154 AGRICULTURAL ANALYSIS 



has cast doubt on the methods of determination based on its 

 weight. Its most probable composition is expressed by the fol- 

 lowing formula, (NH 4 ) 3 PO 4 (MoO 3 ) 12 . For convenience in 

 writing reactions this formula should usually be doubled. Pem- 

 berton has described a volumetric determination of phosphoric acid 

 in the yellow precipitate which is easily conducted and is very 

 rapid. 13 



The method as originally proposed by Pemberton has not al- 

 ways given satisfactory results when compared with the molyb- 

 date gravimetric process, but as perfected by experience has con- 

 stantly grown in favor until now it is regarded as entirely trust- 

 worthy on account of its original merit and of the extended use 

 in later forms. It has, however, attracted so much attention from 

 analysts as to the principles of the original process, that they are 

 given in some detail. 



141. The Process. The principle of the process is based on the 

 separation of the phosphoric acid as ammonium-phosphomolyb- 

 date, freeing the yellow precipitate from any free nitric acid by 

 washing, dissolving the yellow precipitate in an excess of standard 

 alkali and titrating the residual alkali by a standard acid. The 

 process as originally described by Pemberton is carried out as 

 follows : One gram of phosphate rock, or from two to three 

 grams of phosphatic fertilizer, are dissolved in nitric acid and, 

 without evaporation, diluted to 250 cubic centimeters. Without 

 filtering, 25 cubic centimeters are placed in a four-ounce beaker 

 and ammonia added until a slight precipitate begins to form. 

 Five cubic centimeters of nitric acid of one and four-tenths 

 specific gravity are added, and 10 cubic centimeters of saturated 

 solution of ammonium nitrate and enough water to make the 

 volume about 65 cubic centimeters . The contents of the 

 beaker are boiled, and while still hot five cubic centimeters of the 

 aqueous solution of ammonium molybdate added. Additional 

 quantities of the molybdate are used, if necessary, until the 

 whole of the phosphorus pentoxid is thrown out. 



After allowing to settle for a moment the contents of the beaker 

 are poured upon a filter seven centimeters in diameter. The pre- 

 u Journal of the American Chemical Society, 1893, 15 : 382. 



