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–Huckel theory has been applied to the thermodynamic pKa values obtained. The pKa of the acids with respect to enrichment of the methanol by propylene carbonate attains a minimum at xPC= 0.2. The results are discussed in terms of the preferential solvation of species by two types of solvent molecules.