DIRECT DETERMINATION OF PHOSPHORIC ACID 65 



tates are allowed to stand over night before filtering. The de- 

 terminations are completed as usual. 



The results by the citrate method were unexpectedly low. In 

 Veitch's hands this method has always given satisfactory results, 

 even on low percentages. It is probable the low results are due 

 to an excess of citrate. This addition is unnecessary, and bet- 

 ter results obtained when more citrate is not added, lead to the 

 belief that this additional citrate is the cause of the low results. 



The results by the molybdate method are good. It was feared 

 that the organic matter present would prevent the complete pre- 

 cipitation of the ammonium phosphomolybdate. To insure com- 

 plete precipitation the samples were allowed to stand over night 

 before filtering. 



Notwithstanding the oft-repeated statement that salts of or- 

 ganic acids and organic matter generally prevent the complete pre- 

 cipitation of ammonium phosphomolybdate, the molybdate method 

 is used to determine soluble phosphoric acid in the presence of 

 what organic matter may be dissolved by the water used in the 

 extraction. In the Wagner method for basic slag, the precipi- 

 tation is accomplished with molybdate solution in the presence of 

 three grams of citric acid. Lorenz precipitates in the presence 

 of two per cent, of citric acid to prevent contamination with mag- 

 nesia. Jiiptner uses as much as 100 grams of tartaric acid per 

 liter of molybdate solution to prevent the precipitation of iron 

 and the separation of molybdic acid. 40 The successful use of the 

 molybdate method in these cases seems to warrant the conclusion 

 that we are needlessly alarmed at the presence of, at least, some 

 forms of organic matter in phosphate solutions. 



The direct determination of the available phosphoric acid pos- 

 sesses several advantages. Only one determination is required 

 instead of two as by the present method. The probable error is 

 reduced one-half. The soluble, reverted, insoluble, and total phos- 

 phoric acid can also be determined in one sample and with one 

 weighing, where it now takes two samples and two weighings. 



The saving of time effected by this method is of considerable 

 40 Abstract, Experiment Station Record, 1894-5, 6 : 610. 

 3 



