64 AGRICULTURAL ANALYSIS 



consisted in the determination of the water-soluble phosphoric acid 

 by the volumetric method, as modified and carried out by Veitch ; 3S 

 the direct determination of the citrate-soluble by the citrate method 

 in 50 cubic centimeters of the citrate filtrate, and the determina- 

 tion of that removed by washing the citrate-insoluble residue,, 

 using the modified volumetric method. The sum of these three 

 results should equal the available phosphoric acid by the official 

 method. 



It is perhaps sufficient to say that the citrate method at that 

 time and later gave satisfactory results. 39 



The two methods gave practically the same results on availables 

 and on totals. The work also shows very plainly why the Ros& 

 method differs from the official, from 0.09 per cent, to 1.48 per 

 cent, being removed and accounted for in the wash water of the 

 official method that could not be accounted for by the Ross method. 

 Of course, the amount removed by the wash water will vary 

 somewhat in the hands of different analysts, according as they 

 wash the citrate-insoluble much or little. It is the practice of 

 Veitch to wash until the filtrate and washings amount to about 

 250 cubic centimeters. 



A comparison of the official method with the citrate and the 

 molybdate methods, precipitating with magnesia mixture and with 

 molybdate solution, respectively, in the mixed filtrates con- 

 taining the water-soluble and the citrate-soluble, was undertaken. 



The method finally adopted is as follows: The water-soluble 

 extracted as usual, is received in a 500 cubic centimeter flask, 

 graduated roughly at 250 cubic centimeters and containing from 

 five to 10 cubic centimeters nitric acid. The citrate-soluble is then 

 extracted as usual and the filtrate and washings received in the 

 flask" with the water-soluble. After cooling, the volume is com- 

 pleted, shaken, filtered, and in aliquots of 100 cubic centimeters 

 the phosphoric acid is determined by one of two methods, the 

 molybdate or citrate, the precipitants being added directly to the 

 solution without destroying the organic matter, and the precipi- 



38 Journal of the American Chemical Society, 1896, 1 8 : 389. 

 w Division of Chemistry, Bulletin 49, 1897 : 61. 



