Common ion effect may be defined as the suppression of the degree of dissociation of a weak acid or a weak base by the addition of a strong electrolyte containing a common ion (ion common to the weak acid or base or the strong electrolyte).
The case of ionisation of a weak base NH4OH.
At equilibrium if we add solid NH4Cl to the solution,the concentration of NH4+ ions increases.
Due to increase in NH4+ ions, the equilbrium shifts to the left and ionisation of NH4OH reduces.
This principle is made use of in purification of sodium chloride
sodium chloride obtained from sea water or lakes is always impure.
The saturated solution of impure sodium chloride is prepared by dissolving in minimum quantity of water required.
Then HCL gas is passed through this solution (CL- is the common ion).
Due to increase of common chloride ions, the dissociation of sodium chloride is suppressed and sodium chloride is thrown down as precipitate. From the initial saturated solution, now sodium chloride is precipitated.
But the chlorides of impurities still remain in solution. So pure NaCl is obtained using the principle of common ion effect.