CITRATE-SOLUBLE PHOSPHORIC ACID IOJ 



stance has been recognized as essential to the execution of a 

 process involving the separation of the phosphoric acid as phos- 

 phomolybdate. 



It is evident from the data cited in the preceding paragraph, 

 that great accuracy may be secured in this process by adding a 

 sufficient quantity of a solution of a mineral phosphate and pro- 

 ceeding by the citrate method. 



Ross has also proposed to estimate the acid soluble in ammonium 

 citrate directly by first destroying the organic matter by moist 

 combustion with sulfuric acid. 76 He recommends the following 

 process : 



After completion of the 30 minutes' digestion of the sample 

 with citrate solution, 25 cubic centimeters are filtered at once into 

 a dry vessel. If the liquid be filtered directly into a dry burette, 

 25 cubic centimeters can be readily transferred to another vessel 

 without dilution. After cooling, run 25 cubic centimeters of the 

 solution into a digestion flask of 250-300 cubic centimeters capac- 

 ity, add about 15 cubic centimeters of concentrated sulfuric acid 

 and place the flask on a piece of wire gauze over a moderately 

 brisk flame ; in about eight minutes the contents of the flask com- 

 mence to darken and foaming begins ; but this will occasion no 

 trouble, if an extremely high or a very low flame be avoided. 

 In about 12 minutes the foaming ceases and the liquid in the 

 flask appears quite black ; about one gram of mercuric oxid is now 

 added and the digestion is continued over a brisk flame. The 

 operation can be completed in less than half an hour with ease, 

 and in many cases, 25 minutes. After cooling, the contents of the 

 flask are washed into a beaker, ammonia is added in slight excess, 

 the solution is acidified with nitric acid, and after the addition of 

 15 grams of ammonium nitrate, the process is conducted as 

 usual. 



In case as large an aliquot as 50 cubic centimeters of the 

 original filtrate be used, 10 cubic centimeters of sulfuric acid are 

 added, and the digestion is conducted in a flask of 300-500 cubic 

 centimeters capacity ; after the liquid has blackened and foam- 

 ing has progressed to a considerable extent, the flask is removed 

 76 Division of Chemistry, Bulletin 38, 1893 : 16. 



