210 ANALYTICAL CHEMISTRY. 



cipitate a number of metals together, and the reagents used for 

 this purpose are known as group-reagents. 

 They are : 



1. Hydrosulphuric acid, added to the solution previously 

 acidified by hydrochloric acid. Precipitated are : the metals of 

 the arsenic and lead groups as sulphides. 



2. Ammonium sulphide, added after supersaturating with am- 

 monium hydrate. Precipitated are : the metals of the iron 

 group and of the earths as sulphides or hydrates. 



3. Ammonimn carbonate. Precipitated are : the metals of the 

 alkaline earths as carbonates. 



4. In solution are left : the metals of the alkalies and magne- 

 sium. 



The order in which these group-reagents are added cannot 

 be reversed or changed, because ammonium sulphide added 

 first would precipitate not only the metals of the iron group and 

 the earths, but also the metals of the lead group ; ammonium 

 carbonate would also precipitate most of the heavy metals. 



For the same reasons, in separating metals of the different 

 groups, the group-reagents must be added in excess, that is, 

 enough of them must be added to precipitate the total quantity 

 of the metals of one group, before it is possible to test for 

 metals of the next group. Suppose, for instance, a solution to 

 contain a salt of bismuth only. Upon the addition of hydro- 

 sulphuric acid to the acidified solution, a dark brown precipitate 

 (of bismuth sulphide) is produced, indicating the presence of a 

 metal of the lead group. Suppose, further, that hydrosulphuric 

 acid has not been added in a sufficient quantity to precipitate 

 the whole of the bismuth, then ammonium sulphide, as the 

 next group-reagent, would produce a further precipitation in 

 the filtrate, which fact would lead to the assumption that a 

 metal of the iron group was present, which, however, is not 

 the case. 



If the solution contain but one metal, the group-reagents 

 are added successively in small quantities to the same solu- 

 tion, until the reagent is found which causes a precipitation, 

 which reagent is then added in somewhat larger quantity in 

 order to produce a sufficient amount of the precipitate for further 

 examination. 



