Determination of Acid Dissociation Constants by High-Performance Liquid Chromatography

Abstract

Values for the acid dissociation constant K_a of various acidic and basic compounds were determined from the pH-dependent capacity factor k' by reversed-phase high-performance liquid chromatography (HPLC) with octadecyl silane as the stationary phase. Both the neutral and ionic forms of acids and bases were retained on the stationary phase, and k' of the ionic form markedly influenced the calculation of K_a value. Values of K_a determined by HPLC agreed well with those determined by the conventional potentiometric method, indication the availability of this method for the determination of K_a values. The "hydrophobicity" of the ionized forms of these compounds is discussed in terms of the partition coefficient of the neutral form between octanol and water.