The common ion effect can be defined as a shift in equilibrium on adding a substance that provides more of an ionic species already present in the dissociation equilibrium.

Thus, we can say that the common ion effect is a phenomenon based on the Le Chatelier’s principle.

Addition of acetate ions to an acetic acid solution results in decreasing the concentration of hydrogen ions, [H+].

Also, if H+ ions are added from an external source then the equilibrium moves in the direction of undissociated acetic acid i.e., in a direction of reducing the concentration of hydrogen ions, [H+].