explain common ion effect with reference to GrIII precipitation of cations

 

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.   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.