386 ANALYTICAL CHEMISTRY. 



In some cases sulphur is precipitated on the addition of hydrogen 

 sulphide, while a change in color may take place. This change is 

 due to the deoxidizing action of hydrogen sulphide, the hydrogen of 

 this reagent becoming oxidized and converted into water, while sul- 

 phur is liberated. Thus, brown ferric compounds are converted into 

 pale-green ferrous compounds; red solutions of acid chromates 

 become green; and red permanganates or green manganates are 

 decolorized. 



The same deoxidizing action of hydrogen sulphide is the reason 

 why this reagent cannot be employed in a solution containing free 

 nitric acid, which latter compound oxidizes the hydrogen sulphide. 



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

 group. The precipitate produced by hydrogen sulphide in acid solu- 

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

 separated by means of ammonium sulphide, which dissolves the sul- 

 phides 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 hydroxide, as. otherwise, a precipitate 

 of sulphur may be formed. The yellow ammonium sulphide is 

 almost invariably a polysulphide of ammonium, 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, 

 hydrogen sulphide is liberated, and sulphur precipitated : 



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



Ammonium sulphide precipitates the metals of the iron group as 

 sulphides, with the exception of chromium, which is precipitated as 

 hydroxide ; aluminum is precipitated in the same form of combina- 

 tion. 



Ammonium sulphide (or ammonium hydroxide) 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 some of these precipitates is to be recognized are found in 

 Table VI., page 389. 



Addition of ammonium carbonate. The reagent used is the 

 commercial salt, dissolved in water, to which some ammonia water 



