Autoprotolysis constants of water + dimethyl sulphoxide mixtures at 25°C and the related free energies of transfer of OH–

Abstract

The autoprotolysis constants (K_s) of dimethyl sulphoxide (DMSO)+ water mixtures containing 10, 20, 40 and 60 percent of DMSO by weight have been determined at 25°C from e.m.f. measurements of the cell: Pt, H₂(g, 1 atm)∣ NaOH(m₁), NaCl(m₂), solvent ∣ AgCl—Ag. These values were utilized to compute the standard free energy changes, ΔG_t(H₂O—H₂O)_sys accompanying the transfer of H₂O molecules, acting as an acid as well as a base, from water to these mixed solvents, and thence ΔG^°_t(OH^–)_app values for these solvent mixtures using ΔG^°_t(H⁺) values reported in the literature. The observed increasingly positive values of ΔG^°_t(OH^–)_app and also of ΔG^°_t(OH^–), as obtained by deducting the influence of the activity of H₂O in the mixed solvents, i.e., RT ln (aH₂O)^s_w, from ΔG^°_t(OH^–)_app, suggest that the acidity of the mixed solvents decreases with increasing proportion of DMSO, as is expected from the decreasing stabilization of OH^– by H-bonding and the increasing desolvation of OH^– due to weaker ion-dipole interactions in the resulting solvent mixtures.