Common Ion Effect: When a salt of a weak acid is added to the acid itself, the dissociation of the weak acid is suppressed further. Acetic acid is a weak acid. It is not completely dissociated in aqueous solution aid hence the following equilibrium exists.

CH₃COOH (aq) ⇌ H⁺ (aq)+ CH₃OO^- (aq) 

However, the added salt, sodium acetate, completely dissociates to produce Na⁺ and CH₃COO^- ion. 

CH₃COONa (aq) → Na⁺ (aq) + CH₃COO^- (aq) . 

Hence, the overall concentration of CH₃COO^- is increased, and the acid dissociation equilibrium is disturbed. We know from Le chatelier’s principle that when a stress is applied to a system at equilibrium, the system adjusts itself to nullify the effect produced by that stress. So, in order to maintain the equilibrium, the excess CH₃COO^- ions combines with H+ ions to produce much more unionized CH₃COOH i.e., the equilibrium will shift towards the left. In other words, the dissociation of CH₃COOH is suppressed. Thus, the dissociation of a weak acid (CH₃COOH) is suppressed in the presence of a salt (CH₃COONa) containing an ion common to the weak electrolyte. It is called the common ion effect.