50 AGRICULTURAL ANALYSIS 



of considerable difficulty, and it would be vain to suppose that the 

 best possible methods are now known. Especially is this the case 

 with the processes which relate to the estimation of the fluorin, 

 silica, iron, alumina, and lime. The phosphoric acid, however, 

 which is the chief constituent from a commercial point of view, 

 it is believed, can now be determined with a high degree of pre- 

 cision. Often the estimation of some of the less important con- 

 stituents is of great interest in determining the origin of the de- 

 posits, especially in the case of fluorin. While the merchant is 

 content with knowing the percentage of phosphoric acid and the 

 manufacturer asks in addition only some knowledge of the quan- 

 tity of iron, alumina, and lime, the analyst in most cases is only 

 content with a complete knowledge of the constitution of the 

 sample at his disposal. 



56. Dissolving the Phosphoric Acid. It often happens, in the 

 case of a mineral phosphate, that the only determination desired 

 is of the phosphoric acid. In such a case the analyst may pro- 

 ceed as follows: If the qualitative test shows the usual amount 

 of phosphoric acid, two grams of the sample passed through a 

 sieve, with a millimeter, or, better, a half millimeter mesh, are 

 placed in a beaker and thoroughly moistened with water. The 

 addition of water is to secure an even action of the hydrochloric 

 acid on the carbonates present. The beaker is covered with a 

 watch-glass and a little hydrochloric acid is added from time to 

 time until all effervescence has ceased. There are then added 

 about 30 cubic centimeters of aqua regia and the mixture is raised 

 to the boiling-point on a sand-bath or over a lamp. The heating 

 is continued until chlorin is no longer given off and solution is 

 complete. The volume of the solution is then made up to 200 

 cubic centimeters without filtering, filtered, and an aliquot part 

 of the filtrate, usually 50 cubic centimeters, representing half a 

 gram of the original sample, used for the determination of the 

 phosphoric acid according to some one of the accredited methods. 

 The small quantity of insoluble material from phosphates of the 

 usual composition does not introduce any appreciable error into 

 the process when the volume is made up to 200 or 250 cubic 

 centimeters. 



