28. Silica and Insoluble Bodies.—Wyatt describes the following method for determining the total insoluble or siliceous matters in a mineral phosphate[19]. Five grams of the fine sample are placed in a porcelain dish with about thirty cubic centimeters of aqua regia. The dish is covered with a funnel, placed on a sand-bath and, after solution is complete, evaporated to dryness with care to prevent sputtering. When dry the residue is moistened with hydrochloric acid and again dried, rubbing meanwhile to a fine powder. The heat of the bath is then increased to 125° and maintained at this temperature for about ten minutes. When cool, the residue is treated with fifty cubic centimeters of hydrochloric acid for fifteen minutes. The acid is then diluted and filtered on a gooch, which is washed with hot water until the filtrate amounts to a quarter of a liter. The residue in the crucible is dried, ignited, and weighed. This method, unless the solution be subsequently boiled with nitric acid, may not retain all the phosphoric acid in the ortho form.
It is difficult to estimate the total silica by the ordinary methods of mineral analysis. This is due to the fact that in an acid solution of a substance containing silicates and fluorids the whole of the silica or the fluorin, as the case may be may escape as silicofluorid on evaporation. Again, it is not easy to decompose calcium phosphate by fusing with sodium carbonate. If an attempt be made to do this, however, the process should be conducted as follows: A portion of the sample is ground to an impalpable powder in an agate mortar. From one to two grams of the substance are mixed with five times its weight of sodium carbonate and fused with the precautions given in standard works on quantitative analysis. The fused mass is digested in water, boiled, and filtered, and the residue washed first with boiling water and afterwards with ammonium carbonate. The filtrate contains all the fluorin as sodium fluorid and, in addition to this, sodium carbonate, silicate, and aluminate. Mix the filtrate with ammonium carbonate and heat for some time, replacing the ammonium carbonate which evaporates. Separate by filtration the silicic acid hydrate and aluminum hydroxid which are formed and wash them with ammonium carbonate. To separate the last portions of silica from the filtrate, add a solution of zinc oxid in ammonia. Evaporate until no more ammonia escapes and separate, by filtration, the zinc silicate and oxid. Determine the silica in this precipitate by dissolving in nitric acid, evaporating to dryness, taking up with nitric acid and separating the undissolved silica by filtration. In the alkaline filtrate the fluorin may be estimated by the usual method as calcium salt.
29. Estimation of Lime.—One hundred cubic centimeters of the solution (containing one gram of the original substance) are evaporated in a beaker to about fifty cubic centimeters; ten cubic centimeters of dilute sulfuric acid (one to five) are added; and the evaporation is continued on the water-bath until a considerable crop of crystals of gypsum has formed[20]. The solution is then allowed to cool, when it generally becomes pasty, owing to the separation of additional gypsum. When it is cold, 150 cubic centimeters of ninety-five per cent alcohol are slowly added, with continual stirring, and the whole is allowed to stand for three hours, being stirred from time to time. After three hours, it is filtered, with the aid of a filter-pump, into a distillation flask, and the beautifully crystalline precipitate, which does not adhere to the beaker, is washed with ninety-five per cent alcohol. The filter, with the precipitate, is gently removed from the funnel and inverted into a platinum crucible, so that, by squeezing the point of the filter, the precipitate is made to fall into the crucible, and the paper can be pressed down smoothly upon it. On gentle heating of the crucible, the remaining alcohol burns off, and when the paper has been completely destroyed, the heat is raised to the full power of a bunsen for about five minutes. After cooling in a desiccator the crucible containing the calcium sulfate, is weighed. The filtration may also be accomplished on asbestos felt.
30. The Ammonium Oxalate Method.—This method has been extensively used in this country in commercial work, and is best carried out as described by Wyatt.[21] The total filtrates from the iron and alumina precipitates, secured as described in [paragraph 33], are well mixed and concentrated to a volume of about 100 cubic centimeters. There are added about twenty cubic centimeters of a saturated solution of ammonium oxalate, and after stirring, the mixture is allowed to cool and remain at rest for six hours. The supernatant liquid is poured through a filter, the residue washed three times by decantation with hot water and brought upon the filter. The beaker and precipitate are washed at least three times. The precipitate is dried and ignited at low redness for ten minutes. The temperature is then raised by a blast and the ignition continued for five minutes longer, or until the lime is obtained as oxid. The precipitate is likely to contain magnesia. The magnesia is estimated in the filtrates from the lime determination by first mixing them and concentrating to 100 cubic centimeters, which, after cooling, are made strongly alkaline with ammonia. After allowing to stand for twelve hours the ammonium magnesium phosphate is collected and reduced to magnesium pyrophosphate by the usual processes. If one gram of the original material has been used the pyrophosphate obtained, multiplied by 0.36, will give the weight of magnesia contained therein.
31. Lime Method of Immendorff.—The tedious processes required to determine the lime in the presence of iron, alumina, and large quantities of phosphoric acid are well known to analysts. Immendorff has published a method, accompanied by the necessary experimental data, based on the comparative insolubility of calcium oxalate in very dilute solution of hydrochloric acid. He has shown in the data given that the lime is all precipitated in the conditions named and that the precipitate, when properly prepared, is not contaminated with weighable amounts of the other substances found in the original solution[22]. The ease with which oxalic acid can be determined volumetrically with potassium permanganate solution aids greatly in the time-saving advantages of the process.
In a hydrochloric acid solution of a mineral phosphate an aliquot part of the filtrate representing about 250 milligrams of calcium oxid, usually about twenty-five cubic centimeters, should be taken for the analysis. Ammonia is added in slight excess and then the acid reaction restored with hydrochloric until shown plainly by litmus. The solution is then heated and the lime thrown down by adding a solution of ammonium oxalate in excess. In order to secure a greater dilution of the hydrochloric acid after the precipitation has been made, water should be added until the volume is half a liter. Before filtering, the whole should be cooled to room temperature. The precipitate should be washed first with cold and afterwards with warm water. The well-washed precipitate is dissolved in hot dilute sulfuric acid and the solution, while hot, titrated with a standard solution of potassium permanganate set by a solution of ammonio-ferrous sulfate.
If one cubic centimeter of the permanganate represent 0.005 gram of iron it will correspond almost exactly to 0.0035 gram of calcium oxid.
Example.—Sample of rather poor mineral phosphate, five grams in half a liter. Strength of potassium permanganate, one cubic centimeter equivalent to 0.00697 gram of iron and to 0.003484 gram of calcium oxid.
Twenty-five cubic centimeters of the solution, representing one quarter of a gram, in which the lime was precipitated as above described, required 9.6 cubic centimeters of the potassium permanganate to saturate the oxalic acid. Then
9.6 × 0.003484 = 0.0334464 gram,