THE CITRATE METHOD.
63. General Principles.—It has been seen that in the molybdic method there is introduced a process at considerable cost, both of reagents and time, having for its object the separation of the phosphoric acid from all the other acids and bases which may have been present in the original sample. The phosphorus is thus obtained in composition with molybdenum and ammonium in a form easily soluble in ammonia, from which it can be accurately separated by means of a soluble salt of magnesia.
The citrate method has for its object the suppression of this intermediate step and the determination of the phosphoric acid by direct precipitation in presence of iron, lime, and alumina. The principle on which it is based rests on the well-known power of an alkaline ammonium citrate to hold in solution the salts of iron, alumina, and lime, while at the same time it permits of the separation of phosphoric acid, as ammonium magnesium phosphate. In no case can the citrate method be regarded as an exact analytical process, but large experience has shown that the errors of the method are compensatory and that it affords a good and ready method for fertilizer control.
When phosphoric acid solutions which contain no iron, lime, alumina, or manganese, are precipitated in presence of ammonium citrate the results obtained vary markedly with the quantity of magnesia mixture employed. Grupe and Tollens[48] were the first to point out that a portion of the phosphoric acid might remain in solution, but that the precipitate might contain a sufficient excess of magnesia to compensate for the loss. It has been further shown by Glaser[49] that a portion of the phosphoric acid may be lost by volatilization in the citrate method. When the ignition is carried on in a crucible where the cover is coated with magnesia to intercept the volatilized acid, a considerable quantity of it can be recovered by the molybdic method.
Where too little magnesia mixture is employed, therefore, two sources of loss are to be guarded against; viz., a part of the phosphoric acid may remain in solution and another part be volatilized on ignition. The explanation of the volatilization is as follows: In the presence of ammonium citrate, magnesium chlorid may be partly converted into magnesium citrate and ammonium chlorid. There may be a time, therefore, in the precipitation with not too great excess of magnesia mixture, when proportionally there is little magnesium chlorid and much ammonium chlorid present. The formation of a salt represented by the formula Mg(NH₄)₄(PO₄)₂ may take place which, upon ignition, breaks up into Mg(PO₃)₂ and finally passes into Mg₂P₂O₇ with loss of P₂O₅. This theoretical condition has but little weight, however, practically in the analysis of fertilizers, since in these cases a large quantity of lime is always present. But even in these cases traces of volatile P₂O₅ may be discovered.
Wells[50] has shown that the citrate method gives good results in certain conditions but that this accuracy is reached by a fortunate compensation of errors. The ammonium magnesium salt does not precipitate all the phosphoric acid in this process, but contains enough impurities to make up for this loss.
Johnson[51] in conjunction with Osborne has shown that the results by the citrate method practiced in accordance with the details laid down by Vögel, are too low, but that this difficulty could be overcome by using more and stronger magnesia mixture and a larger quantity of strong ammonia solution. The citrate method was found to give unsatisfactory results when iron and alumina were present in any considerable quantity. In the examination of the final ignited precipitate, which should be pure magnesium pyrophosphate, it was found to consist of only 94.98 to 97.83 per cent of that salt. The chief impurity found was calcium oxid, the percentage of which varied from 2.05 to 3.95 in six cases. There was also a considerable percentage of loss due, probably, to magnesia and pyrophosphoric acid.
The presence of large quantities of iron and alumina also impairs the accuracy of the molybdate method when the precipitation of the yellow salt takes place at too high a temperature. When the temperature of precipitation in the method is above 50° the results are likely to be too high while a great excess of nitric acid in the reagent may produce a contrary effect. In the latter case the filtrate from the yellow salt should be mixed with additional quantities of molybdate solution until no further precipitate takes place.
Many methods of conducting the citrate method have been proposed but the best of them are based on the one elaborated at the experiment station of Halle by Bühring, and which will be given in the next paragraph, followed by some other methods in use in other localities.
64. Method of the Halle Agricultural Experiment Station.[52]—The citrate method, as described by Morgen, is the one employed.[53] The principle depends upon the direct precipitation of the phosphoric acid by magnesia mixture. By the addition of a solution of ammonium-citrate the precipitation of lime, iron, alumina, and other bases, is prevented. The precipitate of ammonium magnesium phosphate is converted by ignition into magnesium pyrophosphate and weighed as such. By the use of this method a part of the phosphoric acid sometimes escapes precipitation and a portion of the other bases is sometimes thrown down with the precipitate. Experience has shown that by adhering to certain precautions the weight of impurities in the precipitate may be made to correspond exactly to the weight of the phosphoric acid which escapes precipitation.
Figure. 3.
Shaking Apparatus for Superphosphates.
(1) Soluble Acid.—The soluble phosphates are first brought into solution in such a way that one liter of water contains the soluble phosphoric acid from twenty grams of the substance. Twenty grams are rubbed in a porcelain mortar with water and through a wide-necked funnel washed into a bottle-shaped flask in which a little water has been previously placed. The flasks employed are made of thick glass in order to withstand shaking. After the substance is washed, the flasks are filled to the mark and closed with rubber stoppers. They are then placed upon a shaking rack as indicated in [Fig. 3], which is also furnished with an apparatus for separating the fine meal from basic slag.
On a table, as [shown in the figure], is fastened a movable horizontal board by means of hinges. At the left hand of this movable board is placed an open wooden box in which is a perforated shelf for the purpose of holding the flasks, so as to prevent their striking together during the shaking.
For the best results the substance to be examined should be placed in the flask in a dry state and then 800 cubic centimeters of water added and shaken by means of the machine indicated for half an hour. Afterwards the flasks are filled up to the mark, well shaken, and filtered through double folded filters into ordinary flasks of about 400 cubic centimeters capacity. Before any of the filtrate is collected, the first that runs through should be well shaken in the receiving flasks and rejected. Fifty cubic centimeters of the filtrate thus collected, corresponding to one gram of the substance, should be used for the determination.
(2) Total Acid.—For total phosphoric acid, including the insoluble portions, the material is treated as follows: Five grams of the substance are placed in a 500 cubic centimeter flask with twenty cubic centimeters of nitric acid of 1.42 specific gravity, and fifty cubic centimeters of pure concentrated sulfuric acid, and boiled briskly for half an hour. With substances which contain much organic material, a little paraffin is added to avoid frothing. Such substances also require a larger quantity of nitric acid than that above specified. The flasks are allowed to cool, water added, again allowed to cool, and filled up to the mark at 17°.5. If hydrochloric instead of sulfuric acid be used in making the above solution, when the citrate method is employed, the results are always too high because the precipitate contains lime and alumina in such quantities as to render any compensation for them inaccurate. In addition to this the sulfuric has this great advantage over the hydrochloric acid; viz., in not separating silicic acid, inasmuch as the silicic acid is insoluble in boiling sulfuric acid.
(3) Citrate-Soluble Acid.—Two grams of the sample are digested with 100 cubic centimeters of citrate solution, 1.09 specific gravity, for half an hour at 50° in a beaker. Afterwards the soluble matter is separated by filtration with the aid of a filter-pump and the residue washed with a solution of one part water and one part citrate solution until all the dissolved phosphoric acid is removed from the filter. Generally three or four washings are sufficient. The residue on the filter is dried, ignited, and dissolved in a mixture of two cubic centimeters of nitric and twenty cubic centimeters of sulfuric acid, the solution made up to a volume of 200 cubic centimeters, filtered, and 100 cubic centimeters of the filtrate taken for the determination. The acid in the filtrate is nearly neutralized and fifty cubic centimeters of citrate solution are added, and afterwards twenty-five cubic centimeters of magnesia mixture and twenty cubic centimeters of twenty-four per cent ammonia. After standing for forty-eight hours, the precipitate is separated by filtration, ignited, and weighed in the usual way. The difference between the total phosphoric acid and that in the insoluble residue, after treatment with ammonium citrate, as above, gives the quantity of phosphoric acid soluble in the citrate solution. The difference between the total citrate-soluble and the water-soluble gives the quantity of the reverted phosphoric acid.
The ammonium citrate solution used for the digestion is made as follows: Two hundred and fifty grams of crystallized citric acid are dissolved in half a liter of hot water, diluted with 550 cubic centimeters of water, 276 cubic centimeters of twenty-four per cent ammonia added, and finally, exactly neutralized by adding, little by little, fifty per cent citric acid solution.
The Halle methods of separating the water and citrate-soluble acids appear to be less complete and reliable than those in use by the Official Agricultural Chemists of this country. The precipitation of basic phosphates, when large quantities of water are used at once in separating soluble acid, must tend to diminish the quantity obtained, while the lack of care in assuring the neutrality of the citrate solution might lead to varying results.
(4) Double Superphosphates.—In the case of double superphosphates, which sometimes contain large quantities of pyrophosphate, the solution is made in the usual way so that in 100 cubic centimeters there will be contained two grams of the substance. Usually ten grains are taken and the volume made up to half a liter. Twenty-five cubic centimeters of the filtrate are diluted with seventy-five cubic centimeters of water and the pyro converted to orthophosphoric acid by heating with ten cubic centimeters of strong nitric acid on a sand-bath. The heating should be continued until the volume be reduced to twenty-five cubic centimeters. The strongly acid liquid is made alkaline with ammonia, and afterwards slightly acid with nitric, and the rest of the process is carried on in the usual way.
(5) Phosphoric Acid in the Residue of Superphosphate Manufacture.—In the mixture of superphosphates and gypsum, the residue of the manufacture of double superphosphates, the phosphoric acid is estimated in the following manner: Five grams of the substance are placed in a dish, rubbed up with absolute alcohol, and washed into a 250 cubic centimeter flask. The flask is filled with absolute alcohol to the mark, closed with a stopper, and with frequent shaking, allowed to stand for two hours; it is thereupon filtered as quickly as possible; fifty cubic centimeters of the filtrate corresponding to one gram of the substance, are taken for the estimation. This fifty cubic centimeters is evaporated on a sand-bath to a sirupy consistence, diluted with water, and treated, as in the case of the soluble phosphates above mentioned. In all cases as described above, after the solutions are obtained they are treated with the ammonium citrate solution and the phosphoric acid estimated as in the first instance given.
(6) Solutions Employed.—
(a) The citrate solution is made as follows: 1,500 grams of citric acid are dissolved in water, treated with five liters of twenty-four per cent ammonia, and made up to fifteen liters.
(b) The magnesia mixture is made as follows: 500 grams of magnesium chlorid, 1,050 grams of ammonium chlorid, three and five-tenths liters of twenty-four per cent ammonia, and six and five-tenths liters of distilled water are used.
In the case of the superphosphates fifty cubic centimeters of the citrate solution are employed and with the basic slags 100 cubic centimeters; and in both cases twenty-five cubic centimeters of the magnesia mixture.
(7) Details of the Manipulation.—On the addition of the citrate solution there should be no permanent troubling of the liquid but there should be a total clearing up thereof. In order to facilitate this, after the addition of the citrate solution, the flasks should be gently shaken in order to distribute the solution throughout the mass. Solutions from bone-black superphosphates show sometimes, after the addition of the citrate solution, a more or less strong opalescence, but this opalescence does not influence the results. Should it happen that with superphosphates which are made from raw material containing large excesses of iron or clay, fifty cubic centimeters of the citrate solution are not sufficient to prevent the other bases from being precipitated, an additional quantity up to twenty-five cubic centimeters may be added. The addition of the magnesia mixture must follow as quickly as possible after the addition of the citrate solution to avoid a separation of crystalline calcium phosphate. On the addition of the citrate solution there is always a rise in temperature. Inasmuch as the precipitation of the phosphoric acid with magnesia must take place in the cold, the liquid must be cooled after the addition of the citrate,[54] and the cooling should take place as quickly as possible.
The above method was adopted by the chemical section of the International Agricultural Congress held at Vienna, September, 1890.[55]
Figure. 4.
Shaking Machine for Ammonium Magnesium Phosphate.
In order to hasten the precipitation of the ammonium magnesium phosphate and to prevent the fixation of the precipitate on the walls of the erlenmeyer, the flask should be shaken for half an hour. For this purpose the flasks should be closed with smooth well-fitting rubber stoppers and placed in a shaking machine. The shaking machine of the form given in [Fig. 4], recommended by the Halle station, is very conveniently used for this purpose.
On a vertical axis are carried two stages for holding the flasks. The flasks are prevented from striking each other by means of the partitions shown. The apparatus is conveniently driven by a small water-motor, as indicated, which imparts to the stages a partial back and forth revolution.
After shaking for half an hour, any precipitate adhering to the rubber stoppers is carefully washed off with ammonia water into the flask. The filtration can be made immediately after the shaking or after two or three days; the results are the same.
Figure. 5.
Rössler Ignition Furnace.
The filtration of the ammonium magnesium phosphate is made through perforated crucibles. The asbestos felt is prepared in the following way: The coarse fibers of asbestos are chopped up with a sharp knife on a glass plate and boiled for two hours with strong hydrochloric acid; afterwards, by repeated washing with distilled water they are freed from acid and the too fine particles of asbestos which would tend to make the filter too impervious. After the last wash-water is poured off, the asbestos is suspended in water and used for making the felt on the filter. The preparation of the crucible and the filtration under pressure are accomplished in the usual way.
The ignition of the precipitate is accomplished in a Rössler ignition oven, [Fig. 5]. When the muffle of the furnace shows a white heat or a white-red heat it is at the proper temperature for the estimation. At higher temperatures, the asbestos felt is easily injured. Generally, an ignition of five minutes is sufficient, but with double superphosphates, ten minutes are required.
65. The Swedish Citrate Method.[56]—This method of determining phosphoric acid is founded on the fact that phosphoric acid in the presence of calcium salts, without it being necessary, previously, to convert it into phosphomolybdate, is precipitated directly by magnesia mixture from a solution, to which ammonium citrate has been added, provided first, that the solution contain a sufficient quantity of sulfuric acid, and second, that only as much citrate be added as is required to keep the calcium salts in alkaline solution.[57]
Reagents. (1) Citric Acid Solution.—Prepared by dissolving 500 grams of citric acid in water, and completing to a volume of one liter.
(2) Ten Per Cent Ammonia of 0.959 specific gravity.
(3) Magnesia Mixture, of the usual composition.
The various processes are conducted as follows:
(a) Water-Soluble Phosphoric Acid.—Add twenty cubic centimeters of citric acid solution to fifty cubic centimeters of the water-soluble solution obtained according to the Swedish molybdenum method, and then add thirty-three cubic centimeters of ammonia. When the mixture has cooled, add slowly twenty-five cubic centimeters of the magnesia mixture, and then forty-two cubic centimeters of the ammonia. Keep the solution stirred by means of a closely clipped feather which is pressed tightly against the sides of the beaker; by this process the phosphate is precipitated after half an hour in pure condition and completely, without, in the least, sticking to the wall of the beaker; filter, wash, and ignite, as usually directed.
(b) Insoluble Phosphoric Add.—Moisten, in a porcelain dish, ten grams of the powdered sample with water; add fifty cubic centimeters of concentrated sulfuric acid, and heat for fifteen minutes so high that fumes of sulfuric acid will escape. When the mass has cooled, wash it into a half liter graduated flask, fill to the mark, and shake well. After filtration, the clear filtrate may, after some time, turn turbid by separation of calcium sulfate, but as the ammonium citrate, which is afterwards added, again brings the precipitate into solution, it is of no importance. Take fifty cubic centimeters of the solution, corresponding to one gram of the powdered sample, add twenty cubic centimeters of the citric acid solution, neutralize the mixture approximately, but not exactly, by ammonia; after cooling, add twenty-five cubic centimeters of magnesia mixture; stir the fluid by means of a feather, as described above, till no more precipitate is formed, and finally add thirty-three cubic centimeters of ammonia while stirring for a couple of minutes more; after half an hour the precipitate may be separated by filtration, washed, and ignited, as usually directed.
The above process is essentially the one used with basic slags. When much organic matter is present, by continuing the heating with sulfuric acid for some time, it may be destroyed.
66. Methods Adopted by the Brussels Congress, 1894.—The report of the committee on methods of analysis of phosphoric acid requires the molybdate method to be used in all cases where the quantity to be determined is very small. In other cases the citrate method may be employed.[58]
(1) Soluble Phosphoric Acid.—The soluble phosphoric acid is determined by the method adopted at Brussels in the following manner: Five grams of the sample are rubbed to a powder in a mortar, and then from fifty to sixty cubic centimeters of water added. After allowing to settle for a few minutes the liquid portion is decanted upon a filter. This operation is repeated three or four times. Finally the solid portions are washed upon the filter, and the washing with water is continued until the filtrate amounts to about three-quarters of a liter. A few drops of hydrochloric acid are added until the filtrate is perfectly clear, and the volume is then made up to one liter. Fifty cubic centimeters of the solution are then treated with thirty cubic centimeters of ammonium citrate solution and one-third as much ammonia. Afterwards thirty cubic centimeters of magnesia mixture are added, drop by drop, with constant stirring.
For superphosphates containing more than eighteen per cent of phosphoric acid only one gram is taken, for ordinary superphosphates two grams, and for compound fertilizers four grams. The sample is first treated as above for soluble acid until the filtrate amounts to 200 cubic centimeters, then clarified with a drop of nitric acid, and made up to a quarter of a liter.
(2) Reverted Phosphoric Acid.—The filter containing the residue is then introduced into a quarter liter flask and treated with 100 cubic centimeters of Petermann’s alkaline ammonium citrate solution, vigorously shaken, and left at room temperature for fifteen hours. It is then digested for an hour at 40° and filtered. Fifty cubic centimeters of the filtrate are placed in a flask and, with constant shaking, thirty-five cubic centimeters of magnesia mixture added. The aqueous solution is treated in the same way. The precipitate is collected, ignited and weighed, and multiplied by 0.64 for phosphoric acid. The total acid is determined in the usual way.
67. Dutch Method for Citrate-Soluble Phosphoric Acid.[59]—The reagents necessary are:
(1) Citrate solution, prepared according to Petermann. Dissolve 165 grams of citric acid in 700 cubic centimeters of water, mix with 250 cubic centimeters of ammonia of 0.92 specific gravity, and, after cooling, bring to the volume of one liter.
(2) Magnesia mixture prepared according to Petermann. Dissolve 400 grams of crystallized magnesium chlorid, 800 grams of ammonium chlorid, and 1,600 cubic centimeters of ammonia of 0.96 specific gravity in water, and dilute to five liters.
The quantity to be taken for the analysis is five grams where the fertilizer contains less than six per cent of phosphoric acid (mixed fertilizers); two grams where it contains more than six and less than fifteen per cent (common superphosphates); and one gram where it contains more than fifteen per cent (double superphosphates). Place the weighed substance in a mortar and cover with 100 cubic centimeters of citrate solution. Gently rub up, wash into a half liter flask, and heat in a water-bath for an hour to a temperature between 35° and 38°. Allow to cool, fill up to 500 cubic centimeters, and filter through a dry double filter. If it is not clear at the first filtration, pour through the filter again, repeating this till clearness is attained. Measure 100 cubic centimeters and add seventy-five cubic centimeters of magnesia mixture, allowing the latter to flow into the former very slowly, and constantly stirring during the influx. Allow to stand fifteen hours, filter, wash with ammonia of 0.96 specific gravity, dry, ignite, and weigh.
The per cent of phosphoric acid, except where otherwise indicated, is always to be given as per cent of anhydrous acid (P₂O₅).
68. Comparative Accuracy of the Citrate and Molybdate Methods.—The general use of the citrate method of determining phosphoric acid by the German chemists has led Johnson[60] to review some trials of that method in the Yale laboratory made as early as 1880. These determinations have lately been repeated in comparison with the ordinary molybdate methods with the result that in sixty-seven determinations on bone-dust, superphosphate, cotton-hull ashes, cottonseed-meal, tankage, bone-char, phosphatic guano, and phosphate rock, only three citrate results differed from those obtained by the molybdate method by more than three-tenths of one per cent. The greatest discrepancy between the two methods was 0.41 per cent, and the average difference was 0.09 per cent.
The citrate method was found to give poor results when iron and alumina were present in considerable quantity. Ignited precipitates by the citrate method were found to contain as high as four per cent of lime, and iron and alumina in small quantities when these bodies were abundant in the original substance.
In the molybdate method the rapid precipitation from solutions at 65° was found to give unsatisfactory results and it was found necessary to conduct the process at temperatures between 40° and 50°. With a relative excess of nitric or a relative deficiency of molybdic acid some phosphoric acid may easily escape precipitation. The chief objection to precipitating at 65° is found in the fact that in presence of considerable iron and alumina some of these bodies may be found in the yellow precipitate, whence they pass to the final ammonium magnesium phosphate.
The citrate method, therefore, only gives safe results by compensating errors which in every class of phosphates must be empirically determined.
The molybdate method gives results too high when iron and alumina are present in considerable quantity and the yellow precipitate is obtained at temperatures above 50°. On the other hand, if there, be a great relative excess of nitric acid the results may be too low unless the filtrates from the yellow precipitate be mixed with additional molybdic solution and digested until no further precipitate is formed.
Comparative determinations made, by both methods, by the Association of German Experiment Stations have led to the conclusion that both give practically the same results when each one is conducted with the proper precautions peculiar to it.[61] In the latter part of 1892, at the general meeting of the Association, it was declared that the citrate method, after having been subjected to repeated tests, was found to be satisfactory, changing the composition of the solution so that it might have 1,100 instead of 1,000 grams of citric acid and four liters of twenty-four per cent ammonia to each ten liters. The data afforded by the citrate method, when applied to an artificial mixture of known composition, were more satisfactory than those obtained by the molybdic process.
In this laboratory the citrate method has been found to give nearly agreeing results with the old process. It is much shorter and less expensive; and is recommended most favorably for practical use, suggesting, however, that with every new kind of phosphate or phosphatic fertilizer varying notably in composition from the standard, the work should be checked at first by comparison with the molybdenum method.