isoionic

Isoionic is an adjective used in chemistry to describe conditions or species related to isoionic ions in solution. It is most often encountered in discussions of acid–base equilibria involving salts derived from a weak acid and a weak base. In such systems, the cation BH+ is the conjugate acid of a weak base B, and the anion A− is the conjugate base of a weak acid HA. The solution’s pH is governed by the relative strengths of these ions.

The isoionic point refers to the condition under which the hydrolysis tendencies of the cation and the anion balance each other. Concretely, isoionic conditions occur when the acidity constant of the cation Ka(BH+) equals the basicity constant of the anion Kb(A−). Since Ka(BH+) = Kw / Kb(B) and Kb(A−) = Kw / Ka(HA), this equality corresponds to Ka(HA) = Kb(B). Under ideal, pure-water conditions this often yields a near-neutral pH, so the solution is approximately neutral.

An example is ammonium acetate, a salt formed from a weak base (ammonia) and a weak acid (acetic acid). When the strengths of the conjugate acid and conjugate base are matched, the solution tends toward neutrality; deviations occur with different actual strengths and activity effects in solution.

Isoionic is distinct from the related term isoelectric point, which describes the pH at which a macromolecule, such as a protein, carries no net electrical charge. Isoionic refers to the ionic composition and hydrolysis behavior of the solution rather than the net charge of a biomolecule.