Acid Dissociation Constant (pKa)

Acid Dissociation Constant (pKa) designates the logarithmic measure of a compound's tendency to donate a proton, defining the pH at which equal concentrations of ionised and unionised forms coexist. This fundamental physicochemical property governs solubility, membrane permeability, protein binding, and distribution across physiological pH compartments.

The pharmaceutical industry uses pKa values extensively during drug discovery and formulation to predict bioavailability, select optimal salt forms, and guide excipient choice. Compounds with pKa values in physiologically relevant ranges exhibit pH-dependent absorption, requiring careful consideration of gastrointestinal pH variation. Potentiometric titration and spectrophotometric methods provide accurate experimental measurements. Understanding pKa guides medicinal chemistry optimisation, salt screening, and stability assessment across development programmes.