Solute–solvent effects in the dissociation of thymolsulphonephthalein (an uncharged acid) in aqueous mixtures of protic (methanol) and dipolar aprotic (dimethylformamide) solvents

The thermodynamic first dissociation constants, (sK)H2A of thymolsulphonephthalein, an uncharged acid, have been determined spectrophotometrically at 25 °C in aqueous solutions containing 10, 30, 50, 70, and 80 wt% protic solvent (methanol) and in aqueous solutions containing 20, 40, 60, 70, and 80 wt% dipolar aprotic solvent (NN-dimethylformamide)(DMF). p(sK)H2AN Increases gradually with increase in proportion of methanol in the solvent. In contrast, as DMF is progressively added to water, p(sK)H2AN decreases sharply at the beginning followed by a very small decrease before showing a weak tendency to rise after ca. 40 mol% DMF. The solvent effect on the dissociation of the acid has been discussed in terms of the standard Gibbs energy of transfer of H+ from water to the mixed solvent, ΔGto(H+), and the relative values of the standard Gibbs energies of transfer of HA–, ΔGto(HA–), and of H2A, ΔGto(H2A), in the two solvent systems. The contrasting behaviour of the indicator acid is found to be dictated by the specific solute–solvent interactions of the species involved in the ionization equilibria.