What is common ion effect?

Common ion effect :-The reduction of the degree of dissociation of a salt by the addition of a common-ion is called the common-ion effect.

If a salt of a weak acid is added to a solution of the acid itself, the dissociation of the acid is diminished further.

 For example, the addition of sodium acetate to a solution of acetic acid suppresses the dissociation of acetic acid which is already very small. Consider the equilibrium,

CH₃COOH ⇔ H⁺ + CH₃COO^-

The addition of one of the products of dissociation (example:acetate ion), supplied by the largely dissociated salt (example:sodium acetate) pushes the equilibrium to the left. In other words, the dissociation of acetic acid is suppressed. Similarly, the addition of hydrogen ions furnished by the addition of a largely dissociated acid such as hydrochloric acid , also suppresses the dissociation of acetic acid. Thus, the common ion effect is the suppression of the dissociation of a weak acid or a weak base on the addition of its own ions.