SEPARATION OF METALS IN DIFFERENT GROUPS. 211 



Acidifying the solution. Hydrosulphuric acid has to be added 

 to the acidified solution for two reasons, viz. : In a neutral or 

 alkaline solution some metals of the arsenic group (which are to 

 be precipitated) would not be precipitated by hydrosulphuric 

 acid ; some of the metals of the iron group (which are not to 

 be precipitated) would be thrown down. 



The best acid to be used in acidifying is hydrochloric acid ; 

 but this acid forms insoluble compounds with a few of the 

 metals of the lead group, causing them to be precipitated. 

 Completely precipitated by hydrochloric acid are mercurous 

 and sil\ 7 er compounds ; partially precipitated are compounds of 

 lead, chloride of lead being somewhat soluble in water. The 

 precipitate formed by hydrochloric acid may be examined by 

 Table III., page 214. 



Hydrochloric acid added to a solution may in a few cases 

 (other than those just mentioned) cause a precipitate, as, for 

 instance, when added to solutions containing certain compounds 

 of antimony or bismuth (the precipitated oxychlorides of these 

 metals are soluble in excess of the acid), to metallic oxides or 

 hydrates which have been dissolved by alkaline hydrates (for 

 instance, hydrate of zinc dissolved in potassium or ammonium 

 hydrate), to solution of alkaline silicates, when silica separates, 

 etc. 



Addition of hydrosulphuric acid. This reagent is employed 

 either in the gaseous state (by passing it through the heated 

 solution) or as sulphuretted hydrogen water. The latter reagent 

 answers in those cases where but one metal is present ; if, how- 

 ever, metals of the arsenic and lead groups are to be separated 

 from metals of other groups, the gas must be used. 



In some cases sulphur is precipitated on the addition of 

 hydrosulphuric acid, while a change in color may take place. 

 This change is due to the deoxidizing action of hydrosulphuric 

 acid, the hydrogen of this reagent becoming oxidized and con- 

 verted into water, while sulphur is liberated. Thus, brown 

 ferric compounds are converted into pale green ferrous com- 

 pounds ; red solutions of acid chromates become green; and red 

 permanganates or green manganates are decolorized. 



The same deoxidizing action of hydrosulphuric acid is the 

 reason why this reagent cannot be employed in a solution con- 



