Acids, Bases, and Salts in Mixed-Aqueous Solvents

Abstract

The dependence of the dissociation constants of acids and bases and their tendency to form salts upon solvent composition in mixed-aqueous solvents are reviewed, along with the activities of the solvent components. The acid–base equilibria are dominated by preferential solvation of the ions by water molecules in the mixtures, compounded by the very high water activities across most of the solvent composition range; the neutral components are normally preferentially solvated by the organic component of the solvent mixture. Neutral acids, such as carboxylic acids and phenols, show only modest increases in pKa (1–2 pK units) in solvent mixtures containing up to 60–70 wt % organic component; thereafter is a much steeper, solvent-dependent increase to the value in the pure organic solvent. Cationic acids, such as protonated amines, anilines, and pyridines, display a universal decrease in pKa (∼1 pK unit) up until around 80 wt % organic solvent, almost independent of the nature of the base or the solvent; beyon...