THE METHOD OF GRAFTIAU 1 27 



The dried precipitate is cooled in a desiccator and weighed, cov- 

 ered with a watch glass, replaced upon the burner, converted with 

 a stronger heat into the phosphomolybdate anhydrid, and cooled 

 and weighed as before. The weight of this precipitate multiplied 

 by 0.03946 gives the quantity of phosphoric anhydrid (P 2 O 5 ). 



This method was applied to superphosphates and basic slags. 

 In the case of basic slags it was found that the results by the fore- 

 going method were accurate, even without the removal of the 

 silicic acid. In the illustrations given it appears that after the re- 

 moval of the silicic acid there was found 19.30 per cent, of 

 P 2 O 5 by the direct gravimetric method, while by the weighing of 

 the phosphomolybdate precipitate without the separation of the 

 silica in two cases the results were 19.35 per cent. P 2 O 5 and 19.26 

 per cent. P 2 O 5 . 



In a determination of the phosphoric acid soluble in a citrate 

 solution of basic slag by the direct method, there was found 14.7 

 per cent, of P 2 O 5 . In the estimation of the same acid after the 

 removal of the silicic acid there was found 14.42 per cent, of 

 P^Os by the direct method. Without the removal of the silicic 

 acid by direct weighing of the phosphomolybdate precipitate, there 

 was found 14.43 P er cent, of P 2 O 5 . 



The general conclusions of the investigation are : 



1. The methods tried for the estimation of the phosphoric acid 

 in fertilizers, depending upon the simplification of the method of 

 direct precipitation of the phosphoric acid as MgNH 4 PO 4 , give al- 

 most constantly too high results. 



2. The estimation of phosphoric acid as 24MoO 3 P 2 O 5 , accord- 

 ing to the method of P. Neumann, gives without exception, very 

 exact results, and the use of the different solvents for the phos- 

 phoric acid in these fertilizers, as well as the presence of dissolved 

 silicic acid in the hydrochloric acid and citrate solutions, are 

 without influence on the accuracy of the results. 



Neumann's method is at least more simple and as readily ap- 

 plicable as the common method for estimating phosphoric acid as 

 Mg 2 P 2 7 . 



114. The Method of Graftiau. Graftiau has proposed some 

 slight modifications of the method of determining phosphoric 



