



Separation and Estimation of Phosphoric Acid. 183 



is to be evaporated to perfect dryness in a water bath. If the 

 mass still smell of free acid, water is to be added and the solu- 

 tion is again to be evaporated to dryness. This process must be 

 repeated if necessary till the dry mass no longer smells of nitric 

 acid. Either hot or cold water may then be added, and the whole 

 may be filtered, care being taken to use the smallest possible fil- 

 ter. The mass upon the filter is then to be washed out, till a 

 few drops of the filtrate evaporated upon platina foil no longer 

 leave a residue after igniting the foil. The filtrate now contains 



& u " uj & 



the bases united to nitric acid, together with much nitrate of sub- 

 ox yd of mercury and a small portion of nitrate of the protoxyd. 

 Chlorohydric acid is to be added and the precipitate if abundant 

 is to be filtered oft'. It consists of Hg 2 CI and should leave no 

 residue after ignition. If however the precipitate obtained by 

 muriatic acid be insignificant, ammonia may be directly added 

 and the partly black and the partly white precipitate obtained 

 must then be rapidly filtered and protected as much as possible 

 from the air during the washing, in order that no carbonate of 

 lime may be formed. This method of getting rid of the mercury 

 can be employed only when there are no bases present which are 

 precipitated by ammonia. Chlorid of ammonium may be added 

 to prevent the precipitation of magnesia. If the decomposition 

 of the phosphates has been complete, the precipitate upon the fil- 

 ter will be entirely volatilized by heat or leave at best a few mil- 

 ligrammes of oxyd of iron. If however this be not the case the 

 process has not been carefully conducted or the whole of the 

 excess of nitric acid has not been driven off. The residue is 

 then to be farther examined; if it consists of earthy phosphates, 

 these may again be decomposed by nitric acid and mercury; if 

 of earthy carbonates or of oxyd of iron, these may be dissolved 

 in chlorohydric acid and added to the filtrate obtained by am- 

 ttionia. From this filtrate the bases may then be separated in the 

 usual manner. 



Instead of separating the mercury from the solution of the 

 bases by means of ammonia, it is frequently more simple and ad- 

 vantageous to evaporate the whole in a platinum capsule and then 

 to ignite the residue in a platinum crucible. In this manner we 

 avoid the necessity of subsequently driving off the large mass of 

 ammoniacal salts formed in the process first described. If how- 

 ever nitrates of the alkalies be present as well as those of the alka- 

 line earths, small quantities of solid carbonate of ammonia must 

 be added from time to time during the process of ignition to con- 

 otherwise 

 residue is 



vert the decomposed alkaline nitrates into carbonates, 



the platinum crucible will be attacked. The ignited 



to be dissolved in chlorohydric acid and the bases separated from 



each other in the usual order. It now remains to determine the 



quantity of phosphoric acid in the residue insoluble in water. 



% 



