THE PROCESS 157 



32 cubic centimeters of standard alkali to neutralize the excess 

 of acid. 



The alkali consumed by the yellow precipitate represents 50 32 

 = 18 cubic centimeters. The sample, therefore, contains 18 per 

 cent, of phosphoric acid. 



Comparison with Official Method. A comparison of the Pem- 

 berton volumetric with the official method of the Association of 

 Official Agricultural Chemists has been made by Day and Bry- 

 ant. 16 The comparisons were made on samples containing from 

 1.45 to 37.28 per cent, of phosphoric acid and resulted as follows: 



Percent. Percent. 



Substance. PjO 5 , Official. PO 6 , Pemberton. 



No. i. Florida rock 1.45 1.32 



"2. " " 440 4-53 



" 3. Sodium phosphate 19.78 *9-99 



" 4- " " I9-72 19-73 



" 5. Florida rock 37-28 37-22 



This near agreement shows the reliability of the method. The 

 comparison of the Pemberton volumetric method with the official 

 gravimetric method was investigated by Kilgore, reporter of the 

 Association of Official Agricultural Chemists, in i894. 17 The 

 individual variations were found to be greater than in the regular 

 method, but the average results were nearly identical therewith. 

 The method works far better with small percentages of phosphoric 

 acid than with large. Where the average of the results by the 

 official methods gave 12.25 P er cent., the volumetric process gave 

 11.90 per cent., whereas in the determination of a smaller per- 

 centage the results were 2.72 and 2.73 per cent., respectively. 

 Kilgore proposes a variation of the method which differs from 

 the original in two principal points. 18 First, the temperature of 

 precipitation in the Pemberton process is 100 ; but in the modi- 

 fied form from 55 to 60. At the higher temperature there is 

 danger of depositing molybdic acid. 



The second difference is in the composition of the molybdate 

 solution employed. The official molybdate solution contains about 



16 Journal of the American Chemical Society, 1894, 16 : 282. 



17 Division of Chemistry, Bulletin 43, 1894 : 68. 



18 Division of Chemistry, Bulletin 43, 1894 : 91. 



