62 AGRICULTURAL ANALYSIS 



with water, precisely in the way above described, except that in 

 case the substance contains much animal matter (bone, fish, etc.), 

 the residue insoluble in ammonium citrate is to be treated by one 

 of the processes described under i, 2, or 3, paragraph 61. 



69. Citrate-Soluble Phosphoric Acid. The sum of the water- 

 soluble and citrate-insoluble subtracted from the total gives the 

 citrate-soluble phosphoric acid. 



70. Time Required for the Precipitation of Phosphoric Acid. 

 The length of time required for the complete precipitation of 



the phosphoric acid by molybdate mixture is perhaps much less 

 than generally supposed. At 65 the precipitation, as shown by 

 de Roode, is complete in five minutes. 32 In a given case the 

 weight of pyrophosphate obtained after five minutes was 0.0676 

 gram, and exactly the same weight was found after 24 

 hours. In view of these facts analysts would often be able to 

 save time by omitting the delay usually demanded by the set- 

 ting aside of the yellow precipitate for a few hours in order to 

 secure a complete separation of the phosphoric acid. In the 

 method of the official chemists it is directed that the digestion at 

 65 be continued for one hour, and this time may possibly be 

 shortened with advantage. In all cases, however, where there 

 is any doubt in regard to the complete separation, some of the 

 molybdate solution should be added to the filtrate and, with 

 renewed digestion, it should be noted whether any additional 

 precipitate be formed. 



71. Examination of the Pyrophosphate. In fertilizer control 

 it is not usually thought necessary to examine the magnesium 

 pyrophosphate for impurities. Among those most likely to be' 

 found is silica. It is proper, in all cases where accuracy is re- 

 quired, to dissolve the precipitate in nitric acid, boil for some 

 time to convert the pyro- into orthophosphate, and reprecipitate- 

 with molybdate and magnesia mixture. This treatment will sep- 

 arate the silica, which remains practically insoluble after the first 

 ignition. It has been observed by some analysts that the results 

 obtained by the official method are a trifle too high and also that 

 on re-solution the second precipitate of pyrophosphate weighs. 



32 Journal of the American Chemical Society, 1895, 17 : 43. 



