WATER AND CITRATE-SOLUBLE PHOSPHORIC ACID 1 19 



The apparatus has proven extremely satisfactory in the pre- 

 cipitation of ammonium magnesium phosphate. The precipi- 

 tate is very crystalline, and where the stirring is continued for 

 some minutes, after the magnesia solution has all been added, 

 no amorphous precipitate is observed on longer standing. 



107. Precipitation of the Water and Citrate-Soluble Phosphoric 

 Acid. The importance of rapid precipitation with vigorous stir- 

 ring when the molybdate solution is employed has also been 

 pointed out by Ledoux and likewise the desirability of keeping 

 the temperature low (i6). 8T 



In the use of the aqueous and citrate extract of superphos- 

 phates the precipitation is conducted as follows : 



The aqueous or citrate extract, or both combined, is made up to 

 a volume of 250 cubic centimeters, or each is made up to that vol- 

 ume. A part of the solution representing 0.2 gram of the sample 

 is boiled with 15 cubic centimeters of strong nitric acid of 1.4 

 specific gravity for five minutes to convert all phosphoric acid in- 

 to the ortho type. After cooling, 15 cubic centimeters of am- 

 monia of 0.92 specific gravity is added, leaving the mixture 

 slightly acid, and then 100 cubic centimeters of the molybdic solu- 

 tion, prepared by dissolving 150 grams of molybdic acid in 600 

 cubic centimeters of ammonia of 0.96 density and pouring the 

 solution into 1070 cubic centimeters of nitric acid of 1.22 density. 

 The precipitation is made with vigorous stirring, best by a me- 

 chanical agitator, for 30 minutes at a temperature not exceed- 

 ing 1 6. The precipitate is perfectly pure and the phosphoric 

 acid may be determined by direct weighing, by titration or by 

 the gravimetric process. 



Many of the above precautions were previously pointed out 

 by Pellet who especially called attention in 1889 to tne volumet- 

 ric method of Thilo, afterwards developed by Pemberton and Kil- 

 gore. 88 



87 Bulletin de 1' Association beige des Chimistes, 1901, 15 : 125. 



88 Annales de Chimie analytique, 1900, 5 : 244; 1901, 6 : 248. 

 Bulletin de 1' Association des Chimistes de Sucrerie et de Distillerie, 



1893-94, 11 : 152; 1896-97, 14 : 423- 



Bulletin de 1'Association beige des Chimistes, 1888-89, 3 : 5 r > 73- 



