definite crystalline form of this salt shows that it is not merely an intimate mechanical mixture, but a true molecular form. As a type of the supposed arrangement of its particles the graphic formula proposed by Kormann may be consulted; viz.,
The crystals of this salt, as may be seen by inspection of the analytical data, contain other bodies than calcium, oxygen, and phosphorus. It would be of interest to push the investigation of their constitution further and see if crystals of pure tetracalcium phosphate could be obtained, and under what conditions they would be contaminated by other metallic oxids. Usually, by the color of the crystals, it will be easy to determine something of the nature, if not the extent of the contamination.
73. Solubility of Phosphatic Slags.—The high agricultural value of basic slags led to an early study of their solubility in ammonium citrate, citric acid, and other organic solutions. Even finely ground mineral phosphates and bones are soluble to some extent in ammonium citrate, as was pointed out as long ago as 1882.[63] The most common solvents used for basic slags are ammonium citrate and citric acid. The ammonium citrate should be the same as that used for the determination of reverted phosphoric acid and the citric acid solution commonly used contains five grams in a hundred cubic centimeters. The slags of different origin and even of different age vary greatly in respect of the quantity of soluble matter they contain. It is believed, however, that a very fair idea of the agricultural value of a slag may be obtained by determining its degree of solubility in one of the menstrua named.
74. Separation by Sifting.—The relative availability of a slag, as in the case of a mineral phosphate, is determined by the percentage of fine material it contains. Sieves of varying apertures are used to determine this percentage. A one-half millimeter or a one-quarter millimeter circular aperture is best, and the percentage of the total material passing through is determined. A method used in Germany consists in sifting the slag in a sieve twenty centimeters in diameter the meshes of which are from 0.14 to 0.17 millimeter square and which measure diagonally from 0.22 to 0.24 millimeter.
75. Solution of Phosphatic Slags.—Sulfuric acid has been found to be an excellent solvent for basic slags preparatory to the determination of phosphoric acid. There is, however, no unanimity of opinion concerning the best method or means of solution. Aqua regia and nitric acid are objected to because they may convert any phosphorus in combination with the iron into phosphoric acid and thus increase the quantity present.[64] But iron phosphid is seldom or ever found in slags and therefore this objection is not always tenable. Sulfuric acid has also been deemed objectionable because the gypsum separated is likely to carry with it some of the other substances to be determined.
Hydrochloric acid is also excluded by some from the list of solvents because it dissolves so many of the foreign elements in the slag and thus tends to complicate the subsequent determination, especially of magnesia. Further than this a hydrochloric acid solution is not suited to the use of the citrate method now so commonly employed. When hydrochloric acid is used, moreover, the dissolved silica must be removed and thus the time required for making a phosphoric acid determination is much increased.
If the sample be sufficiently fine the occlusion of undissolved phosphate particles by the gypsum formed when sulfuric acid is used is not to be feared and the disturbance of volume by the gypsum is pretty nearly constant and can be allowed for. When five grams of slag are used the mean volume of gypsum in the solution is about two cubic centimeters.
76. Estimation of Total Acid.—In the determination of total phosphoric acid in a slag, twenty-five cubic centimeters of the strongest sulfuric acid are placed in an erlenmeyer having a wide neck, and with careful shaking five grams of the fine slag meal gradually added. The flask is heated over a naked flame until solution is complete. When the mass is cold it is washed into a quarter liter flask; again allowed to cool, filled with water to the mark, and two cubic centimeters of water corresponding to the volume of gypsum undissolved, are added, well mixed, and filtered. In fifty cubic centimeters of the filtrate the phosphoric acid is determined by either the molybdic or citrate methods already described.
77. Alternate Method.—The following method may also be used: Ten grams of the substance are heated with fifty cubic centimeters of concentrated sulfuric acid until white vapors have been evolved for some time. The operation lasts for about fifteen minutes and can be carried on in a half liter flask or in a porcelain dish. Without regarding the undissolved material the volume of the liquid is now made up to half a liter and filtered. The filtered liquid becomes turbid after some time through the separation of calcium sulfate, but this turbidity should not be regarded. To fifty cubic centimeters of the solution, corresponding to one gram of substance, twenty cubic centimeters of citric acid solution (500 grams citric acid to the liter) are added, and it is afterwards nearly neutralized by the addition of ten per cent ammonia and the liquid, which is warmed by this operation, cooled. There are now added twenty-five cubic centimeters of the ordinary magnesium chlorid mixture and the solution stirred until turbidity is produced, one-third of its volume of ten per cent ammonia added, and again stirred for about a minute.
Instead of the addition of the citric acid and ammonia the ammonium citrate prepared as follows, may be added: 1,500 grams of citric acid are dissolved with water, made up to three liters and five liters of twenty-four per cent ammonia and seven liters of water added. The rest of the operation is carried on in the usual manner.
78. Halle Method for Basic Slag.—The total phosphoric acid is estimated at the Halle Station by the following process:[65]
Ten grams of the substance are moistened in a porcelain dish with a few drops of water and about five cubic centimeters of a one to one solution of sulfuric acid added, and after the mass has hardened, which takes place very soon, fifty cubic centimeters of concentrated sulfuric acid are added and stirred with a glass rod until it is evenly distributed throughout the whole mass. In stirring this mixture the greatest care must be taken, otherwise the substance would remain attached to the sides of the dish, which during later heating would cause loss through spurting. The complete solution now takes place after a few hours’ heating on a sand-bath. During the cooling the jelly-like mass must be stirred with a glass rod, and after it is cool, by means of a washing-bottle, gently along the sides of the dish, water is added, and when the mixture becomes hot it is again cooled and washed into a half liter flask, which is made up to the mark at a temperature of 17°.5 and filtered. When the acid filtrate stands for some time there is often a separation of gypsum which, however, does not in any way influence the subsequent analysis, which is made in the usual manner.
Fifty cubic centimeters of the filtrate, representing one gram of the original substance, are placed in an erlenmeyer. In the case of double superphosphates which often contain large quantities of pyrophosphates, twenty-five cubic centimeters of the filtrate just obtained, equivalent to five grams of the substance, are diluted with seventy-five cubic centimeters of water, ten cubic centimeters of nitric acid of 1.42 specific gravity added, and heated on a sand-bath to convert the pyro into orthophosphates. The heating should be continued until the liquid is reduced to its original volume of twenty-five cubic centimeters. The strongly acid liquid is saturated with ammonia and with the addition of a drop of rosolic acid as an indicator, again acidified with nitric acid, and treated as with superphosphates.
79. Dutch Method for Basic Slag.—Heat ten grams of the sample with fifty cubic centimeters of sulfuric acid (1.84 specific gravity) till white vapors are evolved, shaking or stirring constantly. After cooling make the fluid up to 500 cubic centimeters with water, taking no account of the undissolved substance. Filter, and to fifty cubic centimeters of the filtrate add 100 cubic centimeters of the ammoniacal citrate solution, and after cooling, twenty-five cubic centimeters of magnesia mixture. Stir or shake for a sufficient time. After the lapse of two hours the precipitate is to be separated by filtration and treated in the usual manner.
80. Estimation of Citrate-Soluble Phosphoric Acid in Basic Slag.—Experience has shown that the manurial value of basic slags does not depend alone on their content of phosphoric acid. Slags may contain tri- as well as tetracalcium phosphate, and even this latter salt may exist in states of differing availability. In determining the availability of basic slag for manurial purposes its solubility in ammonium citrate is considered the best standard. But this solubility will evidently be influenced by the basicity of the sample, or in other words, by the quantity of lime present. A slag rich in calcium oxid would deport itself differently with a given ammonium citrate solution from one in which the lime had been chiefly converted into carbonate. If possible, therefore, all samples should be reduced to the same state of basicity before the action of any given solvent is determined.
Wagner proposes to neutralize the basicity of a slag in the following manner:[66] Five grams of the slag are placed in a half liter flask which is then filled up to the mark with a one per cent solution of citric acid and shaken for half an hour. After filtering, fifty cubic centimeters are titrated with a standard soda solution using phenolphthalein as indicator. This gives the quantity of citric acid necessary to neutralize the slag. To a second portion of five grams of the sample in a half liter flask are added 200 cubic centimeters of water and enough five per cent citric acid solution to neutralize the lime and then 200 cubic centimeters of acid ammonium citrate made as indicated below. After filling to the mark with water it is shaken for half an hour and filtered. To fifty cubic centimeters of the filtrate are added 100 cubic centimeters of molybdic solution and heated to 80°. After cooling, the precipitate is filtered and the phosphoric acid estimated in the usual way.
The acid ammonium citrate solution used is made as follows: Dissolve 160 grams of citric acid with enough ammonia to represent about twenty-eight grams of nitrogen and make up with water to one liter. The exact method is given in 82.
The molybdic solution is made by dissolving 125 grams of molybdic acid in a slight excess of two and a half per cent of ammonia, adding 400 grams of ammonium nitrate, diluting to one liter and pouring the solution into one liter of nitric acid having a specific gravity of 1.19. After allowing to stand at room temperature for one day the mixture is filtered and is then ready for use.
81. Wagner’s Shaking Apparatus.—The latest directions given by Wagner for determining the phosphoric acid in slags and raw phosphates soluble in citrate solutions, are the following:[67] Five grams of the material as it is sent into commerce without grinding or sifting, are placed in a half liter flask and covered with nearly a quarter liter of water, and then 200 cubic centimeters of citrate solution added, prepared as described below. The flask is filled to the mark with water. The flasks, which are of the shape shown in the figure, are closed with rubber stoppers, and without delay placed for half an hour in a rotating apparatus, ([Fig. 6]) which is turned on its axis from thirty to forty times a minute. If a shaking apparatus be used instead of the one mentioned, 200 cubic centimeters of the citrate solution should be placed in a half liter flask, filled to the mark with water, and the contents poured into a liter flask containing the phosphate. This flask should be placed in a nearly horizontal position in the apparatus and the agitation be continued for half an hour. On removal from the apparatus the mixture is filtered and fifty cubic centimeters thereof treated with double the quantity of molybdic solution at 80° and the precipitate separated after cooling. The precipitate is carefully washed with one per cent nitric acid mixture, after which the filter is broken and the precipitate washed into a beaker with two per cent ammonia and the filter washed therewith until about 100 cubic centimeters have been used. If the solution is turbid from the presence of silicic acid it should be precipitated a second time by addition of molybdic solution until the acid reaction is restored. The ammoniacal solution of the yellow precipitate is treated, drop by drop, with constant stirring, with fifteen cubic centimeters of magnesia mixture, and set aside for two hours. The precipitate is collected, washed, ignited, and weighed in the usual manner. The direct precipitation of the phosphoric acid by the magnesia solution in presence of citrate is not advisable because of the almost general presence of silicic acid which would cause the results to be too high.
Figure. 6.
Wagner’s Digestion Apparatus for Slags.
The chief objection to this method of Wagner lies in the failure to control the temperature at which the digestion with citrate solution is made. Huston has shown, as will be described further on, that the temperature exercises a great influence in digestion with citrate. Since the laboratory temperature, especially in this country, may vary between 10° and 35°, it is evident that on the same sample the Wagner method would give very discordant results at different seasons of the year.
82. Solutions Employed in the Wagner Method.—1. Ammonium Citrate.—In one liter there should be exactly 150 grams of citric acid and 27.93 grams of ammonia, equivalent to twenty-three grams of nitrogen. The following example illustrates the preparation of ten liters of the solution: In two liters of water and three and a half liters of eight per cent ammonia, 1,500 grams of citric acid are dissolved and the cooled solution made up exactly to eight liters. Dilute twenty-five cubic centimeters of this solution to 250 cubic centimeters and treat twenty-five cubic centimeters of this with three grams of calcined magnesia and distill into forty cubic centimeters of half normal sulfuric acid. Suppose the ammonia nitrogen found correspond to twenty cubic centimeters of fourth normal soda-lye. Then in the eight liters are contained
- (20.0 × 0.0035 × 8000)
- ————————— = 224 grams
- 2.5
of ammonia nitrogen. Then in order to secure in the ten liters the proper quantity of ammonia there must be added two liters of water containing 230 - 224 = six grams of nitrogen or seven and three-tenths grams ammonia; viz., ninety-four cubic centimeters of 0.967 specific gravity.
2. Molybdate Solution.—Dissolve 125 grams of molybdic acid in dilute two and five-tenths per cent ammonia, avoiding a large excess of the solvent. Add 400 grams of ammonium nitrate, dilute with water to one liter and pour the solution into one liter of nitric acid of 1.19 specific gravity. Allow the preparation to stand for twenty-four hours at 35° and filter.
3. Magnesia Mixture.—Dissolve 110 grams of pure crystallized magnesium chlorid and 140 grams of ammonium chlorid in 700 cubic centimeters of eight per cent ammonia and 130 cubic centimeters of water. Allow to stand several days and filter.
83. Estimation of Lime.—When the lime is to be determined in basic slags some difficulty may be experienced by reason of danger of contamination of the oxalate precipitate with iron and especially manganese, which is often present in slags.
Holleman[68] proposes to estimate the lime in basic slag by a modification of the methods of Classen and Jones. The manipulation is as follows: Fifty cubic centimeters of the solution of slag, equivalent to one gram of substance, are evaporated to a small volume, twenty cubic centimeters of neutral ammonium oxalate solution (one to three) added to the residue and heated on a water-bath with frequent stirring, until the precipitate is pure white and free from lumps. The time required is usually about ten minutes. The precipitate is collected on a filter and washed with hot water until the filtrate contains no oxalic acid. The precipitated calcium oxalate must be snow-white. The filter is broken and the calcium oxalate washed through, first with water and finally with warm, dilute hydrochloric acid (one to one). The calcium oxalate is dissolved by adding fifteen cubic centimeters of concentrated hydrochloric acid, the solution evaporated to a volume of about twenty-five cubic centimeters and ten cubic centimeters of dilute sulfuric acid (one to five), and 150 cubic centimeters of ninety-six per cent alcohol added. After standing three hours or more the precipitate is separated by filtration and washed with ninety-six per cent alcohol until the washings show no acid reaction with methyl orange. The calcium sulfate precipitated is dried to constant weight. This method gives a pure precipitate of calcium sulfate, containing only traces of manganese.
84. Estimation of Caustic Lime.—The lime mechanically present in basic slags is likely to be found as oxid or hydroxid, especially when the sample is of recent manufacture. In the form of oxid the lime may be determined by solution in sugar. In this process one gram of the fine slag meal is shaken for some time with a solution of sugar, as suggested by Stone and Scheuch.[69] The dissolved lime is separated as oxalate by treatment of the solution with the ammonium salt. The calcium oxalate may be determined by ignition in the usual way or volumetrically by solution in sulfuric acid and titration of the free oxalic acid with potassium permanganate solutions. The standard solution of permanganate should be of such a strength as to have one cubic centimeter equivalent to about 0.01 gram of iron. The iron value of the permanganate used multiplied by 0.5 will give the quantity of calcium oxid found.
85. Detection of Adulteration of Phosphatic Slags.—The high agricultural value of phosphatic slags has led to their adulteration and even to the substitution of other bodies. Several patents have also been granted for the manufacture of artificial slags of a value said to be an approximation to that of the by-products of the basic pig iron process.
(1) Method of Blum.—One of the earliest methods of examining basic slag for adulterations is the method of Blum.[70] This method rests upon the principle of the determination of the carbon dioxid in the sample. The basic phosphatic slag is supposed to contain no carbon dioxid. This is true only in case it is freshly prepared. The tetrabasic phosphate, after being kept for some time, gradually absorbs carbon dioxid from the air. As high as nineteen per cent of carbon dioxid have been found in slags which have been kept for a long while. When the slag has absorbed so much of carbon dioxid and water from the air as to be no longer profitable for market, it can be restored to its original condition by ignition.
(2) Method of Richter-Forster.—One of the common adulterants of tetrabasic phosphate is aluminum phosphate. The method of detecting this when mixed with the slag is described by Richter-Forster.[71] The method depends on the fact that soda-lye dissolves the aluminum phosphate, although it does not dissolve any calcium phosphoric acid from the slag. Two grams of the sample to be tested are treated with ten cubic centimeters of soda-lye of from 7° to 8° C. in a small vessel with frequent shaking for a few hours at room temperature. After filtration the filtrate is made acid with hydrochloric and afterwards slightly alkaline with ammonia. With pure basic slag there is a small trace of precipitate produced, but this is due to a little silica which can be dissolved in a slight excess of acetic acid. If, however, the basic slag contain aluminum phosphate, a dense jelly-like precipitate of aluminum phosphate is produced.
(3) Method of Jensch.—Edmund Jensch[72] determines the tetrabasic phosphate in slags by solution in organic acids, and prefers citric acid for this purpose. This method was also recommended by Blum[73].
It is well known that the tetrabasic phosphate in slags is completely soluble in citric acid while the tribasic phosphate is only slightly, if at all, attacked. The neutral ammonium salts of organic acids do not at first attack the tribasic phosphate at all, and they do not completely dissolve the tetrabasic phosphate. The solution used by Jensch is made as follows: Fifty grams of crystallized citric acid are dissolved in one liter of water. A weaker acid dissolves the tetrabasic phosphate too slowly and a stronger one attacks the tribasic phosphate present.
Schucht recommends the following method of procedure:[74] One gram of the slag, finely ground, is treated in a beaker glass with about 150 cubic centimeters of Jensen’s citric acid solution and warmed for twelve hours in an air-bath at from 50° to 70° with frequent shaking. Afterwards it is diluted with 100 cubic centimeters of water, boiled for one minute and filtered. The filter is washed thoroughly with hot water and the phosphoric acid is estimated in the filtrate in the usual way. With artificial mixtures of basic slags and other phosphates the quantity of basic slag can be determined by the above method.
(4) Method of Wrampelmeyer.—According to Wrampelmeyer the most convenient method for discovering the adulteration of basic slag is the use of the microscope.[75] All finely ground natural phosphates are light colored and with a strong magnification appear as rounded masses. In basic slags the particles are mostly black but there are often found red-colored fragments having sharp angles which refract their light in a peculiar way so that, with a very little experience, they can be recognized as being distinctive marks of pure basic slag.
In artificial mixtures of these two phosphates, which we have made in our laboratory, we have been able to detect with certainty as little as one per cent of added mineral phosphate.
One form of adulterating natural mineral phosphates has been mixing them with finely pulverized charcoal or soot to give them the black appearance characteristic of the basic slags. This form of adulteration is at once disclosed by simple ignition or by microscopic examination.
(5) Loss on Ignition.—If all doubts cannot be removed by the use of the microscope, the loss on ignition should be estimated. Natural phosphates all give a high loss on ignition, ranging from eight to twenty-four per cent, while a basic slag gives only a very slight loss on ignition, especially when fresh. A basic slag which has stood for a long while and absorbed carbon dioxid and moisture, may give a loss on ignition approximating, in a maximum case, the minimum loss on ignition from a natural phosphate.
In experiments made in this laboratory in testing for loss on ignition, we have uniformly found that natural mineral phosphates will lose from nearly one to two and one-half times as much on ignition as a basic slag which has been kept for two years. A basic slag in the laboratory more than two years old gave, as loss on ignition, 4.12 per cent. Several samples of finely ground Florida phosphates gave the following percentages of loss on ignition, as compared with a sample of slag.
Odorless phosphate 4.12.
Florida phosphates 8.06, 6.90, 9.58, 6.40, 10.38, and 10.67 respectively.
There are some mineral phosphates, however, which are ignited before being sent to the market. We have one such sample in our laboratory from Florida which gave, on ignition, a loss of only one and four-tenths per cent. In this case it is seen that the application of the process of ignition would not discriminate between a basic slag and a mineral phosphate.
It may often be of interest to know what part of the loss, on ignition, is due to water in form of moisture. In such cases the sample should first be dried to constant weight in a steam-bath and then ignited. In the following data are found the results obtained here with samples treated as above indicated and also ignited directly. Number one is a basic slag two years old and the others Florida phosphates.
| Heated to 100° C. then ignited. | Ignited directly. | ||||
|---|---|---|---|---|---|
| Loss at 100° C. | Loss on ignition. | Total loss. | Loss on ignition. | ||
| No. 1 | (Slag) | 2.57 | 1.77 | 4.34 | 4.12 |
| No. 2 | (Rock) | 2.61 | 5.19 | 7.80 | 8.06 |
| No. 3 | “ | 1.09 | 5.77 | 6.86 | 6.90 |
| No. 4 | “ | 0.42 | 9.20 | 9.62 | 9.58 |
| No. 5 | “ | 1.81 | 4.83 | 6.64 | 6.40 |
| No. 6 | “ | 4.36 | 6.52 | 10.88 | 10.83 |
| No. 7 | “ | 3.31 | 7.01 | 10.32 | 10.67 |
(6) Presence of Sulfids.—Another point noticed in this laboratory is that the basic slags uniformly contain sulfids which are decomposed upon the addition of an acid with an evolution of hydrogen sulfid.
(7) Presence of Fluorin.—In applying the test for fluorin, it has been uniformly found here that the mineral phosphates respond to the fluorin test while the basic slags, on the contrary, respond to the hydrogen sulfid test. This test, however, was applied only to the few samples we have had and may not be a uniform property.
The absence of fluorin might not prove the absence of adulteration, but its presence would, I believe, certainly prove the fact of the adulteration in that particular sample.
The fluorin test is applied by Böttcher in the following manner:[76] From ten to fifteen grams of the slag are placed in a beaker ten centimeters high and from five to six centimeters in diameter, with fifteen cubic centimeters of concentrated sulfuric acid, stirred with a glass rod, and covered with a watch-glass on the under side of which a drop of water hangs. If there be formed upon the drop of water a white murky rim, it is proof that a mineral phosphate containing fluorin has been added. After from five to ten minutes you can notice on the clean watch-glass the etching produced by the hydrofluoric acid. According to Böttcher an adulteration of ten per cent of raw phosphate in slag can be detected by this method.
(8) Solubility in Water.—Solubility in water is also a good indication, natural phosphates being totally insoluble in water, while a considerable quantity of the basic slag will be dissolved in water on account of the calcium oxid or hydroxid which it contains. If the loss on ignition is low, and the volume-weight and water-solubility high, the analyst may be certain that the sample is a pure slag.
In comparative tests made in our laboratory with a sample of basic slag and seven samples of Florida phosphate, the percentages of material dissolved by water and by a five per cent solution of citric acid were found to be as follows:
| Water-soluble. Per cent. | Sol. in five per cent citric acid. Per cent. | ||
|---|---|---|---|
| Odorless | phosphate | 0.97 | 16.10 |
| Florida | phosphate | 0.01 | 4.15 |
| “ | “ | 0.09 | 4.66 |
| “ | “ | 0.02 | 3.43 |
| “ | “ | 0.08 | 3.61 |
| “ | “ | 0.02 | 3.79 |
| “ | “ | 0.05 | 4.46 |
| “ | “ | 0.02 | 4.24 |
From the above data it is seen that the solvent action of water especially would be of value inasmuch as it dissolves only a mere trace of the mineral phosphates, approximating one per cent of the amount dissolved from basic slag. In the case of the citric acid it is found that the amount of materials soluble in this solvent for basic slag is fully four times as great as for the mineral phosphates. Both of these processes, therefore, have considerable value for discriminating between the pure and adulterated article of basic slag.
(9) Specific Gravity.—The estimation of the volume specific gravity is also a good indication for judging of the purity of the slag. This is best done by weighing directly a given volume. Basic slag will have a volume-weight of about one and nine-tenths, while natural phosphates will have about one and six-tenths.
(10) Conclusions.—From the above résumé of the standard methods which are in use for determining the adulteration of basic slag, it is seen that there are many cases in which grave doubt might exist even after the careful application of all the methods mentioned. If we had only to consider the adulteration of basic slag with certain of the mineral phosphates, that is, tricalcium phosphate, the problem would be an easy one, but when we add to this the fact that iron and aluminum phosphates are employed in the adulteration, and that artificial slags may be so used, the question becomes more involved.
In doubtful cases one after another of the methods should be applied until there is no doubt whatever of the judgment which should be rendered.
VOLUMETRIC DETERMINATION
OF PHOSPHORIC ACID.
86. Classification of Methods.—The time required for a gravimetric determination of phosphoric acid has led analysts to try the speedier if less accurate processes, depending on the use of volumetric methods. The chief difficulty with these methods has been in securing some sharp method of distinguishing the end reaction. In most cases it has been found necessary to remove a portion of the titrated solution and prepare it for final testing by subsidence or filtration. As is well known, this method of determining the end reaction is less accurate and more time-consuming than those processes depending on a change of color in the whole mass. All the volumetric processes now in general use may be divided into two classes; viz., (1) the direct precipitation of phosphoric acid and the determination of the end reaction by any appropriate means, and (2) the previous separation of the phosphoric acid, usually by means of a citro-magnesium or molybdenum mixture, and in the latter case the subsequent titration of the yellow ammonium phosphomolybdate either directly or after reduction to a lower form of oxidation. In respect of extent of application by far the most important volumetric method is the one depending on titration by a uranium salt after previous separation by ammoniacal magnesium citrate. A promising method after previous separation by molybdenum is the one proposed by Pemberton, but it has not yet come into general use. For small quantities of phosphoric acid or of phosphorus, such as are found in steels and irons, the method of Emmerton, either as originally proposed or as modified by Dudley and Noyes, is in frequent use. Where volumetric methods are applied to products separated by molybdic solution, the essential feature of the analytical work is to secure a yellow precipitate of constant composition. If this could be uniformly done such methods would rival the gravimetric processes in accuracy. Hence it is highly important in these methods that the yellow precipitate should be secured as far as possible, under constant conditions of strength of solution, duration of time, and manner of precipitation. In these cases, and in such only, can the quicker volumetric methods be depended on for accurate results.
The direct volumetric precipitation of the phosphoric acid by a uranium salt or otherwise is practiced only when the acid is combined with the alkalies and when iron and alumina are absent and only small quantities of lime present. This method has therefore but little practical value for agricultural purposes. In all volumetric analyses the accuracy of the burettes, pipettes, and other graduated vessels should be proved by careful calibration. Many of the disagreements in laboratories where the analytical work is conducted equally well can be due to no other cause than the inaccuracy of the graduated vessels which are found in commerce. Burettes should not only be calibrated for the whole volume but for at least every five cubic centimeters of the graduation.
URANIUM METHOD AS PRACTICED
BY THE FRENCH CHEMISTS.
87. The Uranium Method.—Since the phosphoric acid of practical use for agricultural purposes is nearly always combined with lime, alumina, and iron, its volumetric estimation by means of a standard solution of a uranium salt is to be preceded by a preliminary separation by means of an ammoniacal magnesium citrate solution. The principle of the method was almost simultaneously published by Sutton,[77] Neubauer,[78] and Pincus.[79] The phosphoric acid may also be separated by means of molybdic solution or by tin or bismuth.[80] In practice, however, it has been found that when the uranium method is to be used the magnesium citrate separation is the most convenient. Since this is the method practiced almost universally in France, the method there used will be given in detail. It is based essentially on the process described by Joulie.[81]
88. Preparation of Sample.—(1) Incineration.—Since the organic matters present in a phosphatic fertilizer often interfere with the employment of uranium as a reagent, it is necessary to incinerate the sample taken for analysis.[82]
(2) Solution of the Material.—All phosphates, with the exception of certain aluminum phosphates, amblygonite for example, are easily dissolved in nitric and hydrochloric acids more or less dilute, especially on ebullition. The best solvent, however, for calcium phosphates for the uranium method is incontestably hydrochloric acid which also very easily dissolves the iron and aluminum phosphates, which are often found present with calcium phosphates.
(3) Nitric Acid.—In many laboratories nitric acid is preferred in order to avoid, in part, the solution of ferric oxid which interferes with the determination of phosphoric acid in certain processes. Since it does not act in this way for the citro-magnesium uranium method, it is preferable to employ hydrochloric acid, especially because it dissolves the iron completely and permits thus the operator to judge of the success of the solvent action by the completely white color of the residue.
(4) Pyritic Phosphates.—Certain phosphates contain pyrites which hydrochloric acid does not dissolve, and there is left consequently, a residue more or less colored. In this case it is necessary to add some nitric acid and to prolong the boiling until the pyrite has disappeared, since it might retain a small quantity of phosphoric acid in the state of iron phosphate.
(5) Sulfuric Acid.—Some chemists decompose the phosphates by means of dilute sulfuric acid. This method, which is certainly able to give good results for certain products and for certain processes, presents numerous inconveniences which tend to render its use objectionable for volumetric purposes. The calcium sulfate which is formed, requires prolonged washings which lead to chances of fatal error.
If an aluminum phosphate be under examination, containing only very little or no lime, sulfuric acid is to be preferred to hydrochloric and nitric acids, since it attacks amblygonite, which, as has been before stated, resists the action of the other two acids. But these are cases which are met with very rarely, and which can always be treated by the general method by previously fusing the material with a mixture of sodium and potassium carbonate.
In the great majority of cases the decomposition by hydrochloric acid is very easily accomplished by simply boiling in a glass vessel, and without effecting the separation of the silica. This operation is only necessary after the substance has been fused with alkaline carbonates, or, in case of substances which contain decomposable silicates giving gelatinous silica with hydrochloric acid.
There are two methods [see (6) and (7)] of securing a solution of the sample taken which varies from one to five, and even ten grams, according to the apparent homogeneity of the material to be analyzed.
(6) Solution by Filtration and Washing.—The ordinary method can be employed consisting in decomposing the substance by an acid, filtering, and washing the residue upon the filter, and combining all the wash-waters to make a determinate volume. Afterwards an aliquot fraction of the whole is taken for the precipitation. This method is long, and presents some chances of error, when the insoluble residue is voluminous and contains silica which obstructs the pores of the paper and renders the filtration difficult.
(7) Volumetric Solution.—It is advisable to substitute volumetric solution for solution by filtration and washing, which is accomplished by decomposing the substances in a graduated flask, the volume being afterwards made up to the mark with distilled water after cooling. The solution is then filtered without washing, and by means of a pipette an aliquot part of the original volume is taken for precipitation. Thus all retardations in the process are avoided, and likewise the chances of error from washing on the filter. It is true that this method may lead to a certain error due to the volume of the insoluble matter which is left undecomposed, but since this insoluble matter is usually small in quantity, and since it is always possible to diminish the error therefrom by increasing the volume of the solution, this cause of error is much less to be feared than those due to the difficulties which may occur in the other method. Let us suppose, in order to illustrate the above, that we are dealing with a phosphate containing fifty per cent of insoluble sand which may be considered as an extreme limit. In working on four grams of the material in a flask of 100 cubic centimeters capacity, there will be an insoluble residue of two grams occupying a volume of about one cubic centimeter, the density of the sand being generally nearly two. The one hundred cubic centimeter flask will then contain only ninety-nine cubic centimeters of the real solution, and the error at the most would be 0.01. This error could be reduced to one-half by dissolving only two grams of the material in place of four, or by making the volume up to 200 instead of 100 cubic centimeters.
In general it may be said that the errors which do not exceed 0.01 of the total matter under treatment, are negligible for all industrial products. The method of volumetric solution does not present any further inconvenience. It deserves to be and has been generally adopted by reason of its rapidity in all the laboratories where many analyses are to be made. In the volumetric method great care should be taken not to make up to the volume until after the cooling to room temperature, which may be speedily secured by immersing the flask in cold water. Care should also be exercised in taking the sample for analysis by means of the pipette immediately after filtration, and filtration should take place as soon as the volume is made up to the standard. By operating in this way the possible variations from changes of volume due to changes of temperature are avoided.
(8) Examination for Arsenic Acid.—When the sample examined contains pyrites, arsenic is often present. When the decomposition has been effected by means of nitric acid, arsenic acid may be produced. This deports itself in all circumstances like phosphoric acid, and if it is present in the matter under examination it will be found united with the phosphoric acid and determined therewith afterwards. It is easy to avoid this cause of error by passing first a current of sulfurous acid through the solution, carrying it to the boiling-point in order to drive out the excess of sulfurous acid, and afterwards precipitating the arsenic by a current of hydrogen sulfid. After filtration, the rest of the operation can be carried on as already described.
89. Precipitation of the Phosphate by Magnesium Citrate.—By means of an accurate pipette a quantity of the solution representing from 0.125 to 0.250 gram or more is taken, according to the presumed richness of the product to be examined. In order that the following operations may go on well, it is necessary that the quantity of phosphoric acid contained in the sample should be about fifty milligrams. The sample being measured is run into a beaker, and there are added, first, ten cubic centimeters of magnesium citrate solution, and second, a large excess of ammonia. If the quantity of the magnesium citrate solution be sufficient, the mixture should at first remain perfectly limpid and only become turbid at the end of some moments and especially after the mixture is stirred.
If there should be an immediate turbidity produced it is proof that the quantity of magnesium citrate solution employed has been insufficient, and it is necessary to begin again by doubling its amount. Good results cannot be obtained by adding a second portion of the magnesium citrate solution to the original, since the iron and aluminum phosphates which are once formed are redissolved with difficulty. Many chemists at the present time abstain from using the magnesium citrate solution and replace it by a solution of citric acid and one of magnesium sulfate, which they pour successively into the sample under examination. This is a cause of grave errors which it is necessary to point out. Joulie has indeed recognized the fact that the precipitation of the phosphoric acid is not completed in presence of ammonium citrate except it is employed in conjunction with a sufficient excess of magnesia. But the foreign matters which accompany the phosphoric acid require different quantities of ammonium citrate in order to keep them in solution, and it is important to increase the magnesium solution at the time of increasing the citric acid in order to maintain them always in the same proportion. This is easily accomplished by measuring the two solutions, but it is much more easily done by uniting them and adding them together.
90. The Magnesium Citrate Solution.—The formula originally proposed by Joulie, and modified by Millot, and adopted by the French Association of Chemists, is as follows: Citric acid, 400 grams; pure magnesium carbonate, forty grams; caustic magnesia, twenty grams; distilled water, half a liter. After solution, add enough of ammonia to render strongly alkaline, requiring about 600 cubic centimeters. Make the volume up with distilled water to one and a half liters. If the solution be turbid, it is proof that the magnesia or the carbonate employed contains some phosphoric acid which is to be separated by filtration, and the solution can then be preserved indefinitely.
91. Time of Subsidence.—When the phosphoric acid is precipitated by the mixture above mentioned, it is necessary to allow it to subside for a certain time under a bell jar in order to avoid the evaporation of the ammonia. In order to give plenty of time for this subsidence, it is well to make the precipitations in the afternoon and the filtrations the following morning. There are thus secured twelve to fifteen hours of repose, which is time amply sufficient for all cases.
92. Filtration and Washing.—Filtration is performed easily and rapidly upon a small filter without folds placed in a funnel with a long stem of about two millimeters internal diameter. Placed in a series of six or eight, they allow the filtration to take place in regular order without loss of time, the first filter being always empty by the time the last one is filled. The supernatant liquid from the precipitate should first be decanted on the filter, avoiding the throwing of the filtrate on the filter which would greatly retard the process, especially if it should contain a little silica, as often happens.
When the clear liquid is thus decanted as completely as possible, the rest of the precipitate is treated with water to which one-tenth of its volume of ammonia has been added, and the washing is continued by decantation as at first, and afterwards by washing upon the filter until the filtered solution gives no precipitate with sodium phosphate. Four washings are generally sufficient to attain this result.
If the operations which precede have been well-conducted, the total phosphoric acid contained in the material under examination is found upon the filter-paper, except the small portion which remains adhering to the beaker in which the precipitation has been made. The determination of the phosphoric acid comprises the following operations: First, solution of the ammonium magnesium phosphate and second, titration by means of a standard solution of uranium.
93. Solution of the Ammonium Magnesium Phosphate.—The phosphate which has been collected upon the filter is dissolved by a ten per cent solution of pure nitric acid. This solution is caused to pass into the beaker in which the precipitation was made in order to dissolve the particles of phosphate which remain adherent to its sides; and this solution is then thrown upon the filter. The filtrate is then received in a flask of about 150 cubic centimeters capacity, marked at seventy-five cubic centimeters. After two or three washings with the acidulated water, the filter itself is detached from the funnel and introduced into the vessel which contains the solution.
The whole of the filtrate being collected in the flask it is saturated by one-tenth ammoniacal water until a slight turbidity is produced. One or two drops of dilute nitric acid are now added until the liquor becomes limpid, and the flask is placed upon a sand-bath in order to carry the liquid to the boiling-point. After ebullition there are added five cubic centimeters of acid sodium acetate in order to cause the free nitric acid to disappear and immediately the titration, by means of a standard solution of uranium, is undertaken.
94. Acid Sodium Acetate.—The acid sodium acetate is prepared as follows: Crystallized sodium acetate, 100 grams; glacial acetic acid, fifty cubic centimeters; distilled water, enough to make one liter.
95. Standard Solution of Uranium.—A solution of uranium is to be prepared as follows: Pure uranium nitrate, forty grams; distilled water, about 800 cubic centimeters. Dissolve the uranium nitrate in the distilled water and add a few drops of ammonia until a slight turbidity is produced, and then a sufficient amount of acetic acid to cause this turbidity to disappear. The volume is then completed to one liter with distilled water.
The uranium nitrate often contains some uranium phosphate and some ferric nitrate. It is important that it be freed from these foreign substances. This is secured by dissolving it in distilled water and precipitating it by sodium carbonate, which redissolves the uranium oxid and precipitates the iron phosphate and oxid.
The filtered liquor is saturated with nitric acid, and the uranium oxid reprecipitated by ammonia. It is then washed with distilled water by decantation and redissolved in nitric acid, as exactly as possible, evaporated, and crystallized.
The crystals are taken up with ether, which often leaves still a little insoluble matter. The solution is filtered, and the ether evaporated. The salt which remains is perfectly pure. It frequently happens when the uranium nitrate has not been properly purified that the solution prepared as has been indicated above, deposits a light precipitate of phosphate which alters its strength and affords a cause of error. Only those solutions should be employed which have been prepared some days in advance, and which have remained perfectly limpid.
The solution of uranium thus obtained contains uranium nitrate, a little ammonium nitrate, a very small quantity of uranium acetate, some ammonium acetate, and a little free acetic acid. Its sensibility is the more pronounced as the acetates present in it are less in quantity. It is important, therefore, never to prepare the solution with uranium acetate.
96. Typical Solution of Phosphoric Acid.—In order to titrate a solution of uranium, it is necessary to have a standard solution of phosphoric acid; that is to say, a solution containing a precise and known quantity of that acid in a given volume. This solution is prepared by means of acid ammonium phosphate, a salt which is easily obtained pure and dry. Sometimes as it may contain a small quantity of neutral phosphate which modifies the relative proportions of phosphoric acid and ammonia, and it is indispensable to have its strength verified. The titer of the typical solution should be such that it requires for the precipitation of the phosphoric acid which it contains, a volume of the solution of uranium almost exactly equal to its own, in order that the expansions or contractions which the two liquors undergo, by reason of changes in the temperature of the laboratory, should be without influence upon the results.
The solution of uranium prepared as has been indicated above, precipitates almost exactly five milligrams of phosphoric acid per cubic centimeter; the typical solution of phosphoric acid is prepared with eight and one-tenth grams of acid ammonium phosphate pure and dry, which is dissolved in a sufficient quantity of distilled water to make one liter.
The acid ammonium phosphate containing 61.74 per cent of anhydrous phosphoric acid, the quantity above gives exactly five grams of that acid in a liter, or five milligrams in a cubic centimeter.
97. Verification of the Strength of the Standard Solution of Phosphoric Acid.—The strength of the standard solution of phosphoric acid is verified by evaporating a known volume, fifty cubic centimeters for example, with a solution of ferric hydroxid containing a known quantity of ferric oxid. The mass having been evaporated to dryness, and ignited in a platinum crucible, gives an increase in the weight of the iron oxid exactly equal to the amount of anhydrous phosphoric acid contained therein, both the nitric acid and ammonia being driven off by the heat.
To prepare the solution of ferric hydroxid, dissolve twenty grams of iron filings in hydrochloric acid. The solution is filtered to separate the carbon, and it is converted into ferric nitrate by nitric acid, and the solution diluted with distilled water, and the ferric oxid precipitated by a slight excess of ammonia. The precipitate, washed by decantation with distilled water until the wash-water no longer gives a precipitate with silver nitrate, is redissolved in nitric acid, and the solution is concentrated or diluted, as the case may be, to bring the volume to one liter.
In order to determine the quantity of ferric oxid which it contains, fifty cubic centimeters are evaporated to dryness, ignited, and weighed.
A second operation like the above is carried on by adding fifty cubic centimeters of the standard solution of phosphoric acid, and the strength of the solution thus obtained is marked upon the flask.
If the operation have been properly carried on, three or four duplicates will give exactly the same figures. If there are sensible differences, the whole operation should be done over from the first.
98. Titration of the Solution of Uranium.—In a 150 cubic centimeter flask marked at seventy-five cubic centimeters, are poured ten cubic centimeters of the standard solution of phosphoric acid measured with an exact pipette; five cubic centimeters of the acid sodium acetate are added, and distilled water enough to make about thirty cubic centimeters, and the whole carried to the boiling-point. The titration is then carried on by allowing the solution of uranium to fall into the flask from a graduated burette, thoroughly shaking after each addition of the uranium, and trying a drop of the liquor with an equal quantity of a ten per cent solution of potassium ferrocyanid upon a greased white plate. Since the quantity of the uranium solution present will be very nearly ten cubic centimeters at first, nine cubic centimeters can be run in without testing. Afterwards, the operation is continued by adding two or three drops at a time until the test upon the white plate with the potassium ferrocyanid shows the end of the reaction. When there is observed in the final test a slight change of tint, the flask is filled up to the mark with boiling distilled water and the process tried anew. If in the first part of the operation the point of saturation have not been passed, it is still usually necessary to add a drop or two of the uranium solution in order to produce the characteristic reddish coloration, and this increase is rendered necessary by the increase in the volume of the liquid. Proceeding in this manner two or three times allows the attainment of extreme precision, inasmuch as the analyst knows just when to look for the point of saturation.
Correction.—The result of the preceding operation is not absolutely exact. It is evident indeed that in addition to the quantity of uranium necessary for the exact precipitation of the phosphoric acid, it has been necessary to add an excess sufficient to produce the reaction upon the potassium ferrocyanid.
This excess is rendered constant by the precaution of operating always upon the same volume; namely, seventy-five cubic centimeters. It can be determined then once for all by making a blank determination under the same conditions but without using the phosphoric acid.
The result of this determination is that it renders possible the correction which it is necessary to make by subtracting the quantity used in the blank titration from the preceding result in order to obtain the exact strength of the uranium solution.
The operation is carried on as follows: In a flat-bottomed flask of about 150 cubic centimeters capacity and marked at seventy-five cubic centimeters, by means of a pipette, are placed five cubic centimeters of the solution of sodium acetate; some hot distilled water is added until the flask is filled to the mark, and it is then placed upon a sand-bath and heated to the boiling-point. It is taken from the fire, the volume made up to seventy-five cubic centimeters with a little hot distilled water, and one or two drops of the solution of uranium are allowed to flow into the flask from a graduated burette previously filled exactly to zero. After each drop of the solution of uranium the flask is shaken and the liquid tried upon a drop of potassium ferrocyanid, as has been previously indicated. For a skilled eye, four to six drops are generally necessary to obtain the characteristic coloration; that is from two-tenths to three-tenths of a cubic centimeter. Beginners often use from five-tenths to six-tenths, and sometimes even more.
The sole important point is to arrest the operation as soon as the reddish tint is surely seen, for afterwards the intensity of the coloration does not increase proportionally to the quantity of liquor employed.
It is well to note that at the end of some time the coloration becomes more intense than at the moment when the solutions are mixed, so that care must be taken not to pass the saturation-point. This slowness of the reaction is the more marked as there is more sodium or ammonium acetate in the standard solutions. This is the reason that it is important to introduce always the same quantity; namely, five cubic centimeters. This is also the reason why the uranium acetate should not be employed in preparing the standard solution of uranium which ought to contain the least possible amount of acetate in order that the necessary quantity which is carried into each test should be as small as possible and remain without appreciable influence. If it were otherwise, the sensibility of the reaction would be diminished in proportion as a larger quantity of uranium solution was employed, giving rise to errors which would be as much more important as the quantities of phosphoric acid to be determined were greater. The correction for the uranium solution having been determined it is written upon the label of the bottle containing it.
Causes of Errors.—In the work which has just been described, some causes of error may occur to which the attention of analysts should be called.
The first is the error which may arise from the consumption of the small quantity of uranium phosphate which is taken with a stirring rod when the liquid is tested with potassium ferrocyanid. It is very easy to be assured that the end of the reaction has really been reached. For this purpose it is only necessary to note the quantity of the solution already employed and to add to it afterwards four drops; shake, and make a new test with a drop of the potassium ferrocyanid placed near the spot which the last one occupied. If a decidedly reddish tint does not appear at the moment of removing the glass rod, it is to be concluded that the first appearance was an illusion, and the addition of uranium is to be continued. If, on the contrary, the coloration appear of a decided tint, the preceding number may be taken for exact. It is then always beneficial to close the titration by this test of four supplementary drops which will exaggerate the coloration and confirm the figure found.
The second cause of error, and one moreover which is the most frequently met with, consists in passing the end of the reaction by adding the uranium too rapidly. In place of giving then a coloration scarcely perceptible, the test with the drop of potassium ferrocyanid gives a very marked coloration. In this case the analysis can still be saved. For this purpose the analyst has, at his disposal, a tenth normal solution prepared with 100 cubic centimeters of the standard solution of phosphoric acid diluted to one liter with distilled water. Ten cubic centimeters of this tenth normal solution are added, and the titration continued. At the end, the amount of additional phosphoric acid used is subtracted from the total.
A third cause of error is found in the foam which is often found in the liquid, due to the shaking. This foam may retain a portion of the last drops of the solution of uranium which fall upon its surface and prevent its mixture with the rest of the liquid. If the glass stirring rod in being removed from the vessel pass through this froth charged with uranium, the characteristic coloration is obtained before real saturation is reached. Consequently it is necessary to avoid, as much as possible, the formation of the foam, and especially to take care never to take the drop for test after agitation except in the middle of the liquid where the foam does not exist.
Suppose the titration has been made upon ten cubic centimeters of the normal solution of phosphoric acid in the conditions which we have just indicated, and the figure for the uranium obtained is 10.2 cubic centimeters; if now the correction, which may be supposed to amount to two-tenths cubic centimeter, be subtracted there will remain ten cubic centimeters of the uranium solution which would have precipitated exactly fifty milligrams of phosphoric acid.
The quantity of phosphoric acid which precipitates one cubic centimeter of the solution will be consequently expressed by the proportion ⁵⁰/₁₀ = five milligrams, which is exactly the strength required. In the example which has just been given, the inscription upon the flask holding the standard solution would be as follows: Solution of uranium, one cubic centimeter equals five milligrams of phosphorus pentoxid; correction, two-tenths cubic centimeter.
99. Titration of the Sample.—The strength of the solution of uranium having been exactly determined, by means of this solution the strength of the sample in which the phosphoric acid has been previously prepared as ammonium magnesium phosphate is ascertained. In this case the quantity of phosphoric acid being unknown, it is necessary to proceed slowly and to duplicate the tests in order not to pass beyond the point of saturation. From this there necessarily results a certain error in consequence of the removal of quite a number of drops of the solution of the sample before the saturation is complete. It is therefore necessary to make a second determination in which there is at once added almost the quantity of the solution of uranium determined by the first analysis. Afterwards the analysis is finished by additions of very small quantities of uranium until saturation is reached. Suppose, for instance, that the sample was that of a mineral phosphate, five grams of which were dissolved in 100 cubic centimeters, and of which ten cubic centimeters of the solution prepared as above required 15.3 cubic centimeters of the standard solution of uranium. We then would have the following data:
Mineral phosphate, five grams of the material dissolved in twenty cubic centimeters of hydrochloric acid.
Water, sufficient quantity to make 100 cubic centimeters.
Quantity taken, ten cubic centimeters = 0.50 gram of the sample taken.
| Solution of uranium required | 15.3 | cubic | centimeters. |
| Correction | 0.2 | “ | “ |
| Actual quantity of uranium solution | 15.1 | “ | “ |
Strength of the solution of uranium, one cubic centimeter = fivemilligrams P₂O₅.
Then P₂O₅ in 0.50 gram of the material = 5 × 15.1 = 75.50 milligrams.
| Then the per cent of P₂O₅ = | 75.5 x 100 | = 15.10. |
| 50 |
The sample under examination ought always to be prepared in duplicate, either by making a single precipitation and re-solution of the ammonium magnesium phosphate which is made up to a certain volume and an aliquot portion of which is taken for the analysis, or by making two precipitations under the conditions previously described. When the content of phosphoric acid in the material under examination is very nearly known, the double operation may be avoided, especially if it be required to have rapid and only approximate analyses, such as those which are made for general control and for the conduct of manufacturing operations. But when analyses are to be used to serve as the basis of a law or for the control of a market, they should always be made in duplicate, and the results ought not to be accepted when the numbers obtained are widely different, since the agreement of the two numbers will show that the work has been well executed.
This method of analysis, much longer to describe than to execute, gives results perfectly exact and always concordant when it is well carried out, provided that the standard solutions, upon which it rests for its accuracy, are correctly prepared and frequently verified in the manner indicated.
The strength of the solution of uranium ought to be verified every three or four days. The strength of the standard solution of phosphoric acid should be verified each time that the temperature of the laboratory undergoes any important change. A solution prepared, for example, in winter when the temperature of the laboratory is from 15° to 18° would no longer be exact in summer when the temperature reaches 28° or 30°.
100. Condition of Phosphoric Acid in Superphosphates.—Superphosphates are the products of the decomposition of phosphates by sulfuric or hydrochloric acid. They contain phosphoric acid combined with water, with lime, with magnesia, and with iron and alumina in various proportions.
These combinations may be classed in three categories: First, those compounds soluble in water; second, those insoluble in water, but very soluble in ammoniacal salts of the organic acids such as the citrate and oxalate; and third, phosphates not soluble in any of the above-named reagents.
In the products soluble in water are met free phosphoric acid, monocalcium phosphate, acid magnesium phosphate, and the iron and aluminum phosphates dissolved in the excess of phosphoric acid. In the products insoluble in water but soluble in the ammonium citrate are found bicalcium phosphate and iron and aluminum phosphates, which together constitute the phosphates called reverted.
These compounds reduced to a very fine state of division in the process of manufacture are considered to contain phosphoric acid of the same economic value.
101. Determination of the Total Phosphoric Acid in Superphosphates and Fertilizers.—The process is carried on exactly as for an ordinary phosphate, and with all the care indicated in connection with the sampling, the incineration, the solution by means of hydrochloric acid, and the separation of the phosphoric acid in the state of ammonium magnesium phosphate, and finally in the titration by uranium.
102. Determination of Soluble and Reverted Phosphoric Acid.—To make this determination a method unique and applicable to all cases consists in extracting, at first, the soluble constituents in distilled water, and following this operation by digestion in the ammonium citrate. The products soluble in water can be determined either separately or at the same time as the products soluble in the ammonium citrate according to the taste of the people interested, without its being necessary to modify very greatly the method of operation.
The determination of the soluble phosphoric acid comprises first, the solution of the soluble constituents in distilled water; second, the solution of the reverted phosphates in ammonium citrate; third, the precipitation of the phosphoric acid dissolved in the two preceding operations, and its determination.
103. Preparation of the Sample for Analysis.—The sample sent to the chemical expert is prepared as has been indicated; that is to say, it is poured on a sieve of which the meshes have a diameter of one millimeter, and sifted upon a sheet of white paper. The parts which do not pass the sieve are broken up either by the hand or in a mortar and added, through the sieve, to the first portions. The product is well mixed and, in this state, the mass presents all the homogeneity desirable for analysis.
Some fertilizers are received in a pasty state which does not permit of their being sifted. It is necessary in such a case to mix them with their own weight either of precipitated calcium sulfate dried at 160° or with fine sand washed with hydrochloric acid and dried, which divides the particles perfectly and permits of their being passed through the meshes of the sieve.
104. Extraction of the Products Soluble in Distilled Water.—The substance having been prepared as has just been indicated, one and a half grams are placed in a glass mortar. Twenty cubic centimeters of distilled water are added, and the substance gently suspended therein. After standing for one minute, the supernatant part is decanted into a small funnel provided with a filter-paper and placed in a flask marked at 150 cubic centimeters. This operation is repeated five times and is terminated by an intimate breaking up of the matter with distilled water. When the volume of 100 cubic centimeters of the filtrate has been obtained, the residue in the mortar is placed on the filter, and the washing is continued until the total volume reaches 150 cubic centimeters. The filtrate is shaken in order to render the liquor homogeneous, and is transferred to a precipitating glass of about 300 cubic centimeters capacity.
105. Solution of the Reverted Phosphates by Ammonium Citrate.—The filter from the above process is detached from the funnel and is introduced into a flask marked at 150 cubic centimeters together with sixty cubic centimeters of alkaline ammonium citrate prepared in the following manner:
- Pure citric acid, 400 grams.
- Ammonia of 22°, 500 cubic centimeters.
The ammonia is poured upon the citric acid in the form of crystals in a large dish. The mass becomes heated, and the solution takes place rapidly. When it is complete and the solution is cold it is poured into a flask of one liter capacity, and the flask is filled up to the mark with strong ammonia. It is preserved for use in a well-stoppered bottle. The solution must be strongly alkaline.
The flask in which the filter-paper is introduced, together with the ammonium citrate, is stoppered and shaken violently in order to disintegrate the filter-paper and put the reverted phosphates in suspension. There are added then about sixty cubic centimeters of distilled water, and the flask is shaken and left for twelve hours at least, or at most for twenty-four hours. The volume is made up to 150 cubic centimeters with distilled water, and, after mixture, the solution is filtered.
There are thus obtained two solutions which can be precipitated together or separately according to circumstances. The most usual process is to combine the two equal volumes of twenty-five, fifty, or one hundred cubic centimeters, representing one-quarter, one-half, or one gram of the material according to its presumed richness, in a precipitating flask to which are added from ten to twenty cubic centimeters of the solution of magnesia made up as follows:
| Magnesium carbonate, | 50 | grams. | |
| Ammonium chlorid, | 100 | “ | |
| Water, | 500 | cubic | centimeters. |
| Hydrochloric acid, | 120 | “ | “ |
After complete solution of the solid matters in the above, add 100 cubic centimeters of ammonia of 22° strength, and distilled water enough to make one liter.
The solutions are thoroughly mixed in a precipitating glass, an excess of ammonia added, and allowed to stand for twelve hours under a bell jar. The phosphoric acid contained in the liquor is separated as ammonium magnesium phosphate. It is collected upon a small filter, washed with a little ammoniacal water, redissolved, and titrated with the uranium solution in the manner already indicated.
Example: The following is an example of this kind of a determination:
(1) One and one-half grams of the superphosphate and distilled water enough to make 150 cubic centimeters.
(2) Filter-paper with reverted phosphates, sixty cubic centimeters of ammonium citrate, and a sufficient quantity of distilled water to make 150 cubic centimeters.
| Aqueous solution | (1) | 25 cc |  | = 0.25 grams of the sample. |
| Citrate solution | (2) | 25 cc |
Add magnesium solution twenty cubic centimeters and ammonia in excess, and allow from twelve to twenty-four hours of digestion, then filter and wash, dissolve and titrate.
Required of solution of uranium 8.55 cubic centimeters (1 cubic centimeter = 5 milligrams P₂O₅).
Correction 0.20.
Remainder 8.35 × 0.005 = 0.04175 gram P₂O₅ for 0.25 gram of the sample. Then 0.04175 ÷ 0.25 = 16.7 per cent.
From the above data there would be 16.7 per cent of phosphoric acid soluble in water and in ammonium citrate.
If it be desirable to have separately the phosphoric acid soluble in water, a separate precipitation is made of the aqueous solution alone by means of the magnesium citrate solution. The precipitate washed with ammoniacal water is redissolved and titrated in the manner indicated.
In subtracting from the figures obtained with the two solutions together the number obtained for the phosphoric acid soluble in water, the number representing the phosphoric acid soluble in ammonium citrate alone, is obtained.
It is to be noted that the determinations with uranium require always two successive titrations. It would therefore be an advantage in all operations to precipitate a weight of ammonium magnesium phosphate sufficient for allowing this precipitate to be dissolved and made up to 100 cubic centimeters on which amount it would be possible to execute two, three, or four determinations, and thus to obtain a figure absolutely incontestable.
106. Conclusions.—It has been seen from the above data that the French chemists have worked out the uranium volumetric method with great patience and attention to detail. Where many determinations are to be made it is undoubtedly possible for an analyst to reach a high degree of accuracy as well as to attain a desirable rapidity, by using this method. For a few determinations, however, the labor of preparing and setting the standard solutions required would be far greater than the actual determinations either by the molybdate or citrate gravimetric methods. For control work in factories and for routine work connected with fertilizer inspection, the method has sufficient merit to justify a comparison with the processes already in use by the official chemists of this country.
The use of an alkaline ammoniacal citrate solution, however, for the determination of reverted acid renders any comparison of the French method with our own impossible. On the other hand the French method for water-soluble acid is based on the same principle as our own; viz., washing at first with successive small portions of water, and thus avoiding the decomposition of the soluble phosphates, which is, likely to occur when too great a volume of water is added at once.
In the matter of the temperature and time as affecting the solubility of reverted acid, the French method is also distinctly inferior to our own. The digestion is allowed to continue from twelve to twenty-four hours, at the pleasure of the analyst, and meanwhile it is subjected to room temperature. It is not difficult to see that this treatment in the same sample would easily yield disagreeing results between twelve hours at a winter temperature and twenty-four hours at summer heat.
THE DETERMINATION OF PHOSPHORIC ACID
BY TITRATION OF THE YELLOW PRECIPITATE.
107. Pemberton’s Volumetric Method.—In order to shorten the work of determining the phosphoric acid, numerous attempts have been made to execute the final determination directly on the yellow precipitate obtained by treating a solution of a phosphate with ammonium molybdate in nitric acid. The composition of this precipitate appears to be somewhat variable, and this fact has cast doubt on the methods of determination based on its weight. Its most probable composition is expressed by the following formula, (NH₄)₃PO₄(MoO₃)₁₂. For convenience in writing reactions this formula should usually be doubled. Pemberton has described a volumetric determination of phosphoric acid in the yellow precipitate which has the merit of being rapid.[83]
In this laboratory the method has not given very satisfactory results when compared with the molybdate gravimetric process. It has however attracted so much attention from analysts as to merit description, and the details of the process are therefore given.
108. The Process.—One gram of phosphate rock, or from two to three grams of phosphatic fertilizer, are dissolved in nitric acid, and, without evaporation, diluted to 250 cubic centimeters. Without filtering, twenty-five cubic centimeters are placed in a four-ounce beaker and ammonia added until a slight precipitate begins to form. Five cubic centimeters of nitric acid of one and four-tenths specific gravity are then added, and afterwards ten cubic centimeters of saturated solution of ammonium nitrate and enough water to make the volume about sixty-five cubic centimeters. The contents of the beaker are boiled, and while still hot, five cubic centimeters of the aqueous solution of ammonium molybdate added. Additional quantities of the molybdate are added, if necessary, until the whole of the phosphorus pentoxid is thrown-out.
After allowing to settle for a moment the contents of the beaker are poured upon a filter seven centimeters in diameter. The precipitate is thoroughly washed with water, both by decantation and on the filter. The filter with its precipitate is transferred to a beaker and titrated with standard alkali, in the presence of phenolphthalein. Each cubic centimeter of alkali employed should correspond to one milligram of phosphorus pentoxid, (P₂O₅).
The reagents employed have the composition indicated below:
Ammonium Molybdate.—Ninety grams of the crystals of ammonium molybdate are placed in a large beaker and dissolved in a little less than one liter of water. The beaker is allowed to stand over night and the clear liquor decanted. Any undissolved acid is brought into solution in a little ammonia water and added to the clear liquor. If a trace of phosphoric acid be present a little magnesium sulfate is added and enough ammonia to produce a slight alkaline reaction. The volume of the solution is then made up to one liter. Each cubic centimeter of this solution is capable of precipitating three milligrams of phosphorus pentoxid.
Standard Potassium Hydroxid.—This solution is made of such strength that one cubic centimeter is equivalent to one milligram of phosphorus pentoxid. Treated with acid of normal strength, 100 cubic centimeters are required to neutralize 32.37 cubic centimeters thereof.
Standard Acid.—This should have the same strength, volume for volume, as the standard alkali solution. It is made by diluting 323.7 cubic centimeters of normal acid to one liter.
Indicator.—The indicator to be used is an alcoholic solution of phenolphthalein, one gram in 100 cubic centimeters of sixty per cent alcohol, and half a cubic centimeter of this should be used for each titration.
Thomson has shown[84] that of the three hydrogen atoms in phosphoric acid two must be saturated with alkali before the reaction with phenolphthalein is neutral. Therefore, when the yellow precipitate is broken up by an alkali, according to the reaction to follow, only four of the six molecules of ammonium are required to form a neutral ammonium phosphate as determined by the indicator employed. The remaining two molecules of ammonium unite with the molybdenum forming also a salt neutral to the indicator.
Phenolphthalein is preferred because, as has been shown by Long, its results are reliable in the presence of ammonium salts unless they be present in large quantity, and if the solution be cold and the indicator be used in sufficient quantity.[85] To prepare the indicator for this work, one gram of phenolphthalein is dissolved in 100 cubic centimeters of sixty percent alcohol. At least one-half of a cubic centimeter of the solution is used for each titration.
The advantages claimed for the method are its speed and accuracy. Much time is saved by avoiding the necessity for the removal of the silica by evaporation. The results of analyses with and without the removal of the silica are practically identical. When the silica is not removed it is noticed that the filtrate from the yellow precipitate has a yellow tint.
The reaction is represented by the following formula:
(NH₄)₆(PO₄)₂(MoO₃)₂₄ + 46KOH = (NH₄)₄(HPO₄)₂ + (NH₄)₂MoO₄
+ 23K₂MoO₄ + 22H₂O.
From this reaction it is seen that the total available acidity of one molecule of the yellow precipitate titrated against phenolphthalein is equivalent to twenty-three molecules of potassium hydroxid.
Calculation of Results.—The standard alkali is of such strength that one cubic centimeter is equal to one per cent of phosphoric acid when one gram of material is employed and one-tenth of it taken for each determination. In a given case one gram of a sample was taken and one-tenth of the solution used. Fifty cubic centimeters of alkali were added to the yellow precipitate. It required thirty-two cubic centimeters of standard alkali to neutralize the excess.
The alkali consumed by the yellow precipitate was 50 - 32 = 18. The sample therefore contained eighteen per cent of phosphoric acid.
Comparison with Official Method.—A comparison of the Pemberton volumetric with the official method of the Association of Agricultural Chemists has been made by Day and Bryant.[86] The comparisons were made on samples containing from 1.45 to 37.28 per cent of phosphoric acid and resulted as follows:
| Substance. | Per cent P₂O₅, Official. | Per cent P₂O₅, Pemberton. | ||
|---|---|---|---|---|
| No. 1. | Florida | rock | 1.45 | 1.32 |
| “ 2. | “ | “ | 4.40 | 4.53 |
| “ 3. | Sodium | phosphate | 19.78 | 19.99 |
| “ 4. | “ | “ | 19.72 | 19.73 |
| “ 5. | Florida | rock | 37.28 | 37.22 |
This near agreement shows the reliability of the method. The comparison of the Pemberton volumetric method with the official gravimetric method was investigated by the reporter of the Association of Official Agricultural Chemists in 1894.[87] The individual variations were found to be greater than in the regular method but the average results were nearly identical therewith. The method works far better with small percentages of phosphoric acid than with large. Where the average of the results by the official methods gave 12.25 per cent, the volumetric process gave 11.90 per cent, whereas in the determination of a smaller percentage the results were 2.72 and 2.73 per cent, respectively. Kilgore proposes a variation of the method which differs from the original in two principal points.[88] First the temperature of precipitation in the Pemberton process is 100°; but in the modified form from 55° to 60°. At the higher temperature there is danger of depositing molybdic acid.
The second difference is in the composition of the molybdate solution employed. The official molybdate solution contains about sixty grams of molybdenum trioxid in a liter while the Pemberton solution contains sixty-six grams. There is therefore not much difference in strength. The absence of nitric acid, however, from the Pemberton solution favors the deposition of the molybdic acid when heat is applied. Kilgore, therefore, conducts the analysis as follows: The solution of the sample is made according to the official nitric and hydrochloric acid method for total phosphoric acid. For the determination, twenty or forty cubic centimeters are taken, corresponding to two-tenths or four-tenths gram of the sample. Ammonia is added until a slight precipitate is produced and the volume is then made up, with water, to seventy-five cubic centimeters. Add some ammonium nitrate solution, from ten to fifteen cubic centimeters, but this addition is not necessary unless much of the nitric acid has been driven off during solution. Heat in water-bath to 60° and precipitate with some freshly filtered official molybdate solution. Allow to stand for five minutes, filter as quickly as possible, wash four times by decantation using from fifty to seventy-five cubic centimeters of water each time, and then wash on a filter until all acid is removed. The solution and titration of the yellow precipitate are accomplished as in the Pemberton method. The agreement of the results obtained by this modified method was much closer with the official gravimetric method than those obtained by the Pemberton process.
109. Estimation of Phosphoric Acid as a Lead Compound.—In the volumetric lead method, as described by Wavelet, the phosphoric acid is precipitated by the magnesium citrate solution as in the uranium method of Joulie, as practiced by the French chemists, and the washing of the precipitate and its solution in nitric acid are also conducted as in that method.[89] After solution in nitric acid ammonia is added to neutrality and the solution is then made acid with acetic. The phosphoric acid is precipitated in the acid solution by a standard solution of lead nitrate, the precipitate having the formula P₂O₅3PbO.
The end reaction is determined by placing a drop of the titrated mixture on a white greased dish in contact with a drop of a five per cent solution of potassium iodid. When all the phosphoric acid is precipitated the least excess of the lead salt is revealed by the characteristic yellow precipitate of lead iodid.
The author of the process claims that the lead phosphate is insoluble in the excess of acetic acid and that the phosphate itself does not give any yellow coloration with potassium iodid. The process is quite as exact as the uranium method and the end reaction is far sharper and the standard reagents are easily made and preserved.[90] The method described merits, at least, a comparative trial with the uranium process, but cannot be recommended as exact until further approved by experience.
The reagents employed have the following composition:
| (1) | Disodium phosphate | solution | containing | 10.085 | grams | per | liter |
| (2) | Sodium acetate | “ | “ | 50.000 | “ | “ | “ |
| (3) | Lead nitrate | “ | “ | 40.000 | “ | “ | “ |
| (4) | Potassium iodid | “ | “ | 50.000 | “ | “ | “ |
| The titrations should be conducted in the cold. | |||||||
110. Water-Soluble Phosphoric Acid.—Glaser has modified the volumetric method of Kalmann and Meissels for the volumetric estimation of water-soluble phosphoric acid so as to avoid the double titration required by the original method.[91] If methyl orange be used as an indicator in the original method, the determination does not at once lead to the tricalcium salt, but the liquid still contains, after neutralization, some monocalcium phosphate, which is determined by a further titration with phenolphthalein. In the modified method the total phosphoric acid is estimated in one operation as a tricalcium salt. This is secured, by adding, at the proper time, an excess of calcium chlorid. Two grams of the superphosphate are shaken with water several times, and, after settling, filtered, and the insoluble residue finally washed on the filter until the total volume of the filtrate is a quarter of a liter. Of this, fifty cubic centimeters are taken and titrated with tenth normal soda-lye, with addition of two drops of methyl orange, until the acid reaction has entirely disappeared. There is then added some neutral calcium chlorid solution in excess. If iron and alumina be present, a precipitate is produced of which no account need be made. The acid reaction is thus restored. Five drops of the phenolphthalein solution are added and the titration continued until the alkaline reaction is noted throughout the whole mass. Each cubic centimeter of the soda-lye corresponds, in the first titration, to 7.1, and in the second to 3.55 milligrams of phosphoric acid.
111. Estimation of Phosphoric Acid in the Presence of a Large Excess of Iron.—The method given below, due to Emmerton, depends upon the precipitation of a phosphomolybdate, of constant composition, in the presence of a large excess of iron, as in the analysis of iron and steel and iron ores.[92] The molybdenum trioxid obtained is reduced by zinc to Mo₁₂O₁₉. The action of permanganate on this compound is shown in the following equation:
5Mo₁₂O₁₉ + 17(K₂OMn₂O₇) = 60MoO₃ + 17K₂O + 34MnO.
Seventeen molecules of permanganate are equal to sixty molecules of molybdenum trioxid. The iron or steel is dissolved in nitric acid, evaporated to dryness, heated, and redissolved in hydrochloric acid, then treated again with nitric acid and evaporated until a clear and concentrated solution is obtained free from hydrochloric acid.
The solution obtained is diluted to forty cubic centimeters with water and washed into a 400 cubic centimeter flask, making the total volume about seventy-five cubic centimeters. Add strong ammonia, shaking after each addition, until the mass sets to a thick jelly from the ferric hydroxid. Add a few more cubic centimeters of ammonia and shake thoroughly, being sure the ammonia is present in excess. Add next nitric acid gradually, with shaking, until the precipitate has all dissolved; add enough more nitric acid to make the solution a clear amber color. The volume should now be about 250 cubic centimeters. Bring the solution to 85° and add, at once, forty cubic centimeters of molybdate solution of the following strength: Dissolve 100 grams of molybdic acid in 300 cubic centimeters of strong ammonia and 100 cubic centimeters of water, and pour the solution into 1,250 cubic centimeters of nitric acid (1.20); close the flask with a rubber stopper, wrap it in a thick cloth, and shake violently for five minutes. Collect the precipitate on a filter, using pump, and wash with dilute nitric acid (1 HNO₃ : 50 H₂O). If a thin film of the precipitate should adhere to the flask it can be removed by the ammonia in the next operation. Wash the molybdate precipitate into a 500 cubic centimeter flask with dilute ammonia (1 H₃N : 4 H₂O), using about thirty cubic centimeters. Add hot dilute sulfuric acid (1 H₂SO₄ : 4 H₂O) and cover the flask with a small funnel. Add ten grams of granulated zinc and heat until rapid action begins, and then heat gently for five minutes. The reduction is then complete. During the reduction the colors, pink, plum, pale green, and dark green, are seen in the molybdate solution, the latter color marking the end of the reaction.
To remove the zinc, pour through a large folded filter, wash with cold water, and fill up the filter once with cold water. But little oxidation takes place in this way. A port-wine color is seen on the filter, but this does not indicate a sufficient oxidation to make an error.
In titrating, the wine color becomes fainter and finally the solution is perfectly colorless and shows a single drop in excess of the permanganate. The permanganate solution, for convenience, is made so that one cubic centimeter is equal to 0.0001 gram of phosphorus. With iron its value is one cubic centimeter equals 0.006141 gram of iron; and one cubic centimeter equals 0.005574 gram of molybdenum trioxid.
112. Variation of Dudley and Noyes.—The method of Emmerton to determine small quantities of phosphoric acid, or of phosphorus in presence of a large excess of iron, has been modified by Dudley and Pease,[93] and by Noyes and Royse.[94] As modified, the method is not intended for fertilizer analysis, but the principle on which it rests may some time, with proper modifications, find application in fertilizer work. The reduction is accomplished in a Jones’ tube,[95] much simplified, so as to render it suitable for common use. The molybdic acid is reduced to a form, or series of forms, corresponding to molybdenum sesquioxid, as in the Emmerton method, and subsequently as in that method, titrated by a set solution of potassium permanganate.
The iron or steel filings, containing phosphorus, are brought into solution by means of nitric acid. For this purpose two grams of them are placed in a half liter flask together with fifty cubic centimeters of nitric acid of 1.18 specific gravity. The mixture is boiled for one minute, and ten cubic centimeters of permanganate solution of one and a quarter per cent added. Boil again until the pink color disappears. Ferrous sulfate solution is next to be carefully added, shaking meanwhile, until the solution clears. Cool to 50° and add eight cubic centimeters of ammonia of 0.90 specific gravity, stopper the flask, and shake until any precipitate which may form is redissolved. Cool or warm, as the case may be, until the solution is as many degrees above or below 60° as the molybdic solution is above or below 27°. Add sixty cubic centimeters of molybdic solution, stopper, and shake on a machine or by hand for five minutes. After remaining at rest for five minutes pour into a nine centimeter filter of fine texture and wash with the acid ammonium sulfate solution in quantities of from five to ten cubic centimeters each time. The filtrate and washings must be perfectly bright. Continue the washings until the filtrate gives no color with hydrogen sulfid.
Dissolve the yellow precipitate with twelve cubic centimeters of 0.96 ammonia diluted with an equal volume of water, and wash the filter with 100 cubic centimeters of water. Finally add to the filtrate and wash-water eighty cubic centimeters of water and ten of strong sulfuric acid. Pass the mixture through the Jones’ reducing tube and follow it with 200 cubic centimeters of water, taking care that no air enter the tube during the operations. The solution collected in the flask should be at once titrated with potassium permanganate.
Solutions used: (1) Nitric acid.—One part of nitric acid of 1.42 specific gravity and two parts of water by volume. The specific gravity of the mixture is about 1.18.
(2) Permanganate solution for oxidizing.—Dissolve 12.5 grams of potassium permanganate in one liter of water.
(3) Ferrous sulfate.—Fresh crystals not effervesced and free from phosphorus.
(4) Ammonia.—The strong ammonia used should have a specific gravity of about 0.90 and the dilute of 0.96 at 15.5°.
(5) Molybdic Solution.—Dissolve 100 grams of molybdic anhydrid in 400 cubic centimeters of ammonia of 0.96 specific gravity and pour the solution slowly, with constant stirring, into one liter of nitric acid of about 1.20 specific gravity. Heat the mixture to 45° and add one cubic centimeter of a ten per cent solution of sodium phosphate, stir vigorously, and allow to stand in a warm place for eighteen hours. Filter before using.
(6) Add Ammonium Sulfate.—To half a liter of water add 27.5 cubic centimeters of 0.96 ammonia and twenty-four cubic centimeters of strong sulfuric acid, and make the volume one liter with water.
(7) Potassium Permanganate for Titration.—Dissolve four grams of potassium permanganate in two liters of water, heat nearly to boiling for an hour, allow to stand for eighteen hours, and filter on asbestos felt. The solution must not come in contact with rubber or other organic matter. The solution may be standardized with thoroughly air-dried ammonium oxalate in solution with a little dilute sulfuric acid and with ammonium ferrous sulfate partly crystallized in small crystals from a slightly acid solution. The crystals should be well washed and quickly air-dried in a thin layer. The factors 1/1 1/4 2/2 and 1/7 should be used respectively to calculate the iron equivalent. The phosphorus equivalent is obtained by multiplying the iron equivalent by 31 ÷ (36 × 56) = 0.01538.
Figure. 7.
Jones’
Reduction
Tube.
Reduction Apparatus.—The reduction of the molybdic acid to molybdenum trioxid is accomplished in a tube first proposed by Jones. The apparatus is shown in [Figure 7]. A piece of moderately heavy glass tubing thirty-five centimeters long with an internal diameter of two centimeters is drawn out at the lower end so as to pass into the stopper of a flask. A circular piece of perforated platinum or porcelain rests on the constricted portion of the tube and this is covered with an asbestos felt. The tube is then nearly filled with powdered zinc which is washed, before using, with dilute sulfuric acid (1 : 20). A, B, C represent different methods of filtering the molybdic solution. In A a platinum cone is placed in the constricted portion of the tube and the asbestos felt placed thereon and the tube then filled with the granulated zinc. In B there is first inserted a perforated disk then some very fine sand and this is covered with another disk. In C there is a perforated disk which is covered with asbestos felt. The filtering arrangement should be such as to prevent any zinc particles from reaching the flask and yet permitting the filtration to go on without much difficulty. A blank determination is first made by adding to 180 cubic centimeters of water, twelve of 0.96 ammonia and ten of strong sulfuric acid. This is poured through the reducing tube and followed with 200 cubic centimeters of water taking care that no air enter the apparatus. Hydrogen peroxid is formed if air enter. Even after standing for a few moments the tube should be washed with dilute sulfuric acid before again using it. The filtrate should be titrated with the permanganate solution and the amount required deducted from the following amounts obtained with the molybdic salt.
Calculations.—The calculations of the amount of phosphorus in a given sample of iron or steel are made according to the following data: In a given case let it be supposed that the permanganate solution is set with a solution of piano wire and it is found that one cubic centimeter of permanganate liquor is equal to 0.003466 gram of metallic iron. It is found that 90.76 parts of molybdic acid will produce the same effect on permanganate as 100 parts of iron. Hence one cubic centimeter of permanganate solution is equivalent to 0.003466 × 0.9076 = 0.003145 gram of molybdic acid. In the yellow precipitate formed, in the conditions named for the analysis it is found that the phosphorus is one and nine-tenths per cent of the molybdic acid present. Therefore one cubic centimeter of permanganate liquor is equal to 0.003145 × 0.019 = 0.0000597 gram of phosphorus. If then, for example, in a sample of iron or steel eight and six-tenths cubic centimeters of permanganate solution, after correction, be found necessary to oxidize the molybdic solution after passing through the Jones’ reducing tube, the amount of phosphorus found is 0.0000597 × 8.6 = 0.051 per cent.
113. The Silver Method.—The separation of the phosphoric acid by silver according to the method of Perrot has been investigated by Spencer, who found the process unreliable.[96] By a modification of the process, however, Spencer obtained fairly satisfactory results. The principle of this method depends on the separation of the phosphoric acid by silver carbonate and the subsequent titration thereof with standard uranium solution after the removal of the excess of silver. The operation is conducted as follows: The fertilizer is first ignited until all organic matter and residual carbon are destroyed. Solution is then accomplished by means of nitric acid and the volume completed to a definite quantity. An aliquot part is taken, after filtration, varying with the supposed strength of the solution so as to contain about 100 milligrams of phosphorus pentoxid. In the slightly nitric acid solution add freshly prepared silver carbonate in excess, that is, sufficient to saturate any free acid present and also to combine with all the phosphoric acid. Wash thoroughly with hot water and then dissolve the mixed phosphate and silver carbonate in nitric acid and remove the silver from the solution with sodium chlorid. The phosphoric acid is determined in the filtrate by means of a standard solution of uranium nitrate in the manner already described. Spencer found that the separation of the phosphoric acid by the silver method was more exact than by the Joulie magnesium citrate process. With practice on the part of the analyst in determining the end reaction the process is both rapid and accurate. The method is also inexpensive, as both the silver and uranium are easily recovered from the waste.
114. Volumetric Silver Method.—Holleman has proposed a modification of the silver method for the volumetric determination of phosphoric acid, which is conducted in the following manner:[97]
In a flask of 200 cubic centimeters capacity, are placed fifty cubic centimeters of the liquid to be analyzed, which should not contain more than two-tenths gram of phosphoric acid. The solution is treated with ten cubic centimeters of a normal solution of sodium acetate and afterwards with a slight excess of decinormal silver solution, four and five-tenths cubic centimeters for each 0.01 gram of phosphoric acid. The solution is then neutralized with tenth normal sodium hydroxid, the amount required having been previously determined by titrating ten cubic centimeters of the liquid to be analyzed, using phenolphthalein as an indicator. Five times the quantity required for the neutralization of the ten cubic centimeters is added, less one-half cubic centimeter. By this treatment the phosphoric acid in the presence of sodium acetate is completely precipitated as silver phosphate. The excess of silver is determined by diluting the mixture to 200 cubic centimeters, filtering, and titrating 100 cubic centimeters of the filtrate with ammonium thiocyanate. The presence of sulfuric and nitric acids does not interfere with the reaction, but of course hydrochloric acid must be absent. Alkalies and alkaline earth metals may be present, but not the heavy metals.
When iron and aluminum are present 100 cubic centimeters of the solution are precipitated with thirty cubic centimeters of normal sodium acetate, the phosphoric acid is determined in fifty cubic centimeters of the filtrate, and the precipitate of iron and aluminum phosphates is ignited and weighed, and its weight multiplied by 2.225 is added to the phosphoric anhydrid found volumetrically. If ammonia be present it must be removed by boiling, as otherwise it affects the titration with phenolphthalein.
For agricultural purposes this method can have but little value inasmuch as the phosphates to be examined almost always have a certain proportion of iron and aluminum. Inasmuch as the amount of these bases has to be determined gravimetrically, there would be no gain in time and no simplification of the processes by the use of the volumetric method as proposed.
TECHNICAL DETERMINATION
OF PHOSPHORIC ACID.
115. Desirability Of Methods.—In the preceding paragraphs, has been given a statement of the principal methods now in use by chemists and others connected with fertilizer control for the scientific and agronomic determinations of phosphoric acid, and its agricultural value.
A résumé of the important methods, in a form suited to use in a factory for preparing phosphatic fertilizers for the market, seems desirable. In these factories the chemists have been accustomed to use their own, or private methods, and there has not been a general disposition among them to publish their methods and experience for the common benefit. For factory processes, a method should be not only reasonably accurate, but also simple and rapid. It is evident, therefore, that the general principles already indicated must underlie any method which would prove useful to factory work. Albert has made a résumé of such methods applicable for factory control, and these are given here for convenience, although they are, in many respects, but condensed statements of methods already described.[98]
116. Reagents.—Molybdate Solution.—One hundred and ten grams of pure molybdic acid are dissolved in ammonia of nine-tenths specific gravity and diluted with water to one liter. The solution is poured into one liter of nitric acid, of one and two-tenths specific gravity, and, after standing a few days, filtered.
Concentrated Ammonium Nitrate Solution.—Seven hundred and fifty grams of pure ammonium nitrate are dissolved in water and made up to one liter.
Magnesia Mixture.—Fifty-five grams of magnesium chlorid; seventy grams of ammonium chlorid; 130 cubic centimeters of ammonia of nine-tenths specific gravity are dissolved and diluted with water to one liter.
Two and One-Half Per Cent Ammonia.—One hundred cubic centimeters of ammonia of nine-tenths specific gravity are diluted with water to one liter.
Joulie’s Citrate Solution.—Four hundred grams of citric acid are dissolved in ammonia of nine-tenths specific gravity and diluted to one liter with ammonia of the same strength.
Wagner’s Citrate Solution.—One hundred and fifty grams of citric acid are exactly neutralized with ammonia, then ten grams of citric acid added and diluted to one liter with water.
Sodium Acetate Solution.—One hundred grams of sodium acetate, crystallized, are dissolved in water, treated with 100 cubic centimeters of acetic acid, and diluted to one liter with water.
Calcium Phosphate Solution.—About ten grams of dry, pure tribasic calcium phosphate are dissolved in nitric acid and diluted with water to one liter. In this solution the phosphoric acid is determined gravimetrically by the molybdate or citrate method, and the value of the solution marked on the flask containing it.
Titrated Uranium Solution.—Two hundred and fifty grams of uranium nitrate are dissolved in water, twenty-five grams of sodium acetate added, and the whole diluted to seven liters. One cubic centimeter of this solution corresponds to about 0.005 gram of phosphorus pentoxid. In order to determine its exact value proceed as follows: Twenty-five cubic centimeters of the calcium phosphate solution which, for example, has been found to contain 0.10317 gram of phosphorus pentoxid, are neutralized in a porcelain dish with ammonia, acidified with acetic, treated with ten cubic centimeters of sodium acetate solution, and warmed. Through a burette as much uranium solution is allowed to flow as is necessary to show in a drop of the solution taken out of the dish, when treated with a drop of pure potassium ferrocyanid, a slight brown color. In order to be certain, this operation is repeated two or three times with new quantities of twenty-five cubic centimeters of calcium phosphate solution. Example:
Twenty-five cubic centimeters of the calcium phosphate solution containing 0.10317 gram of phosphorus pentoxid, gave as a mean of three determinations 23.2 cubic centimeters of the uranium solution necessary to produce the brown color with potassium ferrocyanid. Consequently 0.10317 ÷ 23.2 = 0.00445 gram of phosphorus pentoxid equivalent to one cubic centimeter of uranium solution. If, for instance, a quantity of fertilizer weighing exactly five grams, require ten cubic centimeters of the uranium solution for the complete precipitation of its phosphoric acid, then the quantity of phosphoric acid contained in the fertilizer would be equivalent to 10 × 0.0045, equivalent to 0.0445 gram of phosphorus pentoxid. The fertilizer, therefore, contains eight and nine-tenths per cent of phosphorus pentoxid.
Conduct of the Molybdenum Method.—This method rests upon the precipitation of the phosphorus pentoxid by a solution of ammonium molybdate in nitric acid, solution of the precipitate in ammonia, and subsequent precipitation with magnesia.
Manipulation.—Twenty-five or fifty cubic centimeters of a solution of the phosphate which has been made up to a standard volume and containing about one-tenth gram of phosphorus pentoxid, are placed in a beaker together with 100 cubic centimeters of the molybdate solution and treated with as much ammonium nitrate solution as will be sufficient to give the liquid a content of fifteen per cent of ammonium nitrate. The contents of the beaker are well mixed and warmed for about twenty minutes at from 60° to 80°. After cooling, they are filtered and the precipitate washed on the filter with cold water until a drop of the filtrate saturated with ammonia does not become opaque on treatment with ammonium oxalate. The filtrate is then washed from the filter with two and one-half per cent ammonia solution and precipitated slowly and with constant stirring by the magnesia mixture. After standing for two hours the ammonium magnesium phosphate is separated by filtration, washed with two and one-half per cent ammonia until the filtrate contains no more chlorin, and ignited.
Conduct of the Citrate Method.—The principle of this method depends upon the fact that when a sufficient quantity of ammonium citrate is added to phosphate solutions, iron, alumina, and lime are retained in solution when, on the addition of the magnesia mixture in the presence of free ammonia, the phosphoric acid is completely precipitated as ammonium magnesium phosphate.
Manipulation.—From ten to fifty cubic centimeters of the solution of the phosphate to be determined are treated with fifteen cubic centimeters of the Joulie citrate solution avoiding warming. A few pieces of filter-paper, the ash content of which is known, are thrown in and, with stirring, fifteen cubic centimeters of magnesia mixture slowly added and if necessary also some free ammonia. By the small pieces of filter-paper the collection of the precipitate against the sides of the vessel and on the stirring rod is prevented and in this way the production of the precipitate hastened. After standing from one-half an hour to two hours the mixture is filtered, ignited, and weighed. If it be preferred to estimate the phosphoric acid by titration, the precipitate is dissolved in a little nitric acid, made slightly alkaline with ammonia, and then acid with acetic and then afterwards titrated with the standard uranium solution.
Conduct of the Uranium Method.—The principle upon which this method rests depends upon the fact that uranium nitrate or acetate precipitates uranium phosphate from solutions containing phosphoric acid and which contain no other free acid except acetic. In the presence of ammonium salts the precipitate is uranium ammonium phosphate having the formula PO₄NH₄UrO₂. The smallest excess of soluble uranium salt is at once detected by the ordinary treatment with potassium ferrocyanid.
Manipulation.—In all cases the solution is first made slightly alkaline with ammonia and then acid by a few drops of acetic, so that no free mineral acid may be present.
(1) With liquids free from iron:
If, on the addition of ammonium or sodium acetate, no turbidity be produced, the liquid is free from iron and alumina. In this case from ten to fifty cubic centimeters of the solution containing about one-tenth gram of phosphorus pentoxid are treated with ten cubic centimeters of sodium acetate, and afterwards with a quantity of uranium solution corresponding, as nearly as possible, to its supposed content of phosphorus pentoxid, and heated to boiling. From the heated liquid by means of a glass rod, one or two drops are taken and placed upon a porcelain plate and one drop of a freshly prepared solution of potassium ferrocyanid allowed to flow on it. If no brown color be seen at the point of contact of the two drops, additional quantities of the uranium solution are added and, after boiling, again tested with potassium ferrocyanid until a brown color is distinctly visible. The quantity of the uranium solution thus having been determined, duplicate analyses can be made and the whole quantity of the uranium solution added at once with the exception of the last drops, which are added as before.
(2) Solutions containing iron and alumina.
The solution is treated with the ammonium citrate solution of Joulie, the magnesia mixture added slowly, and the precipitate collected on a filter and washed with two and one-half per cent ammonia. The precipitate is then dissolved in nitric acid, made alkaline with ammonia, and then acid with acetic. This solution is then treated with ten cubic centimeters of sodium acetate and titrated with uranium, as described in (1). As an alternative method, 200 cubic centimeters of the superphosphate solution may be treated with fifty cubic centimeters of sodium acetate, allowed to stand for some time, and filtered through a filter of known ash content. In fifty cubic centimeters of the filtrate, which correspond to forty cubic centimeters of the original solution, phosphoric acid may be determined as described above. The precipitate, consisting of iron and aluminum phosphates, is washed three times on the filter with boiling water, dried, and ignited in a platinum dish. The weight of ignited precipitate, diminished by the weight of the ash contained in the filter and divided by two, gives the quantity of phosphorus pentoxid which it is necessary to add to that obtained by titration.
117. Determination of the Phosphoric Acid in all Phosphates and Basic Slags.—
(1) Total phosphoric Acid:
Five grams of the fine phosphate meal, or slag meal, are moistened in a flask of 500 cubic centimeters content with some water and boiled on a sand-bath with forty cubic centimeters of hydrochloric acid of from 16° to 20° Beaumé. The boiling is continued until only a few cubic centimeters of a thick jelly of silicic acid remain. After cooling, some water is added and the phosphate shaken until the thick lumps of silica are finely divided. The flask is then filled to 500 cubic centimeters and its contents filtered. Fifty cubic centimeters of the filtrate are treated with fifteen cubic centimeters of the Joulie solution and treated in the manner described with magnesia mixture, precipitated, ignited, and weighed. The precipitate can also be dissolved and treated with uranium solution as described.
The method used by Oliveri may also be employed and it is carried out as indicated in the following description:[99]
A weighed quantity of the slag is reduced to a fine powder. To five grams of the sample is added three times its weight of potassium chlorate and the whole is intimately mixed. The mixture is then placed in a porcelain dish and hydrochloric acid is added, little by little, until the potash salt is completely decomposed. It is evaporated until the mass is dry. The material is then treated with fuming nitric acid, and the determination of the phosphorus is made by the ordinary gravimetric method.
By carrying on the operation as described above, a reduction of phosphoric acid is avoided, and the presence of an abundant quantity of potash prevents the formation of basic iron phosphate which is insoluble in nitric acid.
(2) Citrate-Soluble Phosphoric Acid.—One gram of the basic slag or phosphate is placed in a 100 cubic centimeter flask and covered with Wagner’s acid citrate solution making the total volume up to 100 cubic centimeters. With frequent shaking the flask is kept at 40° for an hour, or it may be allowed to stand for twelve hours at room temperature with frequent shaking. In fifty cubic centimeters of the filtrate from this flask the phosphoric acid is determined by the magnesia mixture as described. Since, in the present case, the precipitate of ammonium magnesium phosphate contains some silicic acid it cannot be directly ignited but must be treated in the following manner: The precipitate and the filter are thrown into a porcelain dish, the filter-paper torn up into shreds with a glass rod, the precipitate dissolved in nitric acid, neutralized with ammonia, acidified with acetic, and treated with uranium solution. The phosphoric acid may also be estimated by the gravimetric method by dissolving the precipitate again in hydrochloric or nitric acid, evaporating to dryness, and drying for one hour at from 110° to 120°, dissolving again in hydrochloric acid, filtering, and washing the precipitate well. The filtrate, which is now free from silica, can be treated with Joulie’s solution, precipitated with magnesia mixture, the precipitate washed, ignited, and weighed as described. The molybdenum method is preferred in the estimation of citrate-soluble phosphoric acid, especially in slags. For this purpose fifty cubic centimeters of the filtrate from the solution of one gram of slag in 100 cubic centimeters of Wagner’s citrate liquid are treated with 100 cubic centimeters of molybdenum solution and thirty cubic centimeters of ammonium nitrate solution, warmed for twenty minutes at 80°, filtered after cooling, and the yellow precipitate washed with cold water. The water will gradually dissolve all the silicic acid from the yellow precipitate and carry it into the filtrate. The yellow precipitate is then dissolved in two and one-half per cent liquid ammonia and precipitated with magnesia mixture and the precipitate washed, ignited, and weighed in the way described.
118. Determination of Phosphoric Acid in Superphosphates.—(1) Citrate-Soluble Phosphoric Acid.—Five grams of the superphosphate are rubbed with 100 cubic centimeters of Wagner’s acid citrate solution in a mortar and washed into a flask of 500 cubic centimeters content and diluted to 500 cubic centimeters with water. With frequent shaking the flask is allowed to stand for twelve hours, after which its contents are filtered. Fifty cubic centimeters of the filtrate are treated with ten cubic centimeters of the Joulie solution and fifteen cubic centimeters of the magnesia mixture and, if necessary, made distinctly alkaline with ammonia, vigorously stirred, and, after two hours, filtered. The precipitate is washed, ignited, and weighed as described, or titrated, after solution in nitric acid and the addition of sodium acetate, with uranium solution. Example:
The weighed precipitate has 0.1272 gram Mg₂P₂O₇ then the phosphate contains 12.72 × 2 × 0.64 = 16.28 per cent of citrate-soluble P₂O₅.
(2) Water-Soluble Phosphoric Acid.—Twenty grams of superphosphate are rubbed in a mortar and washed into a flask of one liter content and made up to the mark with water. After two hours’ digestion with frequent shaking, the contents of the flask are filtered through a folded filter. Twenty-five cubic centimeters of the filtrate equivalent to five-tenths gram of the substance are precipitated with magnesia mixture, the precipitate filtered, washed, ignited, and weighed, or the moist filtrate may be dissolved upon the filter with a little nitric acid, treated with sodium acetate, and titrated, as described, with uranium solution.
Example: 14.5 cubic centimeters of the uranium solution are required for the precipitate from twenty-five cubic centimeters of the original solution = 0.5 gram superphosphate; it contains then 14.5 × 0.00445 = 0.0645 gram P₂O₅. Consequently the superphosphate contains 12.90 per cent of water-soluble P₂O₅.
Total Phosphoric Add.—Twenty grams of the superphosphate are boiled with fifty cubic centimeters of hydrochloric acid of from 16° to 18° Beaumé for about ten minutes and, after cooling, made up to one liter with water and filtered. Twenty-five cubic centimeters of the filtrate are treated with ten cubic centimeters of Joulie’s citrate solution, a few pieces of filter-paper thrown in, fifteen cubic centimeters of magnesia mixture added, and the whole thoroughly stirred. After standing two hours the contents of the flask are filtered and the precipitate is washed with dilute ammonia and the filter and the precipitate are placed in a platinum crucible. The crucible is heated slowly until the moisture is driven off and the filter burned. Then the temperature is gradually raised to a white heat. The residue is cooled and weighed. Example:
The precipitate weighs, after the subtraction of the filter ash, 0.1390 gram; then the superphosphate contains 13.90 × 2 × 0.64 = 17.79 per cent phosphoric acid.
MISCELLANEOUS NOTES ON PHOSPHATES
AND PHOSPHATIC FERTILIZERS.
119. Time Required for Precipitation of Phosphoric Acid.—The length of time required for the complete precipitation of the phosphoric acid by molybdate mixture is perhaps much less than generally supposed. At 65° the precipitation, as shown by de Roode, is complete in five minutes.[100] In a given case the weight of pyrophosphate obtained after five minutes was 0.0676 gram, and exactly the same weight was found after twenty-four hours. In view of these facts analysts would often be able to save time by omitting the delay usually demanded by the setting aside of the yellow precipitate for a few hours in order to secure a complete separation of the phosphoric acid. In the method of the official chemists it is directed that the digestion at 65° be continued for one hour, and this time may possibly be shortened with advantage. In all cases, however, where there is any doubt in regard to the complete separation, some of the molybdate solution should be added to the filtrate and, with renewed digestion, it should be noted whether any additional precipitate be formed.
120. Examination of the Pyrophosphate.—In fertilizer control it is not usually thought necessary to examine the magnesium pyrophosphate for impurities. Among those most likely to be found is silica. It is proper, in all cases where accuracy is required, to dissolve the precipitate in nitric acid, boil for some time to convert the pyro- into orthophosphate, and reprecipitate with molybdate and magnesia mixture. This treatment will separate the silica which remains practically insoluble after the first ignition. It has been observed by some analysts that the results obtained by the official method are a trifle too high and also that on re-solution the second precipitate of pyrophosphate weighs less than the first.[101] The difference in most cases is very little but it may become a quantity of considerable magnitude in samples where soluble silica is found in notable quantities. The danger of contamination with iron, alumina, and arsenic has already been mentioned but it is not of sufficient importance to warrant further attention.
121. Iodin in Phosphates.—The presence of iodin has been detected in many natural phosphates and is of interest in the discussion of the problem of their origin.[102] A qualitative test for the detection of iodin may be applied in the following manner: Some finely ground phosphate is mixed with strong sulfuric acid and the gases arising from the reaction are aspired into some carbon disulfid or chloroform. The violet coloration arising indicates the presence of iodin. The gases carrying the iodin may also be brought into contact with starch-paste producing the well-known blue color.
The quantity of iodin present in a phosphate is rarely more than one or two-tenths of one per cent. It can be determined as a silver salt, in the absence of chlorin or by any of the standard methods found in works on qualitative analysis.
Iodin is quite a constant constituent of Florida phosphates.
For a quantitative determination, the sample is treated with an excess of strong sulfuric acid in a closed flask and during the decomposition a stream of air is aspired through the flask and caused to bubble through absorption bulbs containing sodium hydroxid in solution.
The temperature of the decomposition may be raised to about 200°. After the solution of the sample the sodium iodid formed is oxidized by heating with potassium permanganate, acidulated and mixed with a solution of potassium iodid to hold the free iodin in solution. The free iodin is determined in the usual way by titration with standard sodium thiosulfate solution. The reactions preparatory to the titration are represented by the following formulas:
2KI + H₂SO₄ = K₂SO₄ + 2HI.
2HI + H₂SO₄ = 2H₂O + SO₂ + 2I.
6I + 6NaOH = NaIO₃ + 5NaI + 3H₂O.
NaI + 2KMnO₄ + H₂O = NaIO₃ + 2KOH + 2MnO₂.
HIO₃ + 5HI = 6I + 3H₂O.
The titration is represented by the following reaction:
2Na₂S₂O₂ + 2I = 2NaI + 2NaI + Na₂S₄O₄.
The decinormal solution of sodium thiosulfate may be used. Grind the crystals of the salt to a fine powder, dry between blotting papers, and use 24.8 grams of the dried salt per liter. The quantity of iodin found in phosphates is so minute that it is hardly worth while to make a quantitative determination of it.
122. Occurrence of Chromium in Phosphates.—In some phosphates a small quantity of chromium has been found. In a sample of phosphate from the Island of Los Roques in the Caribbean Sea, Gilbert found three-fourths per cent of chromium oxid (Cr₂O₃). The phosphates containing chromium have a greenish color and are characterized by great insolubility in solutions containing organic acids. The chromium is to be determined by the usual methods described in mineral analysis.
123. Estimation of Vanadium.—In the complete analysis of basic slags it becomes necessary to determine the presence of vanadium and its quantity. The method used in this laboratory for the purpose is the volumetric process of Lindemann.[103] It is conducted as follows: Dissolve four grams of the finely powdered slag in sixty cubic centimeters of dilute sulfuric acid (1 : 4), boil for a few minutes, cool, make the volume up to 100 cubic centimeters, filter, and take an aliquot part for the determination. Add decinormal potassium permanganate solution in slight excess to secure the oxidation of the vanadium to vanadium pentoxid. Add, drop by drop, a weak solution of ferrous sulfate until the pink color just disappears. Prepare a ferrous sulfate solution by dissolving 2.183 grams of piano wire in sulfuric acid and making the volume to one liter. Titrate the vanadic mixture with this solution until a drop of the clear liquor removed and brought in contact with potassium ferricyanid shows a distinctive blue-green color.
One cubic centimeter of the ferrous sulfate solution is equivalent to 0.002 gram of vanadium, 0.002888 gram of vanadium dioxid, and 0.003648 gram of vanadium pentoxid. The ferrous sulfate solution may also be made and standardized by any of the approved methods in common use.
The method described by Blair, designed especially for the estimation of vanadium in iron and steel, is conducted in the following manner:[104] Five grams of the drillings are dissolved in fifty cubic centimeters of nitric acid of 1.24 specific gravity. The solution is evaporated to dryness in a porcelain dish and heated thereafter until the nitrates are nearly decomposed. After cooling, the dried mass is transferred to a mortar and finely ground with thirty grams of dry sodium carbonate and three grams of sodium nitrate. The finely ground materials are placed in a platinum dish and fused for an hour at a high temperature. Spread the fused mass over the sides of the dish while cooling, and afterwards dissolve in hot water, filter, and wash until the volume is a little over half a liter. Add nitric acid to decompose carbonates, but not completely, and boil to get rid of carbon dioxid, being careful to keep the mass always slightly alkaline. Add nitric acid, drop by drop, until slightly in excess, and then sodium carbonate to marked alkalinity, boil, and filter. Add a slight excess of nitric acid to the filtrate, and the development of a yellow color will indicate the presence of vanadic acid. Add to the solution a small quantity of mercurous nitrate and then an excess of mercuric oxid, suspended in water to render the solution neutral and insure the complete precipitation of mercurous vanadate. The mercurous salt also precipitates phosphoric, chromic, tungstic, and molybdic acids which may be present. Boil, filter, and wash the precipitate with hot water, dry, and ignite. Fuse the residue with sodium carbonate and a little nitrate. Dissolve the fused mass, after cooling, in a little water and filter. Add to the filtrate, ammonium chlorid in excess, from three to five grams for each 100 cubic centimeters of the solution, and allow to stand, with occasional stirring, for some time. Ammonium vanadate, insoluble in a saturated solution of ammonium chlorid, separates as a white powder. It is necessary to keep the solution alkaline, and a drop of ammonia should be added from time to time for this purpose. The appearance of a yellowish tint at any time indicates that the solution has become acid, and this acidity must be corrected, or else the results will be too low. Separate the ammonium vanadate by filtration; wash first with a saturated solution of ammonium chlorid containing a little free ammonia, and then with alcohol. Dry, ignite, and moisten with a few drops of nitric acid; again ignite to obtain the compound as vanadium pentoxid. This compound contains 56.22 per cent of vanadium. The method of Rosenheim and Holversheet may also be used.[105] It is based on the preliminary precipitation of the vanadic acid as a barium or lead salt. The substance supposed to contain vanadium is first brought into solution in such a manner as to secure it as vanadic acid, which is then precipitated with barium chlorid or lead acetate. The precipitate is boiled with hydrochloric acid and potassium bromid, and the liberated bromin determined by the quantity of iodin set free from potassium iodid. In the absence of bodies, such as molybdic acid, which are reduced by sulfurous acid or hydrogen sulfid, the vanadic acid may also be determined by reducing it with one of these reagents and, after removing the excess by boiling, titrating the vanadium tetroxid with potassium permanganate. When vanadic and phosphoric acids occur together the former may be first reduced to tetroxid with sulfurous acid, and after expelling excess of this reagent, the phosphoric acid may be separated with molybdate solution and removed by filtration. When the amount of vanadic acid is large the phosphoric acid should be separated rapidly at 55°-60°, using a considerable excess of the molybdate; or the vanadic acid may first be determined in the solution volumetrically by the bromin process above described, and afterwards the phosphoric acid obtained by evaporating to dryness with a little sulfuric acid, taking the residue up with water, reducing the vanadic with sulfurous acid and precipitating the phosphoric acid with molybdate solution as described above.
124. Fluorin in Bones.—Fluorin is not only a constituent of mineral phosphates but also of bones. According to the researches of Carnot there is often as much as one-half per cent of calcium fluorid in bones and teeth.[106] Gabriel has suggested a means of determining a minimum limit of fluorin in bones and teeth by the development of etchings in comparison with known quantities of pure calcium fluorid. The minimum quantity of calcium fluorid necessary to produce a distinct etching, in known conditions, having been determined, the test is applied to known weights of ignited bone or teeth. He concludes from his results, that the ash of bones and teeth often contains less than one-tenth per cent of fluorin. Since, however, there is a loss of fluorin from calcium fluorid, on ignition, the whole of the fluorin may not have been available in the tests described.
125. Note on the Separation of Iron and Aluminum Phosphates from the Calcium Compound.—There are many points of difference noted in the descriptions given by authors of the deportment of the iron, and aluminum phosphates in presence of a large excess of the calcium salt. Especially is this true of the statements made by Hess and Glaser[107] in paragraphs [34] and [35]. The subject is of such importance, from an analytical point of view, as to merit a careful study.
In this laboratory a thorough investigation of the mutual deportment of these three phosphates has been made by Brown with the following results:[108] When a mixture containing a known weight of the salts was treated exactly as Hess directs, in no case was there a complete separation of the iron aluminum phosphate from the calcium salt. In order to discover the cause of the failure, pure solutions of calcium and iron aluminum phosphates were treated under identical conditions by the necessary reagents. Fifty cubic centimeters of a solution of calcium phosphate, containing about one gram of the salt, were treated with 100 cubic centimeters of water and fifty cubic centimeters of the commercial ammonium acetate containing 150 grams of the salt in a liter. An immediate precipitate was produced at ordinary temperature, and on heating to 60° it became abundant. The addition of ammonium chlorid, phosphate, and nitrate in successive portions, does not prevent the precipitation. Making the solution more dilute lessens the difficulty when twenty cubic centimeters of a ten per cent solution of ammonium phosphate are first added, followed by the usual quantity of ammonium acetate; a clear crystalline precipitate is sometimes observed. Experience also shows that the trouble is not due to an excess of the ammonium acetate.
In treating a solution of iron aluminum phosphate, in similar circumstances, with the ammonium acetate, it is found that a complete precipitation takes place.
Since diluting the solution of the calcium salt diminishes its tendency to form a precipitate with the ammonium acetate the true method of separation seems to lie in that direction. The calcium salt is held completely in solution when the separation is made in the following way.
The solution containing the mixed phosphates is diluted so as to contain not more than one gram thereof in half a liter. To this is added one drop of dimethylanilin orange, and afterwards ammonium hydroxid, until a very slight precipitate is formed. The mixture is heated to 70° and from twenty to twenty-five cubic centimeters of a twenty-five per cent solution of acid ammonium acetate are added, enough to change the rose color of the indicator to orange. The iron aluminum phosphate is separated by filtration and washed with a hot five per cent solution of ammonium nitrate.
The washed precipitate shows no impurity due to calcium, as proved by dissolving it, reprecipitating and filtering, adding ammonium hydroxid to the filtrate, and heating for a long time. Sometimes a slight troubling of the clear liquid may be observed which may be due to a slight solubility of the iron aluminum phosphate in washing, an accident that may occur if the temperature be allowed to fall below 70°, but no weighable amount of material is obtained. If due to calcium phosphate, a greater dilution in the first precipitation will remove even this mere trace of that salt. In the above conditions the contamination of the iron aluminum precipitate with calcium phosphate may be entirely avoided. We had also undertaken here the problem of separating the phosphoric acid by the citrate method, followed by a destruction of the citric acid in the filtrate by combustion with sulfuric acid according to the kjeldahl process, and final separation of the iron and alumina in the residues when our attention was called to substantially the same process as described by Jean.[109] The method merits a further critical examination.
126. Phosphoric Acid Soluble in Ammonium Citrate.—There is no other point connected with the determination of phosphoric acid which has excited so much discussion and about which there is such difference of opinion as the solubility of phosphates in ammonium citrate. It was clearly established by Huston, in 1882, that the ammonium citrate, as used in fertilizer analysis, would attack normal tricalcium phosphate as it exists in bones.[110]
In a raw bone, finely ground, containing 20.28 per cent of phosphoric acid, the following quantities were found to be soluble in a neutral ammonium citrate solution of 1.09 specific gravity.
| Time of digestion, thirty minutes. | ||||
| Temperature | 30° | 40° | 50° | 60° |
| Per cent P₂O₅ dissolved | 2.76 | 4.01 | 3.39 | 5.88 |
From this it appears that the quantity of acid dissolved increases with the temperature of digestion with the exception of the number obtained at 50°. When the time of digestion was increased there was also found a progressive increase in the amount of acid passing into solution. At 40° for forty-five minutes the per cent dissolved was 4.97, and at 40° for one hour it was 5.92. These early determinations had the effect of calling attention to the thoroughly empirical process which was in use, in many modified forms, by agricultural chemists, the world over for determining so-called reverted phosphoric acid in fertilizers. Since the publication of the paper above named many investigations have been undertaken by Huston and others relating to this matter.[111] The general results of these studies, tabulated by Huston, are given below.[112]
Influence of the Time of Digestion.
- (A) = Temperature, degrees C.
- (B) = Citrate-soluble phosphoric acid.
- (C) = Total phosphoric acid.
| Material. | Authority. | (A) | Time of digestion. | (B) Per cent. | (C) Per cent. | ||
|---|---|---|---|---|---|---|---|
| Bone meal, | F. B. Dancy, |  | 65 | ½ | hour | 10.60 | 19.75 |
| 65 | 1 | “ | 11.28 | 19.75 | |||
| Orchilla guano, | F. B. Dancy, | | 65 | ½ | “ | 6.62 | 21.68 |
| 65 | 1 | “ | 6.85 | ||||
| Navassa rock, | F. B. Dancy | | 65 | ½ | “ | 4.64 | 31.27 |
| 65 | 1 | “ | 4.81 | 31.27 | |||
| Navassa | F. B. Dancy, | | 65 | ½ | “ | 9.00 | 11.47 |
| superphos., | 65 | 1 | “ | 9.21 | 11.47 | ||
| Bone meal, | H. A. Huston, | | 40 | ½ | “ | 4.01 | 20.28 |
| 40 | 1 | “ | 5.92 | 20.28 | |||
| Bone meal, | H. A. Huston and |  | 65 | ½ | “ | 6.17 | 23.58 |
| raw, | W. J. Jones, Jr., | 65 | 1 | “ | 6.49 | 23.58 | |
| 65 | 2 | hours | 8.22 | 23.58 | |||
| 65 | 5 | “ | 9.31 | 23.58 | |||
| Steamed bone, | H. A. Huston and |  | 65 | ½ | hour | 10.59 | 27.67 |
| W. J. Jones, Jr., | 65 | 1 | “ | 12.21 | |||
| 65 | 2 | hours | 14.61 | ||||
| 65 | 5 | “ | 17.94 | ||||
| 65 | 10 | “ | 19.73 | ||||
| Florida | H. A. Huston and |  | 65 | ½ | hour | 0.56 | 19.75 |
| soft rock, | W. J. Jones, Jr., | 65 | 2 | hours | 1.69 | ||
| 65 | 5 | “ | 1.47 | ||||
| Precipitated | H. A. Huston and | | 65 | ¼ | hour | 26.72 | 33.34 |
| calcium | W. J. Jones, Jr., | 65 | ½ | “ | 27.26 | ||
| phosphate from | 65 | 1 | “ | 27.28 | |||
| glue works | 65 | 2 | hours | 27.29 | |||
| Pamunky | H. A. Huston and | | 65 | ½ | hour | 4.43 | 13.84 |
| phosphate,[113] | W. J. Jones, Jr., | 65 | 1 | “ | 8.28 | ||
| 65 | 2 | hours | 10.34 | ||||
| 65 | 5 | “ | 11.80 | ||||
| 65 | 10 | “ | 12.58 | ||||
| Calcined | H. A. Huston and | | 65 | ½ | hour | 21.24 | 45.15 |
| Redonda, | W. J. Jones, Jr., | 1 | “ | 31.70 | |||
| 2 | hours | 36.92 | |||||
| 5 | “ | 41.00 | |||||
| 10 | “ | 42.70 | |||||
| South | H. A. Huston and | | 65 | ½ | hour | 2.82 | 25.51 |
| Carolina | W. J. Jones, Jr., | 1 | “ | 3.13 | |||
| rock, | 2 | hours | 3.57 | ||||
| 5 | “ | 3.88 | |||||
Influence of Temperature.
- (A) = Temperature, degrees C.
- (B) = Citrate-soluble phosphoric acid.
- (C) = Total phosphoric acid.
| Material. | Authority. | Time of digestion. | (A) | (B) Per cent. | (C) Per cent. | ||
|---|---|---|---|---|---|---|---|
| Apatite | T. S. Gladding[114] | | ½ | hour | 40 | 0.30 | |
| Canadian, | ½ | “ | 65 | 0.56 | |||
| Orchilla guano, | T. S. Gladding, | | ½ | “ | 40 | 4.63 | |
| ½ | “ | 65 | 4.81 | ||||
| South Carolina | T. S. Gladding, | | ½ | “ | 40 | 1.09 | |
| river rock, | ½ | “ | 65 | 1.35 | |||
| Navassa rock, | T. S. Gladding, | | ½ | “ | 40 | 2.73 | |
| ½ | “ | 65 | 2.53 | ||||
| Grand | T. S. Gladding, | | ½ | “ | 40 | 1.16 | |
| Connetable, | ½ | “ | 65 | 1.96 | |||
| Redonda, | S. W. Johnson and | | ½ | “ | 40 | 1.70 | 36.68 |
| E. H. Farrington, | ½ | “ | 65 | 1.85 | |||
| South Carolina | S. W. Johnson and | | ½ | “ | 40 | 1.32 | 25.48 |
| rock, | E. H. Farrington, | ½ | “ | 65 | 1.65 | ||
| Orchilla guano, | S. W. Johnson and | | ½ | “ | 40 | 4.92 | 21.05 |
| E. H. Farrington, | ½ | “ | 65 | 5.85 | |||
| Navassa rock, | S. W. Johnson and | | ½ | “ | 40 | 4.10 | 29.90 |
| E. H. Farrington, | ½ | “ | 65 | 4.22 | |||
| Acid Navassa, | S. W. Johnson and | | ½ | “ | 40 | 11.95 | 16.50 |
| E. H. Farrington, | ½ | “ | 65 | 13.53 | |||
| Fine-ground | S. W. Johnson and | | ½ | “ | 40 | 9.40 | 23.50 |
| bone, | E. H. Farrington, | ½ | “ | 65 | 12.90 | ||
| South Carolina | C. V. Sheppard, Jr. | | ½ | “ | 40 | 1.72 | 24.50 |
| land rock, | also H. C. White, | ½ | “ | 65 | 2.11 | ||
| Orchilla guano, | C. V. Sheppard, Jr. | | ½ | “ | 40 | 6.48 | 15.85 |
| also H. C. White, | ½ | “ | 65 | 6.75 | |||
| Calcined | C. V. Sheppard, Jr. | | ½ | “ | 40 | 5.70 | 44.85 |
| Redonda, | also H. C. White, | ½ | “ | 65 | 10.20 | ||
| Raw Redonda, | C. V. Sheppard, Jr. | | ½ | “ | 40 | 4.49 | 43.79 |
| also H. C. White, | ½ | “ | 65 | 7.92 | |||
| Acid phosphate, | C. V. Sheppard, Jr. | | ½ | “ | 40 | 3.55 | 18.25 |
| S. C. 10.35 per cent | also H. C. White, | ½ | “ | 65 | 4.05 | ||
| water-soluble, | |||||||
| Acid Navassa, | C. V. Sheppard, Jr. | | ½ | “ | 40 | 10.85 | 16.20 |
| 2.85 per cent | also H. C. White, | ½ | “ | 65 | 11.00 | ||
| water-soluble, | |||||||
| Bone, | H. A. Huston, | | ½ | “ | 30 | 2.76 | 20.28 |
| ½ | “ | 40 | 4.01 | ||||
| ½ | “ | 50 | 3.39 | ||||
| ½ | “ | 60 | 5.88 | ||||
| Acid phosphate, | Sheppard and | | ½ | “ | 40 | 3.46 | 15.95 |
| 11.41 per cent | Robertson, | ½ | “ | 60 | 3.82 | ||
| water-soluble, | |||||||
| Calcined | H. A. Huston, | | ½ | “ | 40 | 2.18 | 45.46 |
| Redonda, | ½ | “ | 50 | 5.52 | |||
| ½ | “ | 65 | 21.24 | ||||
| ½ | “ | 75 | 32.90 | ||||
| ½ | “ | 85 | 39.52 | ||||
| Calcined | H. A. Huston and | | 5 | hours | 40 | 26.78 | 42.90 |
| Redonda, | W. J. Jones, Jr., | 5 | “ | 65 | 38.19 | ||
| 5 | “ | 85 | 41.57 | ||||
| Pamunky | H. A. Huston and | | 5 | “ | 40 | 3.10 | 13.84 |
| phosphate, | W. J. Jones, Jr., | 5 | “ | 65 | 11.80 | ||
| 5 | “ | 85 | 12.82 | ||||
| Raw bone, | H. A. Huston and | | 2 | “ | 40 | 5.96 | 23.58 |
| W. J. Jones, Jr., | 2 | “ | 65 | 8.22 | |||
| 2 | “ | 85 | 8.71 | ||||
| Steamed bone, | H. A. Huston and | | 5 | “ | 40 | 16.02 | 27.67 |
| W. J. Jones, Jr., | 5 | “ | 65 | 20.22 | |||
| 5 | “ | 85 | 20.66 | ||||
| Precipitated | H. A. Huston and | | 2 | “ | 40 | 24.14 | 33.34 |
| calcium phosphate | W. J. Jones, Jr., | 2 | “ | 65 | 23.45 | ||
| from glue works, | 2 | “ | 85 | 22.46 | |||
| Florida | H. A. Huston and | | 2 | “ | 40 | 0.00 | 19.75 |
| soft rock, | W. J. Jones, Jr., | 2 | “ | 65 | 1.69 | ||
| 2 | “ | 85 | 1.99 | ||||
Influence of Quantity of Material Used.
- (A) = Temperature, degrees C.
- (B) = Quantity of material used.
- (C) = Citrate-soluble phosphoric acid.
- (D) = Total phosphoric acid.
| Material. | Authority. | Time | (A) | (B) Grams. | (C) Per cent. | (D) Per cent. | ||
|---|---|---|---|---|---|---|---|---|
| ⅔ | hour | 40 | 2.0 | 9.94 | 21.68 | ||
| Orchilla guano, | F. B. Dancy, | ⅔ | “ | 40 | 1.0 | 12.14 | ||
| ⅔ | “ | 40 | 0.5 | 13.51 | ||||
| ½ | “ | 65 | 2.0 | 6.62 | ||||
| ½ | “ | 65 | 1.0 | 9.33 | ||||
| Redonda, | S. W. Johnson and | | ½ | “ | 40 | 2.0 | 1.70 | 36.68 |
| E. H. Farrington, | ½ | “ | 40 | 0.4 | 3.46 | |||
| ½ | “ | 65 | 2.0 | 1.85 | ||||
| ½ | “ | 65 | 0.4 | 5.26 | ||||
| South Carolina | S. W. Johnson and | | ½ | “ | 40 | 2.0 | 1.32 | 25.48 |
| rock, | E. H. Farrington, | ½ | “ | 40 | 0.4 | 1.33 | ||
| ½ | “ | 65 | 2.0 | 1.65 | ||||
| ½ | “ | 65 | 0.4 | 3.36 | ||||
| Orchilla guano, | T. S. Gladding, | | ½ | “ | 65 | 2.0 | 5.87 | |
| ½ | “ | 65 | 0.4 | 13.05 | ||||
| Calcined | H. A. Huston, |  | ½ | “ | 65 | 0.5 | 16.80 | 45.46 |
| Redonda, | ½ | “ | 65 | 1.0 | 18.26 | |||
| ½ | “ | 65 | 2.0 | 21.24 | ||||
| ½ | “ | 65 | 3.0 | 23.22 | ||||
| ½ | “ | 65 | 5.0 | 24.66 | ||||
| ½ | “ | 65 | 10.0 | 28.64 | ||||
| Calcined | H. A. Huston, | | 5 | “ | 65 | 0.5 | 41.77 | 45.46 |
| Redonda, | 5 | “ | 65 | 2.0 | 41.53 | |||
| 5 | “ | 65 | 8.0 | 39.86 | ||||
| Pamunky | H. A. Huston and | | 5 | “ | 65 | 0.5 | 11.81 | 13.84 |
| phosphate, | W. J. Jones, Jr., | 5 | “ | 65 | 2.0 | 11.80 | ||
| 5 | “ | 65 | 4.0 | 11.44 | ||||
| Raw bone, | H. A. Huston and | | 2 | “ | 65 | 0.5 | 16.49 | 23.58 |
| W. J. Jones, Jr., | 2 | “ | 65 | 2.0 | 8.22 | |||
| 2 | “ | 65 | 4.0 | 7.22 | ||||
| Steamed bone, | H. A. Huston and | | 5 | “ | 65 | 0.5 | 26.40 | 27.67 |
| W. J. Jones, Jr., | 5 | “ | 65 | 2.0 | 17.94 | |||
| 5 | “ | 65 | 4.0 | 12.12 | ||||
| Precipitated | H. A. Huston and | | 2 | “ | 65 | 0.5 | 33.34 | 33.34 |
| calcium phosphate | W. J. Jones, Jr., | 2 | “ | 65 | 2.0 | 27.29 | ||
| from glue works, | 2 | “ | 65 | 4.0 | 19.49 | |||
| Florida | H. A. Huston and | | 2 | “ | 65 | 0.5 | 5.50 | 19.75 |
| soft rock, | W. J. Jones, Jr., | 2 | “ | 65 | 2.0 | 1.69 | ||
| 2 | “ | 65 | 4.0 | 1.27 | ||||
Influence of Acidity and Alkalinity.
- (A) = Temperature, degrees C.
- (B) = 100 cc neutral citrate + citric acid.
- (C) = 100 cc neutral citrate + ammonia equivalent to citric acid.
- (D) = Per cent of phosphoric acid dissolved.
- (E) = Per cent total phosphoric acid.
| Material. | Authority. | Time | (A) | (B) | (C) | (D) | (E) | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Navassa rock, | T. S. Gladding[115] | | ½ | hour | 65 | 0.00 | 0.00 | 2.53 | |||
| ½ | “ | 65 | 0.733 | gm | 4.87 | ||||||
| ½ | “ | 65 | 0.733 | gm | 1.22 | ||||||
| South Carolina | T. S. Gladding | | ½ | “ | 65 | 0.00 | 0.00 | 1.35 | |||
| rock, | ½ | “ | 65 | 0.733 | “ | 2.89 | |||||
| ½ | “ | 65 | 0.733 | “ | 1.06 | ||||||
| Grand | T. S. Gladding, | | ½ | “ | 65 | 0.00 | 0.00 | 1.97 | |||
| connetable, | ½ | “ | 65 | 0.733 | “ | 1.12 | |||||
| ½ | “ | 65 | 0.733 | “ | 11.44 | ||||||
| Dissolved | H. B. McDonnell, | | ½ | “ | 65 | 0.00 | 0.00 | 2.49 | 11.51 | ||
| bone-black and | ½ | “ | 65 | 0.01 | “ | 2.42 | |||||
| cottonseed-meal, | ½ | “ | 65 | 0.01 | “ | 2.37 | |||||
| Ground bone, | H. B. McDonnell, | | ½ | “ | 65 | 0.00 | 0.00 | 8.66 | 26.62 | ||
| ½ | “ | 65 | 0.01 | “ | 9.18 | ||||||
| ½ | “ | 65 | 0.01 | “ | 8.00 | ||||||
| Calcined | H. B. McDonnell, | | ½ | “ | 65 | 0.00 | 0.00 | 30.61 | 45.11 | ||
| Redonda, | ½ | “ | 65 | 0.01 | “ | 29.42 | |||||
| ½ | “ | 65 | 0.01 | “ | 32.47 | ||||||
| Dissolved | H. A. Huston, | | ½ | “ | 65 | 0.00 | 0.00 | 2.24 | 11.32 | ||
| bone-black and | ½ | “ | 65 | 1.00 | “ | 2.24 | |||||
| cottonseed-meal, | ½ | “ | 65 | 1.00 | “ | 2.21 | |||||
| Ground bone, | H. A. Huston and | | ½ | “ | 65 | 0.00 | 0.00 | 8.78 | 26.35 | ||
| W. J. Jones, Jr., | ½ | “ | 65 | 1.00 | “ | 13.48 | |||||
| ½ | “ | 65 | 1.00 | “ | 5.35 | ||||||
| Calcined | H. A. Huston and | | ½ | “ | 65 | 0.00 | 0.00 | 25.54 | 45.15 | ||
| Redonda, | W. J. Jones, Jr., | ½ | “ | 65 | 1.00 | “ | 18.84 | ||||
| ½ | “ | 65 | 1.00 | “ | 35.20 | ||||||
| South Carolina | H. A. Huston and | | ½ | “ | 65 | 0.00 | 0.00 | 1.81 | 27.67 | ||
| rock, | W. J. Jones, Jr., | ½ | “ | 65 | 1.00 | “ | 4.59 | ||||
| ½ | “ | 65 | 1.00 | “ | 0.74 | ||||||
| Basic slag, | H. A. Huston and | | ½ | “ | 65 | 0.00 | 0.00 | 6.98 | 19.42 | ||
| W. J. Jones, Jr., | ½ | “ | 65 | 1.0 | “ | 10.12 | |||||
| ½ | “ | 65 | 1.0 | “ | 5.49 | ||||||
| Pamunky | H. A. Huston and | | 5 | h’rs | 65 | 0.00 | 0.00 | 11.80 | 13.84 | ||
| phosphate, | W. J. Jones, Jr., | 5 | “ | 65 | 1.0 | “ | 11.79 | ||||
| 5 | “ | 65 | 1.0 | “ | 12.28 | ||||||
| Raw bone, | H. A. Huston and | | 2 | “ | 65 | 0.00 | 0.00 | 8.22 | 23.58 | ||
| W. J. Jones, Jr., | 2 | “ | 65 | 1.0 | “ | 11.20 | |||||
| 2 | “ | 65 | 1.0 | “ | 4.02 | ||||||
| Steamed bone, | H. A. Huston and | | 5 | “ | 65 | 0.00 | 0.00 | 17.94 | 27.67 | ||
| W. J. Jones, Jr., | 5 | “ | 65 | 1.0 | “ | 22.55 | |||||
| 5 | “ | 65 | 1.0 | “ | 9.64 | ||||||
| Precipitated | H. A. Huston and | | 2 | “ | 65 | 0.00 | 0.00 | 24.20 | 33.34 | ||
| calcium phosphate, | W. J. Jones, Jr., | 2 | “ | 65 | 1.0 | “ | 30.70 | ||||
| from glue works, | 2 | “ | 65 | 1.0 | “ | 20.67 | |||||
| Florida soft | H. A. Huston and | | 2 | “ | 65 | 0.00 | 0.00 | 1.69 | 19.75 | ||
| rock, | W. J. Jones, Jr., | 2 | “ | 65 | 1.0 | “ | 3.37 | ||||
| 2 | “ | 65 | 1.0 | “ | 0.72 | ||||||
| Calcined | H. A. Huston and | | 5 | “ | 65 | 0.00 | 0.00 | 40.64 | 44.30 | ||
| Redonda, | W. J. Jones, Jr., | 5 | “ | 65 | 1.0 | “ | 40.05 | ||||
| 5 | “ | 65 | 1.0 | “ | 41.01 | ||||||
In the above tabulations no mention is made of the work of Fresenius, Neubauer, and Luck, on whose researches the citrate method is based, but an examination of their original paper shows that the temperature conditions are not carefully enough controlled to justify us in tabulating their results.[116] An attempt has been made to include in the above tables, work made under well-defined conditions, which will illustrate the various points under consideration. While each authority of value upon the subject is represented, no attempt has been made to include all the work done by any of them. One element that seems to have been generally overlooked in discussing the problem is that nearly all results have been obtained from one-half hour’s treatment of the material. This means simply the study of an incomplete reaction, and one which is interrupted while the solution is very rapidly going on. This, of course, is only clearly brought out by a comparison of long-time and short-time work in the various tables. In the opinion of Huston very much more work will have to be done before it can be assumed that we have any very clear knowledge of this subject, and very likely the final result will be that all kinds of goods cannot be examined by the same method. The fact that half a gram of dicalcium phosphate is instantly soluble in 100 cubic centimeters of citrate solution, at ordinary temperatures, while an equal amount of iron and aluminum phosphate is acted upon very slowly at ordinary temperatures will probably have to be taken into consideration, as well as the fact that dicalcium phosphate is less soluble in hot solutions of ammonium citrate than it is in cold solutions, while the reverse is true of the precipitated iron and aluminum phosphate.
At present, the only conclusion that can be safely drawn from the work, is that it would be unsafe to make any generalization upon the subject until more facts are at hand, except that the present methods are unscientific and, unsatisfactory. As the work progresses, new features present themselves, and in such a way as to show that they must be given careful consideration before drawing any final conclusions in the matter.
127. Arbitrary Determination of Reverted Phosphoric Acid.—The so-called reverted phosphoric acid, that is, the acid insoluble in water and soluble in a solution of ammonium citrate, is the most difficult constituent of commercial fertilizers from the point of view of the scientific analyst. A review of all the standard methods which have been given in the preceding pages for its determination must convince every careful observer that, as a rule, each process is based on arbitrary standards, and can give only concordant results when carried out under strictly unvarying conditions. For this reason there can be no just comparison between the results obtained by different methods, which vary from each other only in slight particulars. When, on the other hand, the processes are radically different, the deviations in data become more pronounced.
In such a condition of affairs the analyst is left to choose between methods. He must be guided in his choice not only by what seems to be the most scientific and accurate process, but also, to a certain extent, by the general practice of his professional brethren. For this country, therefore, it is strongly urged that the methods adopted by the Association of Official Agricultural Chemists, be followed in every detail.
By the phrase “reverted phosphoric acid,” was originally meant an acid once soluble in water, as CaH₄(PO₄)₂, and afterwards changed to a form insoluble in water, but soluble in ammonium citrate as Ca₂H₂(PO₄)₂. But in practice this has never been the true signification of the term. In the manufacture of acid and superphosphates there is formed, more or less of the dicalcium phosphate, either directly or after a time, and this salt which, in no sense can be called reverted, is entirely soluble in ammonium citrate. The iron and aluminum phosphates are also, to a certain degree, soluble in the same reagent. When an acid phosphate, containing various forms of calcium phosphate, is applied to a soil containing iron and alumina, the soluble parts of the compound tend to become fixed by union with those bases, or by precipitation as Ca₂H₂(PO₄)₂. But it is not alone reverted phosphate formed in this way, which the analyst is called on to determine in a fertilizer, although he may have occasion to treat it in soil analysis.
The expression “reverted phosphoric acid,” therefore, in practice not only includes a dicalcium phosphate, which once may have been the monocalcium salt, but also all of that salt originally existing in the superphosphate, and formed directly during its manufacture, as well as any iron and aluminum phosphates present which are soluble in ammonium citrate. The expression “citrate-soluble” is, therefore, to be preferred to “reverted” phosphoric acid.
In the reversion of the phosphoric acid in superphosphates the iron plays a far more important role than the aluminum sulfate. It was formerly supposed that the reversion took place as indicated in the following formula: 2CaH₄(PO₄)₂ + Fe₂O₃ = 2(CaHPO₄, FePO₄) + 3H₂O, while Wagner affirms that the reverted acid compounds consist of varying quantities of ferric oxid, aluminum oxid, phosphorus pentoxid, and calcium oxid, in various states of combination.[117] The more probable reaction is the following: 3CaH₄(PO₄)₂ + Fe₂(SO₄)₃ + 4H₂O = 2(FePO₄, 2H₃PO₄, 2H₂O) + 3CaSO₄. This reaction can be demonstrated by adding to a superphosphate solution one of a ferric salt. In addition to free phosphoric acid, iron phosphate is separated, which gradually passes into an insoluble form by the abstraction of water due to the crystallization of the gypsum. The alumina present in a superphosphate seems to have no direct influence on the process of reversion. Its phosphate salt is not acted on by the acid calcium phosphate. Even when a superphosphate solution is treated with alum no precipitation is produced, except on warming, and this disappears when the mass is again cold.
It is therefore not necessary in the process of manufacture to separate the alumina by digestion with a hot soda-lye before treating the mass with sulfuric acid.
In order to avoid the reversion of the phosphoric acid several plans have been proposed. One of the best is to use a little excess of sulfuric acid in the manufacture. This tends to hold the phosphoric acid in soluble form but is objectionable on account of drying, handling, and shipping the fertilizer. During the digestion, moreover, it is important that the temperature does not rise above 120°. Another method consists in adding to the dissolved rock a quantity of common salt chemically equivalent to its iron content. Ammonium sulfate also helps to hold the phosphoric acid water-soluble.
128. Influence of Movement.—The influence of time and temperature of digestion, and of variations in the composition of the ammonium citrate on the quantity of phosphoric acid dissolved by that reagent has been pointed out. Of great importance also in the process is the character of the movement to which the materials are subjected during the digestion. For this reason various mechanical devices have been constructed to secure uniformity of solution. Inasmuch as the temperature factor must also be faithfully observed, the best of these devices are so arranged as to admit of a uniform motion within a bath of water kept at the desired temperature which, by the Association method, is 65°.
Figure. 8.
Huston’s Digesting Apparatus.
129. Digestion Apparatus for Reverted Phosphates.—The digestion apparatus used by Huston consists of two wheels twenty-five centimeters in diameter, mounted on the same axis, having a clear space of four and one-half centimeters between them.[118] In the periphery of each wheel are cut twelve notches, which are to receive the posts bearing the rings through which the necks of the flasks pass. The posts are held in place by nuts which are screwed down on the faces of the wheel. Should it become necessary to take the apparatus apart, it is only necessary to loosen the nuts and the set screw holding one wheel to the shaft and all the parts can at once be removed. The posts extend ten centimeters beyond the face of the wheels, and the rings are four centimeters in internal diameter. Perforated plates, bearing a cross-bar, and held in place by strong spiral springs attached to the plate and the base of the posts, serve to hold the flasks in place. Each plate has a number stenciled through it for convenience in identifying the flasks when it is time to remove them. Attached to the outside of each post, close to the outer end, is a heavy wire which passes entirely around the apparatus, serving to keep the plates in place after they are removed from the flasks.
The apparatus is mounted on a substantial framework, thirty-six centimeters high and thirty centimeters wide at the base. The space in which the wheel revolves is fourteen centimeters wide. The base bars connecting the two sides are extended seven centimeters beyond one side, and serve for the attachment of lateral bracing. At the top of the framework, at one side, is attached a heavy bar forty-five centimeters long, which serves to carry the cog gearing which transmits the power. The upright shaft carries a cone pulley to provide for varying the speed. The usual speed is two revolutions a minute for the wheel carrying the flasks. The entire apparatus is made of brass. The details of construction are shown in [Fig. 8]. Round-bottomed flasks are used, and rubber stoppers are held in place by tying or by a special clamp shown at the lower right-hand of the figure.
When high temperatures are used, the plates and flasks are handled by the hooks shown at the left and right-hand upper corners of the [figure].
When any other than room temperature is desired, the whole apparatus is immersed in water contained in the large galvanized tank forming the back-ground of the figures. The tank is seventy-five centimeters long, seventy-five centimeters high, and thirty centimeters wide. At one end, near the top, is an extension to provide space for heating the fluid in the flasks before introducing the solid in such cases as may be desired.
The apparatus is held in place by angle irons soldered to the bottom of the tank and a brace resting against the upright bar bearing the gear-wheels.
The water in the tank is heated by injecting steam, or by burners under the tank. As the tank holds about 300 pounds of water the work is not subject to sudden changes of temperature, and little trouble has been experienced in raising and maintaining the temperature of the water, especially when steam is used.
An electric motor, or a small water-motor with only a very moderate head of water, will furnish ample power.
130. Comparison of Results.—The following data show the results obtained by the digester as compared with those furnished by the official method, temperature and time of digestion being the same in each instance.
Ammonium Citrate Solution on Phosphates.
| Substance. | Time of treatment. | Total phos. acid. Per cent. | Removed by official method. Per cent. | Removed by digester. Per cent. | |
|---|---|---|---|---|---|
| Steamed bone, | ½ | hour | 27.67 | 10.59 | 14.52 |
| 1 | “ | 12.21 | 14.82 | ||
| 2 | hours | 14.61 | 17.56 | ||
| 3½ | “ | 16.48 | 18.53 | ||
| 5 | “ | 17.94 | 20.22 | ||
| 7½ | “ | 18.99 | 20.25 | ||
| 10 | “ | 19.73 | 21.18 | ||
| Marl, | ½ | hour | 13.86 | 4.43 | 4.11 |
| 1 | “ | 8.28 | 6.82 | ||
| 2 | hours | 10.34 | 9.76 | ||
| 3½ | “ | 11.00 | 11.31 | ||
| 5 | “ | 11.80 | 11.83 | ||
| 8 | “ | 12.51 | 12.64 | ||
| 10 | “ | 12.58 | 13.00 | ||
| Acidulated bone, | ½ | hour | 19.38 | 12.09 | 12.28 |
| 1 | “ | 12.47 | 12.40 | ||
| 2 | hours | 12.20 | 12.43 | ||
| 3½ | “ | 12.40 | 12.24 | ||
| 5 | “ | 12.43 | 12.26 | ||
| Bone, | ½ | hour | 21.40 | 6.97 | 8.48 |
| Ammoniated dissolved bone, | ½ | “ | 18.22 | 9.28 | 10.63 |
| Cottonseed-meal and | |||||
| castor pomace, | ½ | “ | 2.52 | 0.23 | 0.25 |
| Phospho bone, | ¼ | “ | 16.55 | 7.06 | 7.21 |
In comparing duplicates, the results from the use of the digester are found to be subject to less variation than those from the usual method.
131. Huston’s Mechanical Stirrer.—The stirring apparatus shown in [Fig. 9] differs from those which have heretofore come into use, in requiring but a single belt to drive all the stirring rods, and in having all the parts protected from the laboratory fumes.[119] The details of the belt system are shown in the small diagram in the lower central part of the figure. The apparatus is mounted on a substantial wooden box, 200 centimeters long, thirty centimeters high, and eighteen centimeters wide. The driving pulleys, ten centimeters in diameter, are enclosed in the upper part of the case. The shafts on which these pulleys are mounted extend through the bottom of the enclosing box and carry a wooden disk, eleven centimeters in diameter, to prevent particles of foreign matter from falling into the beakers. The shafts extend two centimeters below these disks, and to the end of the shafts the bent stirring rods are attached by rubber tubing.
The board forming the support of the driving pulleys is extended two centimeters in front of the apparatus, and in this extension twelve notches are cut, in which are held the corks carrying the tubes which contain the solution to be used in precipitating the material in the beakers.
Figure. 9.
Huston’s Mechanical Stirrer.
The ends of these tubes are drawn out to a fine point so as to deliver the liquid at the rate of about one drop per second.
The front of the apparatus is hinged and permits the whole to be closed when not in use, or during the precipitation.
The apparatus has proven extremely satisfactory in the precipitation of ammonium magnesium phosphate. The precipitate is very crystalline, and where the stirring is continued for some minutes, after the magnesia solution has all been added, no amorphous precipitate is observed on longer standing.
132. The Citrate Method Applied to Samples with Small Content of Phosphoric Acid.—It is well established that the citrate method does not give satisfactory results when applied to samples containing small percentages of phosphoric acid, especially when these are of an organic nature, as for instance, cottonseed cake-meal. In this laboratory attempts have been made to remedy this defect in the process so as to render the use of the method possible even in such cases.[120] Satisfactory results have been obtained by adding to the solution of the cake-meal a definite volume of a phosphate solution of known strength. Solutions of ordinary mineral phosphates are preferred for this purpose. The following example will show the application of the modified method:
In a sample of cake-meal, (cottonseed cake and castor pomace) the content of phosphoric acid obtained by the molybdate method, was 2.52 per cent.
Determined directly by the citrate method, the following data were obtained:
Allowing to stand thirty hours after adding magnesia mixture, 1.08 and 1.53 per cent in duplicates.
Allowing to stand seventy-two hours after adding magnesia mixture, 2.17 and 2.30 per cent in duplicates.
In each case fifty cubic centimeters of the solution were taken, representing half a gram of the sample.
In another series of determinations twenty-five cubic centimeters of the sample were mixed with an equal volume of a mineral phosphate solution, the value of which had been previously determined by both the molybdic and citrate methods. The fifty cubic centimeters thus obtained represented a quarter of a gram each of the cake-meal and mineral phosphates. The filtration followed eighteen hours after adding the magnesia mixture. The following data show the results of the determinations:
| Per cent P₂O₅ mineral phosphate. | Per cent P₂O₅ in organic sample. | Per cent P₂O₅ found in mixture × 2. | Per cent P₂O₅ in organic sample. | |
|---|---|---|---|---|
| 1 | 15.37 | 2.52 | 17.90 | 2.53 |
| 2 | 29.16 | 2.52 | 31.68 | 2.52 |
| 3 | 31.37 | 2.52 | 33.83 | 2.45 |
| 4 | 31.58 | 2.52 | 34.20 | 2.62 |
| Mean content of P₂O₅ in organic sample | 2.53 | |||
It is thus demonstrated that the citrate method can be applied with safety even to the determination of the phosphoric acid in organic compounds where the quantity present is less than three per cent. It is further shown that solutions of mineral phosphates varying in content of phosphoric acid from fifteen to thirty-two per cent may be safely used for increasing the content of that acid to the proper degree for complete precipitation. In cases where organic matters are present they should be destroyed by moist combustion with sulfuric acid as in the determination of nitrogen to be described in the next part.
133. Direct Precipitation of the Citrate-Soluble Phosphoric Acid.—The direct determination of citrate-soluble phosphoric acid by effecting the precipitation by means of magnesia mixture in the solution obtained from the ammonium citrate digestion, has been practiced for many years by numbers of European chemists, and the process has even obtained a place in the official methods of some European countries. Various objections have been urged, however, against the general employment of this method in fertilizer analysis on account of the inaccuracies in the results obtained in certain cases, and it has, therefore, been used to but a very limited extent in this country. Since it is impracticable to effect the precipitation with ammonium molybdate in the presence of citric acid the previous elimination or destruction of this substance has been recognized as essential to the execution of a process involving the separation of the phosphoric acid as phosphomolybdate.
It is evident from the data cited in the preceding paragraph, that great accuracy may be secured in this process by adding a sufficient quantity of a solution of a mineral phosphate and proceeding by the citrate method.
Ross has also proposed to estimate the acid soluble in ammonium citrate directly by first destroying the organic matter by moist combustion with sulfuric acid.[121] He recommends the following process:
After completion of the thirty minutes’ digestion of the sample with citrate solution, twenty-five cubic centimeters are filtered at once into a dry vessel. If the liquid be filtered directly into a dry burette, twenty-five cubic centimeters can be readily transferred to another vessel without dilution. After cooling, run twenty-five cubic centimeters of the solution into a digestion flask of 250-300 cubic centimeters capacity, add about fifteen cubic centimeters of concentrated sulfuric acid and place the flask on a piece of wire gauze over a moderately brisk flame; in about eight minutes the contents of the flask commence to darken and foaming begins, but this will occasion no trouble, if an extremely high, or a very low flame be avoided. In about twelve minutes the foaming ceases and the liquid in the flask appears quite black; about one grain of mercuric oxid is now added and the digestion is continued over a brisk flame. The operation can be completed in less than half an hour with ease, and in many cases, twenty-five minutes. After cooling, the contents of the flask are washed into a beaker, ammonia is added in slight excess, the solution is acidified with nitric, and after the addition of fifteen grams of ammonium nitrate, the process is conducted as usual.
In case as large an aliquot as fifty cubic centimeters of the original filtrate be used, ten cubic centimeters of sulfuric acid are added, and the digestion is conducted in a flask of 300-500 cubic centimeters capacity; after the liquid has blackened and foaming has progressed to a considerable extent, the flask is removed from the flame, fifteen cubic centimeters more of sulfuric acid are added, and the flask and contents are heated at a moderate temperature for two or three minutes; the mercuric oxid is then added and the operation completed as before described.
Following are some of the advantages offered by the method described:
(1) It dispenses with the necessity of the execution of the frequently tedious operation of bringing upon the filter and washing the residue from the ammonium citrate digestion, while the ignition of this residue together with the subsequent digestion with acid and filtration are also avoided.
(2) It affords a means for the direct estimation of that form of phosphoric acid which, together with the water-soluble, constitutes the available phosphoric acid, thus enabling the latter to be determined by making only two estimations.
(3) In connection with the advantages above mentioned it permits of a considerable saving of time, as well as of labor required in manipulation.
In addition to the tests with mercuric oxid, both potassium nitrate and potassium sulfate were used in the digestion to facilitate oxidation. With the former, several additions of the salt were necessary to secure a satisfactory digestion, and even then the time required was longer than with the mercury or mercuric oxid digestion. With potassium sulfate, the excessive foaming which took place interfered greatly with the execution of the digestion process.
134. Availability of Phosphatic Fertilizers.—There is perhaps no one question more frequently put to analysts by practical farmers than the one relating to the availability of fertilizing materials. The object of the manufacturer should be to secure each of the valuable ingredients of his goods in the most useful form. The ideal form in which phosphoric acid should come to the soil is one soluble in water. Even in localities where heavy rains may abound, there is not much danger of loss of soluble acid by percolation. As has before been indicated, the soluble acid tends to become fixed in all normal soils, and to remain in a state accessible to the rootlets of plants, and yet free from danger of leaching. For this reason, by most agronomists, the water-soluble acid is not regarded as more available than that portion insoluble in water, yet soluble in ammonium citrate.
In many of the States the statutes, or custom, prescribe that only the water and citrate-soluble acid shall be reckoned as available, the insoluble residue being allowed no place in the estimates of value. In many instances such a custom may lead to considerable error, as in the case of finely ground bones and some forms of soft and easily decomposable tricalcium phosphates. There are also, on the markets, phosphates composed largely of iron and aluminum salts, and these appear to have an available value often in excess of the quantities thereof soluble in ammonium citrate.
As a rule the apatites, when reduced to a fine powder and applied to the soil, are the least available of the natural phosphates. Next in order come the land rock and pebble phosphates which, in most soils, have only a limited availability. The soft fine-ground phosphates, especially in soils rich in humus, have an agricultural value, almost, if not quite equal to a similar amount of acid in the acid phosphates. Fine-ground bones also tend to give up their phosphoric acid with a considerable degree of readiness in most soils. Natural iron and aluminum phosphates, have also, as a rule, a high degree of availability. In each case the analyst must consider all the factors of the case before rendering a decision. Not only the relative solubility of the different components of the offered fertilizer in different menstrua must be taken into consideration, but also the character of the soil to which it is to be applied, the time of application, and the crop to be grown. By a diligent study of these conditions the analyst may, in the end, reach an accurate judgment of the merits of the sample.
135. Direct Weighing of the Molybdenum Precipitate.—It has already been stated that many attempts have, been made to determine the phosphoric acid by direct weighing as well as by titration, as in the Pemberton method. The point of prime importance in such a direct determination is to secure an ammonium phosphomolybdate mixture of constant composition. Unless this can be done no direct method, either volumetric or gravimetric, can give reliable results. Hanamann[122] proposes to secure this constant composition by varying somewhat the composition of the molybdate mixture and precipitating the phosphoric acid under definite conditions. The molybdate solution employed is prepared as follows:
| Molybdic acid | 100 | grams. |
| Ten per cent ammonia | 1.0 | liter. |
| Nitric acid (1.246 sp. gr.) | 1.5 | liters. |
The precipitation of the phosphoric acid is conducted in the cold with constant stirring. It is complete in half an hour. The ammonium phosphomolybdate is washed with a solution of ammonium nitrate and then with dilute nitric acid, dried, and ignited at less than a red heat. It should then have a bluish-black color throughout. Such a body contains 4.018 per cent of phosphoric anhydrid.
Twenty-five cubic centimeters of a sodium phosphate solution containing fifty milligrams of phosphoric acid, treated as above, gave a bluish-black precipitate weighing 1.249 grams, which, multiplied by 0.041018, equaled 50.018 milligrams of phosphorus pentoxid. The method should be tried on phosphates of various kinds and contents of phosphorus pentoxid before a definite judgment of its merits is formed.
CHEMISTRY OF THE MANUFACTURE
OF SUPERPHOSPHATES.
136. Reactions with Phosphates.—In this country the expressions “acid” and “super” phosphates are used interchangeably. A more correct use of the terms would designate by “acid” the phosphate formed directly from tricalcium phosphate by the action of sulfuric acid, while by “super” would be indicated a similar product formed by the action of free phosphoric acid on the same materials. In Germany the latter compound is called double phosphate.
The reaction which takes place in the first instance is represented by the following formula:
3Ca₃(PO₄)₂ + 6H₂SO₄ + 12H₂O = 4H₃PO₄ + Ca₃(PO₄)₂ + 6(CaSO₄·2H₂O);
and 4H₃PO₄ + Ca₃(PO₄)₂ + 3H₂O = 3[CaH₄(PO₄)₂·H₂O].
A simpler form of the reaction is expressed as follows:
Ca₃(PO₄)₂ + 2H₂SO₄ + 5H₂O = CaH₄(PO₄)₂·H₂O + 2[CaSO₄·2H₂O].
If 310 parts, by weight, of fine-ground tricalcium phosphate be mixed with 196 parts of sulfuric acid and ninety parts of water, and the resulting jelly be quickly diluted with a large quantity of water, and filtered, there will be found in the filtrate about three-quarters of the total phosphoric as free acid. If, however, the jelly, at first, formed as above, be left to become dry and hard, the filtrate, when the mass is beaten up with water and filtered, will contain monocalcium phosphate, CaH₄(PO₄)₂.
If the quantity of sulfuric acid used be not sufficient for complete decomposition, the dicalcium salt is formed directly according to the following reaction:
Ca₃(PO₄)₂ + H₂SO₄ + 6H₂O = Ca₂H₂(PO₄)₂·4H₂O + CaSO₄·2H₂O.
This arises, doubtless, by the formation, at first, of the regular monocalcium salt and the further reaction of this with the tricalcium compound, as follows:
CaH₄(PO₄)₂ + H₂O + Ca₃(PO₄)₂ + 7H₂O = 2[Ca₂H₂(PO₄)₂·4H₂O].
This reaction represents, theoretically, the so-called reversion of the phosphoric acid. When there is an excess of sulfuric acid there is a complete decomposition of the calcium salts with the production of free phosphoric acid and gypsum. The reaction is represented by the following formula:
Ca₃(PO₄)₂ + 3H₂SO₄ + 6H₂O = 2H₃PO₄ + 3[CaSO₄·2H₂O].
The crystallized gypsum absorbs the six molecules of water in its molecular structure.
137. Reactions with Fluorids.—Since calcium fluorid is present in nearly all mineral phosphates, the reactions of this compound must be taken into consideration in a chemical study of the manufacture of acid phosphates. When treated with sulfuric acid the first reaction which takes place consists in the formation of hydrofluoric acid: CaF₂ + H₂SO₄ = 2HF + CaSO₄. Since, however, there is generally some silica in reach of the nascent acid, all, or a portion of it, combines at once with this silica, forming silicon tetrafluorid: 4HF + SiO₂ = 2H₂O + SiF₄. This compound, however, is decomposed at once in the presence of water, forming hydrofluosilicic acid: 3SiF₄ + 2H₂O = SiO₂ + 2H₂SiF₆. The presence of calcium fluorid in natural phosphates is extremely objectionable from a technical point of view, both on account of the increased consumption of oil of vitriol which it causes, but also by reason of the injurious nature of gaseous fluorin compounds produced. Each 100 pounds of calcium fluorid entails the consumption of 125.6 pounds of sulfuric acid.
138. Reaction with Carbonates.—Most mineral phosphates contain calcium carbonate in varying quantities. This compound is decomposed on treatment with sulfuric acid according to the reaction: CaCO₃ + H₂SO₄ = CaSO₄ + H₂O + CO₂. When present in moderate amounts, calcium carbonate is not an objectionable impurity in natural phosphates intended for acid phosphate manufacture. The reaction with sulfuric acid which takes place produces a proper rise in temperature throughout the mass, while the escaping carbon dioxid permeates and lightens the whole mass, assisting thus in completing the chemical reaction by leaving the residual mass porous, and capable of being easily dried and pulverized. Where large quantities of carbonate in proportion to the phosphate are present the sulfuric acid used should be dilute enough to furnish the necessary water of crystallization to the gypsum formed. For each 100 parts, by weight, of calcium carbonate, eighty parts of sulfuric anhydrid are necessary, or 125 parts of acid of 1.710 specific gravity = 60° Beaumé.
In some guanos a part of the calcium is found as pyrophosphate, and this is acted upon by the sulfuric acid in the following way: Ca₂P₂O₇ + H₂SO₄ = CaH₂P₂O₇ + CaSO₄.
139. Solution of the Iron and Alumina Compounds.—Iron may occur in natural phosphates in many forms. It probably is most frequently met with as ferric or ferrous phosphate, seldom as ferric oxid, and often as pyrite, FeS₂. The iron also may sometimes exist as a silicate. The alumina is found chiefly in combination with phosphoric acid, and as silicate.
Where a little less sulfuric acid is employed, as is generally the case, than is necessary for complete solution, the iron phosphate is attacked as represented below:
3FePO₄ + 3H₂SO₄ = FePO₄·2H₂PO₄ + Fe₂(SO₄)₃.
When an excess of sulfuric acid is employed, the formula is reduced to the simple one:
2FePO₄ + 3H₂SO₄ = 2H₃PO₄ + Fe₂(SO₄)₃.
A part of the iron sulfate formed reacts with the acid calcium phosphate present to produce a permanent jelly-like compound, difficult to dry and handle. As much as two per cent of iron phosphate, however, may be present without serious interference with the commercial handling of the product. By using more sulfuric acid as much as four or five per cent of the iron phosphate can be held in solution. Larger quantities are very troublesome from a commercial point of view. The reaction of the ferric sulfate with monocalcium phosphate, is as follows:
3CaH(PO₄)₂ + Fe₂(SO₄)₃ + 4H₂O = 2(FePO₄·2H₃PO₄·2H₂O) + 3CaSO₄.
Pyrite and the silicates containing iron are not attacked by sulfuric acid, and these compounds are therefore left, in the final product, in a harmless state. If the pyritic iron is to be brought into solution aqua regia should be employed.
With sufficient acid the aluminum phosphate is decomposed with the formation of aluminum sulfate and free phosphoric acid:
AlPO₄ + 3H₂SO₄ = Al₂(SO₄)₃ + 2H₃PO₄.
140. Reaction with Magnesium Compounds.—The mineral phosphates, as a rule, contain but little magnesia. When present it is probably as an acid salt, MgHPO₄. Its decomposition takes place in slight deficiency or excess of sulfuric acid respectively as follows:
2MgHP₄ + H₂SO₄ + 2H₂O = [MgH₄(PO₄)₂·2H₂O] + MgSO₄
and MgHPO₄ + H₂SO₄ = H₃PO₄ + MgSO₄.
The magnesia, when in the form of oxid, is capable of producing a reversion of the monocalcium phosphate, as is shown below:
CaH₄(PO₄)₂ + MgO = CaMgH₂(PO₄)₂ + H₂O.
One part by weight of magnesia can render three and one-half parts of soluble monocalcium phosphate insoluble.
141. Determination of Quantity of Sulfuric Acid Necessary for Solution of a Mineral Phosphate.—The theoretical quantity of sulfuric acid required for the proper treatment of any phosphate may be calculated from its chemical analysis and by the formulas and reactions already given. For the experimental determination the method of Rümpler may be followed.[123]
Twenty grams of the fine phosphate are placed in a liter flask with a greater quantity of accurately measured sulfuric acid than is necessary for complete solution. The acid should have a specific gravity of 1.455 or 45° B. The mixture is allowed to stand for two hours at 50°. It is then cooled, the flask filled with water to the mark, well shaken, and the contents filtered. Fifty cubic centimeters of the filtrate are treated with tenth normal soda-lye until basic phosphate begins to separate. The excess of acid used is then calculated. Example: Twenty grams of phosphate containing 28.3 per cent of phosphoric acid, 10.0 per cent of calcium carbonate, 5.5 per cent of calcium fluorid, and 2.4 per cent of calcium chlorid were treated as above with sixteen cubic centimeters of sulfuric acid containing 10.24 grams of sulfur trioxid. In titrating fifty cubic centimeters of the filtrate obtained as described above, 10.4 cubic centimeters of tenth normal soda-lye were used, equivalent to 0.0416 gram of sulfur trioxid. Then 10.24 × 50 ÷ 1000 = 0.5120 = total sulfur trioxid in fifty cubic centimeters of the filtrate, and 0.5120 - 0.0416 = 0.4704 gram, the amount of sulfur trioxid consumed in the decomposition.
Therefore the sulfur trioxid required for decomposition is 47.04 per cent of the weight of the phosphate employed. One hundred parts of the phosphate would therefore require 47.04 parts of sulfur trioxid = to 73.6 parts of sulfuric acid of 1.710 specific gravity or 92.1 parts of 1.530 specific gravity.
A more convenient method than the one mentioned above consists in treating a small quantity of the phosphate, from one-half to one kilogram, in the laboratory, or fifty kilograms in a lead box, just as would be practiced on a large scale. A few tests with these small quantities, followed by drying and grinding will reveal to the skilled operator the approximate quantity and strength of sulfuric acid to be used in each case. The quantities of sulfuric acid as determined by calculation from analyses and by actual laboratory tests agree fairly well in most instances. There is, however, sometimes a marked disagreement. The general rule of practice is to use always an amount of sulfuric acid sufficient to produce and maintain water-soluble phosphoric acid in the fertilizer, but the sulfuric acid must not be used in such quantity as to interfere with the subsequent drying, grinding, and marketing of the acid phosphate.
For convenience the following table may be used for calculating the quantity of oil of vitriol needed for each unit of weight of material noted:
One Part by Weight of Each Substance Below Requires:
| Sulfuric Acid by Same Unit of Weight. | |||||
|---|---|---|---|---|---|
| At 48° B. | At 50° B. | At 52° B. | At 54° B. | At 55° B. | |
| Tricalcium phosphate | 1.590 | 1.517 | 1.446 | 1.382 | 1.352 |
| Iron phosphate | 1.630 | 1.558 | 1.485 | 1.420 | 1.390 |
| Aluminum phosphate | 2.025 | 1.930 | 1.839 | 1.756 | 1.721 |
| Calcium carbonate | 1.640 | 1.565 | 1.495 | 1.428 | 1.411 |
| Calcium fluorid | 2.006 | 2.010 | 1.916 | 1.830 | 1.794 |
| Magnesium carbonate | 1.940 | 1.860 | 1.775 | 1.690 | 1.660 |
Example.—Suppose for example a phosphate of the following composition is to be treated with sulfuric acid; viz.,[124]
| Moisture and organic | 4.00 | per cent. |
| Calcium phosphate | 55.00 | “ |
| Calcium carbonate | 3.00 | “ |
| Iron and aluminum phosphate | ||
| nearly all alumina | 6.50 | “ |
| Magnesium carbonate | 0.75 | “ |
| Calcium fluorid | 2.25 | “ |
| Insoluble | 28.00 | “ |
Using sulfuric acid of 50° B., the following quantities will be required for each 100 kilograms.
| Kilos of acid required. | ||
| Calcium | phosphate, fifty-five kilos | 83.44 |
| “ | carbonate three and a half kilos | 5.48 |
| “ | fluorid, two and a quarter “ | 4.52 |
| Aluminum and iron phosphate, six and a half kilos | 12.55 | |
| Magnesium carbonate, three-quarters of a kilo | 1.40 | |
| Total | 107.39 | |
142. Phosphoric Acid Superphosphates.—If a mineral phosphate be decomposed by free phosphoric in place of sulfuric acid the resulting compound will contain about three times as much available phosphoric acid as is found in the ordinary acid phosphate. The reaction takes place according to the following formulas:
(1) Ca₃(PO₄)₂ + 4H₃PO₄ + 3H₂O = 3[CAH₄(PO₄)₂·H₂O].
(2) Ca₃(PO₄)₃ + 2H₃PO₄ + 12H₂O = 3[Ca₂H₂(PO₄)₂·4H₂O].
In each case the water in the final product is probably united as crystal water with the calcium salts produced. The monocalcium salt formed in the first reaction is soluble in water and the dicalcium salt in the second reaction in ammonium citrate. Where fertilizers are to be transported to great distances there is a considerable saving of freight by the use of such a high-grade phosphate, which may, at times, contain over forty per cent of available acid. The phosphoric acid used is made directly from the mineral phosphate by treating it with an excess of sulfuric acid.
AUTHORITIES CITED IN
PART FIRST.
[1] Day, Mineral Resources of the United States 193, pp. 703, et seq.
[2] Massachusetts Agricultural Experiment Station, Bulletin 51, March, 1894.
[3] Brown, Manual of Assaying, p. 24.
[4] Bulletin de l’Association des Chimistes de Sucrèrie, No. 2, pp. 7, et seq.
[5] Proceedings of the Twelfth and Thirteenth Meetings of the Society for the Promotion of Agricultural Science, p. 140.
[6] Chemical Division, U. S. Department of Agriculture, Bulletin 43, p. 341.
[7] Rapport adressé par le Comité des Stations agronomiques au sujet des Methodes à suivre dans l’Analyse des Matières fertilisantes.
[8] Die Landwirtschaftlichen Versuchs-Stationen, Band 38, S. 303.
[9] Vid. op. cit. 6, p. 341.
[10] Zeitschrift für analytische Chemie, 1890, S. 390.
[11] Vid. op. cit. 6, p. 342.
[12] Chemisches Centralblatt, Band 2, S. 813.
[13] Transactions of the American Institute of Mining Engineers, Vol. 21, p. 165.
[14] Phosphates of America, p. 144.
[15] Vid. op. et loc. cit. 13.
[16] U. S. Geological Survey, Bulletin No. 47.
[17] Vid. op. et loc. cit. 14.
[18] Vid. op. et loc. cit. 13.
[19] Vid. op. cit. 14, p. 147.
[20] Transactions of the American Institute of Mining Engineers, Vol. 21, p. 168.
[21] Phosphates of America, p. 153.
[22] Die Landwirtschaftlichen Versuchs-Stationen, Band 34, S. 379.
[23] Zeitschrift für analytische Chemie, 1892, S. 383.
[24] Zeitschrift für angewandte Chemie, 1894, Ss. 679 und 701.
[25] Vid. op. cit. supra, 1889, p. 636.
[26] Vid. op. cit. 24, 1891, p. 3.
[27] Rapports presentèes au Congrès International de Chimie Appliqué, Bruxelles, Août, 1894, p. 26.
[28] Vid. op. et loc. cit. 20.
[29] Le Stazioni Sperimentali Agrarie Italiane, Vol. 23, p. 31.
[30] Crookes’ Select Methods, p. 538.
[31] Journal of Analytical and Applied Chemistry, Vol. 5, p. 671. For additional authorities on these methods consult Meyer and Wohlrab, Zeitschrift für angewandte Chemie, 1891, Ss. 170 und 243. Gruber, Zeitschrift für analytische Chemie, Band 30, S. 206. Shephard, Chemical News, May 29, 1891, p. 251. Vögel, Zeitschrift für angewandte Chemie, 1891, Band 12, S. 357.
[32] Journal of the American Chemical Society, April, 1895.
[33] Vid. op. cit. 21, p. 150.
[34] Transactions of the American Institute of Mining Engineers, Vol. 21, p. 170.
[35] Vid. op. cit. supra, p. 173.
[36] Comptes rendus, Tome 54, p. 468.
[37] Crookes’ Select Methods, p. 500.
[38] For details of method see Fresenius quantitative Analysis.
[39] U. S. Department of Agriculture, Chemical Division, Bulletin 43, p. 341.
[40] Letter to B. W. Kilgore, Reporter for Phosphoric Acid to the Association of Official Agricultural Chemists.
[41] Die Landwirtschaftlichen Versuchs-Stationen, Band 38, S. 304.
[42] Communicated by Dr. Solberg.
[43] From the Official Swedish Methods; translated for the author by F. W. Woll.
[44] Methoden van Onderzock aan de Rijkslandbouw-proefstations, 1893, p. 4.
[45] Zeitschrift für analytische Chemie, 1893, S. 64.
[46] Journal of the American Chemical Society, Vol. 16.
[47] Zeitschrift für angewandte Chemie, 1894, S. 678.
[48] Journal für Landwirtschaft, Band 30, S. 23.
[49] Vid. op. cit. 47, p. 544.
[50] Vid. op. cit. 46, Vol. 16, p. 462.
[51] Vid. op. et loc. cit. supra.
[52] Die Agricultur-Chemische Versuchs-Station, Halle a/S., Ss. 56, et seq.
[53] Chemische Industrie, 1890.
[54] Vid. op. et loc. cit. 52.
[55] Chemiker Zeitung, 1890, No. 75, S. 1246.
[56] Vid. op. cit. 43.
[57] Glaser, Zeitschrift für analytische Chemie, 24, 178 (1885). Laubheimer, Ibid, 25, 416 (1886). Müller, Tagebl. d. Naturforscher-Vers. zu Wiesbaden, 1886, 365. Vögel, Chemiker Zeitung, 1888, 85. Stutzer, Ibid, 492. Seifert, Ibid, 1390. v. Reis, Zeitschrift für angewandte Chemie, 1888, 354. Loges, Reportorium für analytische Chemie, 7, 85 (1887). Kassuer, Zeitschrift für Nahrungsmitteluntersuchung und Hygiene, 2, 22 (1888). C. Müller, Die Landwirtschaftlichen Versuchs-Stationen, 35, 438 (1888).
[58] L’Engrais, Tome 9, p. 928.
[59] Vid. op. et loc. cit. 44.
[60] Journal of the American Chemical Society, Vol. 16, p. 462.
[61] Die Landwirtschaftlichen Versuchs-Stationen, Band 41, S. 329.
[62] Journal of Analytical and Applied Chemistry, Vol. 5, p. 685.
[63] Vid. op. cit. supra, Vol. 3, p. 413.
[64] Zeitschrift für angewandte Chemie, 1886, S. 354.
[65] Vid. op. cit. 52, p. 61.
[66] Vid. op. cit. 55, Vol. 18, p. 1153.
[67] Chemiker Zeitung, 1894, No. 88, p. 1934.
[68] Op. cit. supra, 1892, p. 1471.
[69] Vid. op. cit. 60, p. 721.
[70] Zeitschrift für analytische Chemie, Band 29, S. 408.
[71] Mitteilungen der deutschen Landwirtschafts Gesellschaft, 1890-’91, No. 11, S. 131.
[72] Zeitschrift für angewandte Chemie, 1888, S. 299.
[73] Vid. op. cit. 70, p. 409.
[74] Vid. op. cit. 72, 1890, p. 595.
[75] Vid. op. cit. 61, Tome 43, p. 183.
[76] Chemiker Zeitung, Band 18, S. 565.
[77] Chemical News, Vol. 1, p. 97.
[78] Archive für Wissenschaftliche Heilkunde, Band 4, S. 228.
[79] Journal für praktische Chemie, Band 70, S. 104.
[80] Sutton’s Volumetric Analysis, p. 237.
[81] Bulletin de la Société des Agriculteurs de France, 1876, p. 53.
[82] Manual Agenda des Fabricants de Sucre, 1889, p. 307.
[83] Journal of the American Chemical Society, Vol. 15, p. 382, and Vol. 16, p. 278.
[84] Chemical News, Vol. 47, p. 127.
[85] American Chemical Journal, Vol. 11, p. 84.
[86] Vid. op. cit. 83, Vol. 16, p. 282.
[87] Bulletin 43, Chemical Division, U. S. Department of Agriculture, p. 88.
[88] Vid. op. cit. supra, p. 91.
[89] Repertoire de Pharmacie, 1893, p. 153.
[90] Revue de Chimie Analytique Appliqué, 1893, p. 113.
[91] Chemiker Zeitung, 1894, S. 1533.
[92] Blair, Analysis of Iron and Steel, p. 95.
[93] Journal of Analytical and Applied Chemistry, Vol. 7, p. 108.
[94] Journal of the American Chemical Society, Vol. 17, p. 129.
[95] Vid. op. cit. 92, p. 99.
[96] Eighth Annual Report of Purdue University, p. 238.
[97] Receuil des Travaux Chimiques, Tome 12, pp. 1, et seq. Journal of the Chemical Society (Abstracts), Vol. 64, p. 496.
[98] Zeitschrift für angewandte Chemie, 1891, Ss. 279, et seq.
[99] Le Stazioni Sperimentali Agrarie Italiane, February, 1891.
[100] Journal of the American Chemical Society, Vol. 17, p. 43.
[101] Vid. op. et loc. cit. supra.
[102] L’Engrais, Tome 10, p. 65.
[103] Journal of Analytical and Applied Chemistry, Vol. 5, p. 694. Zeitschrift für analytische Chemie, Band 18, S. 99.
[104] Vid. op. cit. 92, p. 103.
[105] Journal of the Chemical Society (Abstracts), Vol. 58, p. 1343.
[106] Comptes rendus, Tome 114, p. 1189.
[107] Vid. op. cit. 24 and 25.
[108] Report communicated to author by W. G. Brown.
[109] Chemisches Centralblatt, 1895, p. 562.
[110] Wiley, Report on Fertilizers to Indiana State Board of Agriculture, 1882.
[111] Proceedings of the Association of Official Agricultural Chemists, Atlanta, 1884, p. 19. Report of Indiana State Board of Agriculture, 1882, p. 230, and Proceedings of the Association of Official Agricultural Chemists, Atlanta, 1884, p. 30. Huston and Jones. (These gentlemen are now investigating all materials used as sources of phosphoric acid in fertilizers; their results here quoted are from unpublished work, and include but a small part of the work so far done.) American Chemical Journal, March, 1884, p. 1. Proceedings of the Association of Official Agricultural Chemists, Atlanta, 1884, p. 23. Ibid, p. 28. Ibid, p. 38. Ibid, p. 45. U. S. Department of Agriculture, Chemical Division, Bulletin No. 7, p. 18. Ibid, Bulletin No. 28, p. 171. Ibid, Bulletin No. 31, p. 100. Ibid, Bulletin No. 31, p. 99.
[112] Manuscript communication to author.
[113] Pamunky phosphate is the so-called “olive earth” found along the Pamunky river, in Virginia. It is almost all precipitated iron and aluminum phosphates, and the product is peculiar in that the iron is almost all in the ferrous condition.
[114] In the work of T. S. Gladding only fifty cubic centimeters of citrate were used.
[115] In the work of T. S. Gladding only fifty cubic centimeters of citrate were used.
[116] Zeitschrift für analytische Chemie, Band 10, S. 133.
[117] Lehrbuch der Düngerfabrication.
[118] Bulletin 54, Purdue Agricultural Experiment Station, p. 4.
[119] Vid. op. cit. supra, p. 7.
[120] Runyan and Wiley; Paper presented to Washington Section of the American Chemical Society, April 11, 1895.
[121] Bulletin 38, Chemical Division, U. S. Department of Agriculture, p. 16.
[122] Chemiker Zeitung, 1895, S. 553.
[123] Die Käuflichen Dungermittel Stoffe, dritte Auflage, 1889.
[124] Wyatt, Phosphates of America, p. 128.
PART SECOND.
NITROGEN IN FERTILIZERS.
143. Kinds of Nitrogen in Fertilizers.—Nitrogen is the most costly of the essential plant foods. It has been shown in the first volume, [paragraph 23], that the popular notion regarding the relatively great abundance of nitrogen is erroneous. It forms only 0.02 per cent of the matter forming and pertaining to the earth’s crust. The great mass of nitrogen forming the bulk of the atmosphere is inert and useless in respect of its adaptation to plant food. It is not until it becomes oxidized by combustion, electrical discharges, or the action of certain microorganisms that it assumes an agricultural value.
Having already, in the first volume, described the relation of nitrogen to the soil it remains the sole province of the present part to study it as aggregated in a form suited to plant fertilization. In this function nitrogen may claim the attention of the analyst in the following forms:
1. In organic combination in animal or vegetable substances, forming a large class of bodies, of which protein may be taken as the type. Dried blood or cottonseed-meal illustrates this form of combination.
2. In the form of ammonia or combinations thereof, especially as ammonium sulfate, or as amid nitrogen.
3. In a more highly oxidized form as nitrous or nitric acid usually united with a base of which Chile saltpeter may be taken as a type.
The analyst has often to deal with single forms of nitrogenous compounds, but in many instances may also find all the typical forms in a single sample. Among the possible cases which may arise the following are types:
a. The sample under examination may contain nitrogen in all three forms mentioned above.
b. There may be present nitrogen in the organic form mixed with nitric nitrogen.
c. Ammoniacal nitrogen may replace the nitric in the above combination.
d. The sample may contain no organic but only nitric and ammoniacal nitrogen.
e. Only nitric or ammoniacal nitrogen may be present.
144. Determination of the State of Combination.—Some of the sample is mixed with a little powdered soda-lime. If ammoniacal nitrogen be present free ammonia is evolved even in the cold and may be detected either by its odor or by testing the escaping gas with litmus or turmeric paper. A glass rod moistened with strong hydrochloric acid will produce white fumes of ammonium chlorid when brought near the escaping ammonia.
If the sample contain any notable amount of nitric acid it will be revealed by treating an aqueous solution of it with a crystal of ferrous sulfate and strong sulfuric acid. The iron salt should be placed in a test-tube with a few drops of the solution of the fertilizer and the sulfuric acid poured down the sides of the tube in such a way as not to mix with the other liquids. The tube must be kept cold. A dark brown ring will mark the disk of separation between the sulfuric acid and the aqueous solution in case nitric acid be present. If water produce a solution of the sample too highly colored to be used as above, alcohol of eighty per cent strength may be substituted. The coloration produced in this case is of a rose or purple tint.
Nitric nitrogen may also be detected by means of brucin. If a few drops of an aqueous solution of brucin be mixed with the same quantity of an aqueous extract of the sample under examination and strong sulfuric acid be added, as described above, there will be developed at the disk of contact between the acid and the mixed solutions a persistent rose tint varying to yellow.
To detect the presence of organic albuminoid nitrogen the residue insoluble in water, when heated with soda-lime, will give rise to ammonia which may be detected as described above.
145. Microscopic Examination.—If the chemical test reveal the presence of organic nitrogen the next point to be determined is the nature of the substance containing it. Often this is revealed by simple inspection, as in the case of cottonseed-meal. Frequently, however, especially in cases of fine-ground mixed goods, the microscope must be employed to determine the character of the organic matter. It is important to know whether hair, horn, hoof, and other less valuable forms of nitrogenous compounds have been substituted for dried blood, tankage, and more valuable forms. In most cases the qualitative chemical, and microscopic examination will be sufficient. There may be cases, however, where the analyst will be under the necessity of using other means of identification suggested by his skill and experience or the circumstances connected with any particular instance. In such cases the general appearance, odor, and consistence of the sample may afford valuable indications which will aid in discovering the origin of the nitrogenous materials.