234 ANALYTICAL CHEMISTRY. 



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 pre- 

 cipitate also 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 con- 

 tain a salt of bismuth only. Upon the addition of hydrogen sulphide 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 

 hydrogen sulphide has not been added in 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, 

 would not be the case. 



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

 successively in small quantities to the same solution, 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. 



Acidifying 1 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 pre- 

 cipitated) would not be precipitated by hydrogen sulphide ; 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 dilute hydrochloric acid ; 

 but this acid forms insoluble compounds with a few of the metals of 

 the lead group, causing them to be precipitated. Completely pre- 

 cipitated by hydrochloric acid are mercurous and silver 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 238. 



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 bis- 

 muth (the precipitated oxychlorides of these metals are soluble in 

 excess of the acid), to metallic oxides or hydroxides which have been 

 dissolved by alkali .hydroxides (for instance, hydroxide of zinc dis- 

 solved in potassium or ammonium hydroxide), to solutions of alkali 

 silicates, when silica separates, etc. 



