[‚ī·ə·nə′zā·shən ¦kän·stənt] (physical chemistry) Analog of the dissociation constant, where k = [H+][A-]/[HA]; used for the application of the law of mass action to ionization; in the equation HA represents the acid, such as acetic acid.MedicalSeeAcid Dissociation ConstantSeePKASeeKA