Common ion effect

Consider a solution of weak acid $\mathrm{CH_3COOH}$ and its soluble ionic salt $\mathrm{CH_3COONa}$.

$\mathrm{CH_3COOH}$ is weak acid, dissociates only slightly in solution

$\mathrm{CH_3COONa}$ being a strong electrolyte dissociates almost completely in solution.

Both the acid and the salt produce $\mathrm{CH_3COO^{-}}$ ions in solution. $\mathrm{CH_3COONa}$ dissociates completely. Therefore it provides high concentration of $\mathrm{CH_3COO^{-}}$ ions. According to Le-Chatelier principle, the addition of $\mathrm{CH_3COO^{-}}$ from $\mathrm{CH_3COONa}$ to the solution of $\mathrm{CH_3COOH}$, shifts equilibrium of dissociation of $\mathrm{CH_3COOH}$ to left. Thus reverse reaction is favoured in which $\mathrm{CH_3COO^{-}}$ combines with $\mathrm{H^{+}}$ to form unionised $\mathrm{CH_3COOH}$. Hence dissociation of $\mathrm{CH_3COOH}$ is suppressed due to presence of $\mathrm{CH_3COONa}$ containing a common $\mathrm{CH_3COO^{-}}$ ion.

The common ion effect states that the ionisation of a weak electrolyte is suppressed in presence of a strong electrolyte containing an ion common to the weak electrolyte.