212 ANALYTICAL CHEMISTRY. 



taining free nitric acid, which latter compound oxidizes the 

 hydrosulphuric acid. 



Separation of the metals of the arsenic from those of the lead group. 

 The precipitate produced by hydrosulphuric acid in acid solu- 

 tion contains the metals of the arsenic and lead groups. They 

 are separated by means of ammonium sulphide, which dissolves 

 the sulphides of the arsenic group, but does not act on those of 

 the lead group. 



Addition of ammonium sulphide. This reagent should never 

 be added to the acid solution, but the solution should be 

 previously supersaturated by ammonium hydrate, as, other- 

 wise, a precipitate of sulphur may be formed. The yellow 

 ammonium sulphide is almost invariably a polysulphide of am- 

 monium, that is, ammonium sulphide which has combined 

 with one or more atoms of sulphur. If an acid be added to 

 this compound, an ammonium salt is formed, hydrosulphuric 

 acid is liberated, and sulphur precipitated : 



(NH 4 ) 2 S 2 + 2HC1 = 2NH 4 C1 + H 2 S + S. 



Ammonium sulphide precipitates the metals of the iron group 

 as sulphides, with the exception of chromium, which is precipi- 

 tated as hydrate ; aluminium is precipitated in the same form 

 of combination. 



Ammonium sulphide (or ammonium hydrate) causes also the 

 precipitation of metallic salts which have been dissolved in 

 acids, as, for instance, of the phosphates, borates, silicates, or 

 oxalates of the alkaline earths, magnesium, and others. The 

 processes by which the nature of these precipitates is to be 

 recognized are found in Table VI., page 218. 



Addition of ammonium carbonate. The reagent used is the 

 commercial salt, dissolved in water, to which some ammonium 

 hydrate has been added. Heating facilitates complete precipi- 

 tation of the carbonates of the alkaline earths. 



