DIRECT DETERMINATION OF PHOSPHORIC ACID 63 



less than the first. 33 The difference in most cases is very little, 

 but it may become a quantity of considerable magnitude in sam- 

 ples where soluble silica is found in notable quantities. The dan- 

 ger of contamination with iron, alumina, and arsenic has already 

 been mentioned and the precautions suggested should be careful- 

 ly observed. 



72. Insolubility of Silica. It is evident that many of the errors 

 which are incident to the methods of separating phosphoric acid 

 ~by ammonia phosphomolybdate are due to the presence of silica. 

 The fact has been repeatedly pointed out by analysts. Pellet 

 proposes to render the silica insoluble and thus prevent the error by 

 the following procedure : 3 * The weighed phosphate is placed in a 

 platinum capsule and moistened with free hydrochloric acid. The 

 moistened mass is evaporated to dryness after which the silica 

 is no longer soluble in hot hydrochloric acid. Pellet claims that 

 this method, which saves the time of a previous solution and evap- 

 oration to dryness, is quite as effective as the longer method. 



73. Direct Determination of Available Phosphoric Acid. The 

 direct determination of available phosphoric acid is not new, being 

 official in several of the European countries. In this country, 

 however, it has not met with favor, probably because the citrate 

 method is not official here. The necessity of destroying the or- 

 ganic matter before precipitating with molybdate solution pre- 

 cludes the use of the molybdate method. 35 



In 1893 Ross presented a method for the direct determination 

 of the reverted phosphoric acid. 36 While the aim of this method 

 met with hearty approval from the official chemists, the method 

 itself did not, owing to some difficulties met with in the manipu- 

 lation, and more particularly to the fact that it did not give 

 results agreeing with the official method. 37 Agreement could 

 hardly be expected, because the method did not account for 

 the phosphoric acid removed in the water used in washing the 

 citrate-insoluble. The estimation of the available phosphoric acid 



33 Journal of the American Chemical Society, 1895, 17 =43- 



34 Annales de Chitnie analytique, 1906, 11 :33i. 



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



36 Division of Chemistry, Bulletin, 38, 1893 : 17. 



37 Division of Chemistry, Bulletin 43, 1894 : 72 and Bulletin 47, 1896: 81. 



