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

Abstract

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)_H2A^N Increases gradually with increase in proportion of methanol in the solvent. In contrast, as DMF is progressively added to water, p(_sK)_H2A^N 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, ΔG_t^o(H⁺), and the relative values of the standard Gibbs energies of transfer of HA^–, ΔG_t^o(HA^–), and of H₂A, ΔG_t^o(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.