106 AGRICULTURAL ANALYSIS 



of the sample were mixed with an equal volume of a mineral 

 phosphate solution, the value of which had been previously 

 determined by both the molybdate and citrate methods. The 50 

 cubic centimeters thus obtained represented a quarter of a gram 

 each of the cake-meal and mineral phosphates. The filtration 

 followed 1 8 hours after adding the magnesia mixture. The 

 following data show the results of the determinations : 



content of P 2 O 5 in organic sample 2.53 



It is thus demonstrated that the citrate method can be applied 

 with safety even to the determination of the phosphoric acid in 

 organic compounds where the quantity present is less than three 

 per cent. It is further shown that solutions of mineral phosphates 

 varying in content of phosphoric acid from 15 to 32 per cent, 

 may be safely used for increasing the content of that acid to 

 the proper degree for complete precipitation. In cases where 

 organic matters are present they should be destroyed by moist 

 combustion with sulfuric acid, as in the determination of nitrogen 

 to be described in the next part. 



98. Direct Precipitation of the Citrate-Soluble Phosphoric Acid. 

 The direct determination of the citrate-soluble phosphoric acid 

 by effecting the precipitation by means of magnesia mixture in 

 the solution obtained from the ammonium citrate digestion, has 

 been practiced for many years by numbers of European chemists, 

 and the process has even obtained a place in the official methods 

 of some European countries. Various objections have been urged, 

 however, against the general employment of this method in fer- 

 tilizer analysis on account of the inaccuracies in the results ob- 

 tained in certain cases, and it has, therefore, been used to but a 

 very limited extent in this country. Since it is impracticable to 

 effect the precipitation with ammonium molybdate in the presence 

 of citric acid the previous elimination or destruction of this sub- 



