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]. Two grams are weighed into a tared platinum crucible. This, with its lid, is placed in an air-bath at 105°, and heated for at least three hours. The lid is then put on, and the crucible is placed in a desiccator and weighed as soon as cold. The loss in weight is the moisture.