To the hot solutions, for every decigram of phosphorus pentoxid that is present, add fifty cubic centimeters of molybdic solution. Digest at about 65° for an hour, filter, and wash with water or ammonium nitrate solution. Test the filtrate by renewed digestion and the addition of more molybdic solution. Dissolve the precipitate on the filter with ammonia and hot water and wash into a beaker to a bulk of not more than 100 cubic centimeters. Nearly neutralize with hydrochloric acid, cool, and add magnesia mixture from a burette; add slowly (about one drop per second), stirring vigorously. After fifteen minutes add thirty cubic centimeters of ammonia solution of 0.95 density. Let stand for some time; two hours are usually enough. Filter, wash with dilute ammonia, ignite gently at first and then at white heat for ten minutes, and weigh. For the quantity of magnesia mixture to be added see paragraph =21=. =52. Citrate-Soluble Phosphoric Acid.=—The sum of the water-soluble and citrate-insoluble subtracted from the total gives the citrate-soluble phosphoric acid. =53. Preparation of Reagents.=—(1) _Ammonium Citrate Solution._—(_a_) Mix 370 grams of commercial citric acid with 1,500 cubic centimeters of water, nearly neutralize with commercial ammonia, cool, add ammonia until exactly neutral (testing with saturated alcoholic solution of corallin) and bring to a volume of two liters. Test the specific gravity, which should be 1.09 at 20°, before using. (_b_) _Alternate Method._—To 370 grams of commercial citric acid add commercial ammonia, of 0.96 specific gravity, until nearly neutral; reduce the specific gravity to nearly 1.09 and proceed as follows: Prepare a solution of fused calcium chlorid 200 grams to the liter, and add four volumes of strong alcohol. Make the mixture exactly neutral, using a small amount of freshly prepared corallin solution as a preliminary indicator, and test finally by withdrawing a portion, diluting with an equal volume of water, and testing with cochineal solution. Fifty cubic centimeters of this solution will precipitate the citric acid from ten cubic centimeters of the citrate solution. To ten cubic centimeters of the nearly neutral citrate solution add fifty cubic centimeters of the alcoholic calcium chlorid solution, stir well, filter at once through a folded filter, dilute with an equal volume of water, and test the reaction with neutral solution of cochineal. If acid or alkaline, add ammonia or citric acid, as the case may be, to the citrate solution, mix, and test again as before. Repeat this process until a neutral reaction of the citrate solution is obtained. At the end the specific gravity must be 1.09 at 20°. (2) _Molybdic Solution._—See paragraph =22=. (3) _Ammonium Nitrate Solution._—Dissolve 200 grams of commercial ammonium nitrate in water and bring to a volume of two liters. (4) _Magnesia Mixture._—See paragraph =22=. (5) _Dilute Ammonia for Washing._—See paragraph =22=.