MAGNESIUM AND ZINC. IONIC EQUILIBRIA 459 



centrations of any two ions in a mixture exceeds the value of the 

 ion-product in a saturated solution of the compound formed by 

 their union, this compound will be precipitated. Naturally the 

 substances with small solubilities, and therefore small solubility- 

 product constants, are the ones most frequently formed as pre- 

 cipitates. 



Completeness of Precipitation. In the above case, we 

 precipitate zinc sulphide practically completely from solution 

 by adding excess of ammonium sulphide. This substance, like 

 all salts, is highly ionized in solution, and therefore a solution 

 which contains it in excess contains a high concentration of sul- 

 phide ion S = . The amount of zinc ion Zn ++ that can exist simul- 

 taneously in such a solution is negligible, since otherwise the solu- 

 bility-product of zinc sulphide would be exceeded. Zinc is there- 

 fore precipitated quantitatively as zinc sulphide by addition of excess 

 of ammonium sulphide to a solution of any zinc salt. This is a 

 fact which is turned to practical use in quantitative analysis. 



If we attempt to carry out the precipitation with hydrogen sul- 

 phide instead of ammonium sulphide, however, we find that only 

 partial precipitation of the zinc as sulphide occurs (p. 451). How- 

 ever great an excess of H 2 S we add, precipitation is incomplete. 

 The reason for this is immediately apparent from a study of the 

 full ionic equations: 



H 2 S <^2H+ + S- 

 ZnCl 2 ^2Cl- + Zn++ 



ti ti 



2HC1 ZnS * ZnS 

 (dissolved) (solid) 



Hydrogen sulphide is so slightly ionized in solution that the 

 concentration of sulphide ion S = that it supplies is extremely 

 small. A large amount of zinc ion Zn 44 " can therefore remain in 

 solution without the solubility-product of zinc sulphide being 

 exceeded. 



