The common ion effect is responsible for the reduction in solubility of an ionic precipitate when a soluble compound combining one of the ions of the precipitate is added to the solution in equilibrium with the precipitate. It states that if the concentration of any one of the ions is increased, then, according to Le Chatelier's principle, the ions in excess should combine with the oppositely charged ions. Some of the salt will be precipitated until the ionic product is equal to the solubility of the product. In simple words, common ion effect is defined as the suppression of the degree of dissociation of a weak electrolyte containing a common ion.
