Acid Dissociation Constants of Some Mercaptobenzazoles in Aqueous-Organic Solvent Mixtures

The acid dissociation constants of some 2-mercaptobenzazoles in aqueous-organic solvent mixtures are determined potentiometrically at (25 ± 0.1) °C and an ionic strength of I = 0.02 mol dm - 3 (KNO 3 ). The organic solvents are methanol, ethanol, DMF, DMSO, and acetonitrile. The p K a values are discussed in terms of the proportion and nature of the organic cosolvent. It was concluded that hydrogen bonding interactions of the conjugate base with the solvent and the solvent basicity in addition to the electrostatic effect contribute the major effects in the ionization process. Moreover, thermodynamic functions (Δ H, Δ G, and Δ S ) of the ionization process in an aqueous medium containing 0.1 mole fraction of ethanol are also determined and discussed.