OFFICIAL METHOD FOR TOTAL PHOSPHORIC ACID 59 



calcium phosphate from the weight of pyrophosphate are given be- 

 low on the two bases, viz., hydrogen equals i, and oxygen 

 equals 16. 



H=i. 



Mg 2 P 2 7 Xo.63756=P 2 5 

 Mg 2 P 2 7 Xi.39i8=Ca 3 (P0 4 ) 2 

 P 2 5 X2.i8 3 i=Ca 3 (P0 4 ) 2 



O=id 



Mg 2 P 2 7 Xo.6 3 757=P 2 5 

 Mg 2 P 2 7 Xi.3932=Ca 3 (P0 4 ) 2 

 P 2 5 X2.i852=Ca 3 (P0 4 ) 2 



64. Official Method for Total Phosphoric Acid. Having now 

 described the approved methods of bringing into solution all the 

 phosphorus in the form of phosphoric acid, the next step is to 

 separate this acid and bring it into a homogeneous compound in 

 which it may be titrated or weighed. The usual method of 

 separation depends" on the property possessed by phosphoric acid 

 of forming in a strongly acid solution, which prevents the pre- 

 cipitation of the associated bodies, an insoluble compound with 

 molybdic acid. The separation is accomplished as follows : 29 

 Determination. Neutralize an aliquot portion of the solution 

 prepared as above, corresponding to 0.25 gram, 0.50 gram, 

 or one gram, with ammonia, and clear with a few drops of nitric 

 acid. In case hydrochloric or sulfuric acid has been used as 

 solvent, add about 15 grams of dry ammonium nitrate or a solu- 

 tion containing that amount. To the hot solution add 50 cubic 

 centimeters of molybdic solution for every decigram of P 2 O 3 

 that is present. Digest at about 65 for an hour, filter, and wash 

 with cold water, or preferably ammonium nitrate solution. Test 

 the filtrate for phosphoric acid by renewed digestion and 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 bur- 

 ette; add slowly (about one drop per second), stirring vigorously. 

 After 15 minutes add 12 cubic centimeters of ammonia solu- 

 29 Bureau of Chemistry, Bulletin 107, 1907 : 3. 



