Common-ion effect

The shift in an ionization equilibrium caused by adding a substance that supplies an ion already present, usually reducing the weak electrolyte’s ionization.

Le Châtelier's principle: A principle stating that an equilibrium shifts in response to a change in conditions, tending to oppose that change. Adding a shared ion changes concentration, prompting the equilibrium shift behind the effect.
Buffer solution: A solution that resists pH change through a weak acid–base pair and its conjugate partner. A conjugate ion suppresses weak-acid or weak-base ionization, helping stabilize pH.
Acetic acid: A weak monoprotic carboxylic acid with the formula CH₃COOH. Added acetate shifts its ionization equilibrium toward undissociated acetic acid.
Weak electrolyte: A substance that only partially forms ions when dissolved in a solvent. Its incomplete ionization makes its equilibrium especially responsive to a shared ion.
Salting out: The reduction in a solute’s solubility caused by high concentrations of dissolved salts. It can resemble common-ion suppression of solubility but does not require a shared ion.
Chemical equilibrium: A dynamic state in which forward and reverse reactions occur at equal rates, keeping concentrations constant. The common-ion effect is a concentration-driven change in an equilibrium system.
Henderson–Hasselbalch equation: An equation relating buffer pH to the acid dissociation constant and the ratio of conjugate base to acid. The common-ion ratio links buffer composition to pH.
Sodium acetate: An ionic compound, CH₃COONa, that dissolves to produce sodium and acetate ions. Its acetate ions suppress the ionization of acetic acid.
Ion: An atom or molecule with a net electric charge from losing or gaining electrons. The effect depends on an ion being present in both the equilibrium and the added substance.
Salt effect: A change in reaction or equilibrium behavior caused by altering the ionic strength of a solution. It arises from activity changes and can occur even without adding a common ion.

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