184 Separation and Estimation of Phosphoric Acid. 



This residue consists of phosphate and nitrate of suhoxyd of mer- 

 cury and of metallic mercury. The filter with the mercurial 

 salts is to be thoroughly dried ; the mass is to be carefully separa- 

 ted from the filter, brought into a platinum crucible, and mixed 

 with an excess of carbonate of soda, or still better, with a mix- 

 ture of equal equivalents of the carbonates of soda and potash, as 

 this is more easily fused. The filter is to be rolled up into a ball, 

 laid in a cavity in the mixture in the crucible, and covered with 

 a layer of the carbonates. The whole is then to be moderately 

 heated under a flue not however to ignition, the contents of the 

 crucible not being allowed to fuse and the heat being continued 

 about half an hour. In this manner the metallic mercury and 

 the mercurial salts, with the exception of the phosphate of the 

 suboxyd, are volatilized. The crucible is now to be subjected to 

 the strongest heat of a spirit lamp with a double current of air 

 and strong alcohol ; the fused mass is to be treated with hot 

 water, in which it should be perfectly soluble if the operation has 

 been carefully conducted and if no iron were present in the 

 original compound. The solution is to be supersaturated with 

 chlorohydric acid and the phosphoric acid determined by means 

 of ammonia, sulphate of magnesia and sal-ammoniac. 



"By the method described," says Rose, "all phosphoric acid 

 compounds may be so completely decomposed that, on the one 

 hand, all bases may be obtained free from every trace of phos- 

 phoric acid, on the other hand, the phosphoric acid is perfectly 

 separated from all the bases with which it was united, which can- 

 not be effected by any other method." 



The following precautions must be carefully observed in the 

 analysis of phosphates by this process. So much metallic mer- 

 cury must be added to the nitric acid solution, that a distinctly 

 recognizable quantity of the metal remains in the dry evaporated 

 mass. The evaporation must be effected in a water bath and 

 not by the direct application of heat, as in this case portions of 

 the nitrates formed might be decomposed. The nitrates of the 

 stronger bases are not decomposed under the temperature of boil- 

 ing water. If however alumina or peroxyd of iron be present, 

 the process must be slightly modified as the nitrates of these 

 bases are partially decomposed during the evaporation, and the 

 bases themselves remain upon the filter with the salts of suboxyd 

 of mercury. When iron alone is present, the subsequent fusion 

 with the alkaline carbonate effects its perfect separation, if how- 

 ever alumina be also contained in the mass upon the filter, it will 

 be necessary during the fusion with the carbonates to add silica 

 in the manner recommended by Berzelius. If the heating in the 

 platinum crucible previous to its ignition has not been carefully 

 conducted, oxyd of platinum will be mixed with the oxyd of iron 

 and may be separated from this by dissolving the mass in chloro- 



