Solvent effects on acid–base behaviour. Phenols and carboxylic acids in methanol + propylene carbonate solvents

Abstract

The electrometric titration method, using a glass electrode and silver, silver bromide electrode in a cell without a liquid junction, has been used to determine the dissociation constants of five substituted phenols: 2,6-dibromo-4-nitro-, 2,6-dinitro-, 2,4-dinitro-, 2-methyl-4,6-dinitro- and 2,5-dinitro; three substituted benzoic acids: 3,5-dinitro-, 3-nitro- and 4-bromo; and substituted acetic acids: chloro-, phenyl- and trimethyl-acetic acid in methanol and in four methanol+propylene carbonate mixtures (0.1, 0.2, 0.4 and 0.6 mole fractions), at 298 K and at a constant ionic strength of 0.02 mol dm^–3. An activity correction derived from Debye–Hückel theory has been applied to the thermodynamic pK_a values obtained. The pK_a of the acids with respect to enrichment of the methanol by propylene carbonate attains a minimum at x_PC= 0.2. The results are discussed in terms of the preferential solvation of species by two types of solvent molecules.