Dissociation of some phenols in acetone + water mixtures

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 eight substituted phenols, i.e. 2,3-(NO₂)₂-, Cl₅-, 2,5-(NO₂)₂-, 2,4,6-Cl₂-, 2,5-Cl₂-, 4-NO₂-, 2-NO₂- and 3,5-Cl₂-, in four acetone + water mixtures at 0.2, 0.4, 0.6 and 0.8 mole fraction, at 298 K at a constant ionic strength of 0.02 mol dm^–3.

An activity correction derived from Debye–Hückel theory was applied to the thermodynamic pK_a values obtained. The pK_a of the acids was found to vary linearly with the mole fraction of acetone over the range 0.2–0.80.

The variations in pK_a with the solvent composition are discussed in terms of the free energies of transfer of phenols, phenoxide anions and the proton from water to the acetone + water mixtures.