Common ion effect

The decrease in the ionization of a weak electrolyte by the presence of a common-ion from a strong electrolyte, is called the common ion effect. Ionisation of CH₃COOH (weak acid) is decreased by the addition of CH₃COONa (CH₃COO- being the common ion)
CH₃COOH <======> CH₃COO^- + H⁺ ………………………..(A)
CH₃COONa -----------> CH₃COO^- + Na⁺
In the presence of CH₃COO^- equilibrium (A) shifts in backward direction.
Ionisation of H₂S (weak acid) is decreased by the addition of HCl (H⁺ being the common ion)
H₂S <=======> 2H⁺ + s^2-
HCl <=======> H⁺ + Cl^-
Ionisation of NH₄OH (weak base) is decreased by the addition of NH₄Cl (NH4 ⁺ being the common ion)
NH₄OH <=======> NH₄⁺ + OH^-
NH₄Cl --------------> NH₄⁺ + Cl^-
Solubility of a sparingly soluble salt is decreased by the addition of common ion. Presence of AgNO₃ or KCl decreases the solubility of AgCl.
AgCl <=======> Ag⁺ + Cl^-
AgNO₃ <========> Ag⁺ (common ion) + NO₃^-
KCl <=======> K⁺ + Cl^-(common ion)
The common ion effect is thus based on Le-Chatelier’s principle in which the stress on the equilibrium that results from raising one of the product concentrations is relieved by shifting the equilibrium to left.