120 AGRICULTURAL ANALYSIS 



108. Veitch's Method for Available Phosphoric Acid. The gen- 

 eral acceptance of the term "available acid'' as including both the 

 quantity soluble in water and afterwards the additional quantity 

 soluble in ammonium citrate solution has led to the suggestion 

 that a single determination of the total amount of the dissolved 

 acid is, for practical purposes, fully as valuable as the determina- 

 tion of the two extracts separately. The usual objection to the 

 direct determination of the citrate-soluble acid with the preliminary 

 separation with molybdate solution has been the supposed difficul- 

 ty of precipitating the phosphoric acid in the presence of a large 

 quantity of organic matter, viz., the excess of the citrate used in 

 extracting the phosphoric acid. Experience has shown that ac- 

 curate separation can be secured, even in the presence of this 

 form of organic matter. This fact led Veitch to combine the 

 two extracts and determine the so-called available acid in one 

 operation. This process is as follows : so 



The two extracts, viz., with water and with ammonium citrate, 

 are placed in a 500 cubic centimeter flask with 10 cubic centi- 

 meters of nitric acid and the volume completed to the mark. The 

 phosphoric acid is determined in aliquots of 100 cubic centimeters, 

 whether by the molybdate or direct citrate of magnesia method. 

 The precipitates are allowed to stand 18 hours before filtering. 



Comparisons with the official method with many varieties of 

 phosphate fertilizers containing from five to 15 per cent, of avail- 

 able acid, show close agreement when the molybdate separation 

 is used, while by direct precipitation with citrate of magnesia, 

 the results are somewhat lower. 



The method therefore possesses certain advantages. Only one 

 determination is required instead of two, and ths probable error 

 in manipulation is reduced one-half. 



109. Availability of Phosphatic Fertilizers. There is perhaps 

 no one question more frequently put to analysts by practical 

 farmers than the one relating to the availability of fertilizing 

 materials. The object of the manufacturer should be to secure 

 each of the valuable ingredients of his goods in the most useful 

 form. The ideal form in which phosphoric acid should come 



89 Journal of the American Chemical Society, 1899, 21 : 1090. 



