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

Abstract

The autoprotolysis constants (Ks) 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, ΔGt(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)sw, 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.