commonion

Commonion is a term used in chemistry to describe the presence of a common ion in a solution. A common ion is an ion that originates from more than one chemical source in the same solution, such as an ion released by the dissolution of one salt and also present as a counterion of another dissolved species. The common ion effect refers to the shift in chemical equilibria that occurs when a common ion concentration is increased.

By Le Chatelier's principle, adding a common ion drives the equilibrium toward the side that consumes that ion. In the dissolution of a sparingly soluble salt AB(s) ⇌ A+ + B-, introducing an additional source of Cl-, for example, decreases dissolution and lowers solubility. For a weak acid HA, adding the conjugate base A- suppresses ionization, reducing the amount of HA that dissociates. Similar logic applies to bases and salts in solution.

Common ion considerations are used to predict solubility and to control precipitation, which is important in qualitative inorganic analysis, purification, and selective separation. They also influence buffer composition, where the presence of a common ion can affect buffering capacity.

Caveats include the role of activity coefficients and ionic strength; if complexation, ion pairing, or strong interactions occur, the simple common ion picture may be altered. Related concepts include the solubility product, Le Chatelier's principle, and acid–base equilibrium.