CITRATE AND MOLYBDATB METHODS 103 



parison with the ordinary molybdate methods, with the result 

 that in 67 determinations on bone-dust, superphosphate, 

 cotton-hull ashes, cottonseed-meal, tankage, bone-char, phosphatic 

 guano, and phosphate rock, only three citrate results differed from 

 those obtained by the molybdate method by more than three- 

 tenths of one per cent. The greatest discrepancy between the two 

 methods was 0.41 per cent., and the average difference was 0.09 

 per cent. 



Attention has already been called to the fact that the citrate 

 method was found to give poor results when iron and alumina 

 were present in considerable quantity. Ignited precipitates by the 

 citrate method were found to contain as high as four per cent, of 

 lime, and iron and alumina in small quantities when these bodies 

 were abundant in the original substance. 



In the molybdate method the rapid precipitation from solutions 

 at 65 was found to give unsatisfactory results and it was found 

 necessary to conduct the process at temperatures between 40 and 

 50. With a relative excess of nitric or a relative deficiency of 

 molybdic acid some phosphoric acid may easily escape precipita- 

 tion. The chief objection to precipitating at 65 is found in the 

 fact that in presence of considerable iron and alumina some of 

 these bodies may be found in the yellow precipitate, whence they 

 pass to the final ammonium magnesium phosphate. 



The citrate method, therefore, only gives safe results by com- 

 pensating errors which in every class of phosphates must be em- 

 pirically determined. 



The molybdate method gives results too high when iron and 

 alumina are present in considerable quantity and the yellow pre- 

 cipitate is obtained at temperatures above 50. On the other hand, 

 if there be a great relative excess of nitric acid the results may 

 be too low unless the filtrates from the yellow precipitate be mixed 

 with additional molybdic solution and digested until no further 

 precipitate is formed. 



Comparative determinations made by both methods by Maercker 

 for the Association of German Experiment Stations have led to 

 the conclusion that both give practically the same results when 



