Question

Role of solubility product and common ion effect in analytical chemistry

Answer 1

COMMON ION EFFECT

 

 DEFINITION

The degree of ionization of an electrolyte is suppressed by the addition of a strong electrolyte containing common ion. This effect is known common ion effect.In other words:
The phenomenon of lowering the degree of ionization of a weak electrolyte by adding a solution of a strong electrolyte having a common ion is called common ion effect.

APPLICATION OF COMMON ION EFFECT  Knowledge of common ion effect is very useful in analytical chemistry. It is frequently applied in qualitative analysis.
An electrolyte is precipitated only when the concentration of its ions exceeds the solubility product (KSP). The precipitation is obtained only when the concentration of any one ion is increased. Thus by adding a common ion, the solubility product can be increased.

PRECIPITATION OF THE CATIONS OF GROUP II

  Sulphides of basic radicals of groups II are precipitated by passing H2S gas through the acidified solution by HCl.Ionization of H2S:

H2S  2H+ + S-2

Here HCl provides common ion H+ which shifts the above equilibrium to the left as given by 
Le-Chatelier's principle.

HCl H+ + Cl-

Addition of HCl suppresses the ionization of H2S and lowers the concentration of S-2 ions, just enough to exceeds the KSP of II group sulphides. In this way only cations of group II are precipitated as CuS, PbS, CdS etc. but precipitation of the sulphides of group IV is prevented because they have high KSPvalues as compared to the sulphides of group II.PRECIPITATION OF THE CATIONS OF GROUP IV Cations of groups IV are precipitated as sulphides by passing H2S gas through the solution in the presence of NH4OH.Ionization of NH4OH: