Common ion effects and its application

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 (K
SP). 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 IV

Cations of groups IV are precipitated as sulphides by passing H2S gas through the solution in the presence of NH4OH.
Ionization of NH4OH:
NH4OH NH4+ + OH-
In this analysis NH4OH provides OH- ions which combines with H+ ions of H2S to form H2O.
H2S 2H+ + S-2 : H+ + OH- H2O
Removal of H+ ions from product side shifts the equilibrium to right and the concentration of S-2 increases which is enough to exceed the KSP of the sulphides of group IV. In this way CoS, NiS or ZnS can easily be precipitated.
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 KSP values as compared to the sulphides of group II.

PRECIPITATION OF THE CATIONS OF GROUP III
Cations of groups III are precipitated as hydroxides by passing NH4OH in the presence of NH4Cl. Here
NH
4Cl provides common ion NH4+ which suppresses the ionization of NH4OH.
NH4OH NH4+ + OH- : NH4Cl NH4+ + Cl-
Common ion NH4+ shifts the equilibrium to left side and the concentration of OH- ions decreases. Under these circumstances, the KSP of the hydroxides of Al, Fe and Cr is only exceeded and they are precipitated as Al (OH)3, Fe (OH)3 and Cr (OH)3 but the hydroxides of Zn, Ni and Co are not precipitated as they have high values of KSP.

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:
NH4OH NH4+ + OH-
In this analysis NH4OH provides OH- ions which combines with H+ ions of H2S to form H2O.
H2S 2H+ + S-2 : H+ + OH- H2O
Removal of H+ ions from product side shifts the equilibrium to right and the concentration of S-2 increases which is enough to exceed the KSP of the sulphides of group IV. In this way CoS, NiS or ZnS can easily be precipitated.