20. Direct Estimation of the Phosphoric Acid.—It often happens, in the case of a mineral phosphate, that the only determination desired is of the phosphoric acid. In this instance the analyst may proceed as follows: If the qualitative test shows the usual amount of phosphoric acid, two grams of the sample passed through a sieve, with a millimeter mesh, are placed in a beaker and thoroughly moistened with water. The addition of water is to secure an even action of the hydrochloric acid on the carbonates present. The beaker is covered with a watch-glass and a little hydrochloric acid is added from time to time until all effervescence has ceased. There are then added about thirty cubic centimeters of aqua regia and the mixture raised to the boiling-point on a sand-bath or over a lamp. The heating is continued until chlorin is no longer given off and solution is complete. The volume of the solution is then made up to 200 cubic centimeters without filtering, filtered, and an aliquot part of the filtrate, usually fifty cubic centimeters, representing half a gram of the original sample, taken for the determination of the phosphoric acid according to the method of the Official Agricultural Chemists. The small quantity of insoluble material does not introduce any appreciable error into the process when the volume is made up to 200 or 250 cubic centimeters.
21. Method of the Official Agricultural Chemists for Total Phosphoric Acid.—To the hot solution, for every decigram of phosphorus pentoxid which may be present, add fifty cubic centimeters of the molybdic solution. Digest at 65° for an hour, filter, and wash with water or ammonium nitrate solution[11]. Test the filtrate by renewed digestion with additional molybdate reagent. Dissolve the precipitate on the filter with ammonia in hot water and wash into a beaker, making the volume of filtrate and washings not more than 100 cubic centimeters. Nearly neutralize with hydrochloric acid, cool, and add magnesia mixture from a burette at the rate of about one drop a second, stirring vigorously, meanwhile. The quantity of magnesia mixture to be added is not prescribed in the official method but it should always be in excess of the amount necessary for complete precipitation. For each decigram of phosphorus pentoxid, from eight to ten cubic centimeters should be used. Fifteen minutes after the last of the magnesia mixture has been stirred in, thirty cubic centimeters of ammonia of 0.95 specific gravity are added and the beaker set aside for two hours or longer. The ammonium magnesium phosphate is separated by filtration, dried, ignited gently at first, and finally over a blast-lamp and weighed as magnesium pyrophosphate. The factors for calculating the phosphorus pentoxid and tricalcium phosphate from the weight of pyrophosphate are given below on the two bases; viz., hydrogen equals 1, and oxygen equals 16.
H = 1.
Mg₂P₂O₇ × 0.63976 = P₂O₅
Mg₂P₂O₇ × 1.3964 = Ca₃(PO₄)₂
P₂O₅ × 2.1827 = Ca₃(PO₄)₂
O = 16.
Mg₂P₂O₇ × 0.63792 = P₂O₅
Mg₂P₂O₇ × 1.3926 = Ca₃(PO₄)₂
P₂O₅ × 2.1831 = Ca₃(PO₄)₂
22. Preparation of Solutions.—Molybdic Solution.—Dissolve 100 grams of molybdic acid in 400 grams or 417 cubic centimeters of ammonia, of 0.96 specific gravity, and pour the solution thus obtained into 1,500 grams or 1,250 cubic centimeters of nitric acid, of 1.20 specific gravity. Keep the mixture in a warm place for several days, or until a portion heated to 40° deposits no yellow precipitate of ammonium phosphomolybdate. Decant the solution from any sediment and preserve in glass-stoppered vessels.
Magnesia Mixture.—Dissolve twenty-two grams of recently ignited calcined magnesia in dilute hydrochloric acid, avoiding an excess of the latter. Add a little calcined magnesia in excess, and boil a few minutes to precipitate iron, alumina, and phosphoric acid; filter, add 280 grams of ammonium chlorid, 700 cubic centimeters of ammonia of specific gravity 0.96, and water enough to make a volume of two liters. Instead of the solution of twenty-two grams of calcined magnesia, 110 grams of crystallized magnesium chlorid may be used.
Dilute Ammonia for Washing.—One volume of ammonia, of 0.96 specific gravity, mixed with three volumes of water, or usually one volume of concentrated ammonia with six volumes of water.
23. Use of Tartaric Acid in Phosphoric Acid Estimation.—In the presence of iron the molybdate mixture is likely to carry down some ferric oxid with the yellow precipitate. To prevent this, and also hinder the separation of molybdic acid in the solution on long standing, tartaric acid has been recommended.
Jüptner has found that the presence of tartaric acid does not interfere with the separation of the yellow precipitate, as some authorities assert.[12] Even 100 grams of the acid in one liter of molybdate solution produce no disturbing effect. Molybdate solution treated with tartaric acid did not show any separation of molybdic acid when kept for a year at room temperatures. The presence of tartaric acid, therefore, is highly recommended by him to prevent the danger of obtaining both ferric oxid and molybdic acid with the yellow precipitate.
24. Water and Organic Matters.—The sample, according to the practice of Chatard, should be ground fine enough to leave no residue on an eighty mesh sieve, and should be thoroughly mixed by passing it three times through a forty mesh sieve[13].