Studies in some ‘isodielectric’ media. Part IV. Solvent effect on the proton transfer equilibria of some acids in methanol–propylene glycol at 25 °C

Abstract

The dissociation constants (_sK) of benzoic acid, protonated (hydroxymethyl)methylamine (TrisH⁺), and the p-nitroanilinium ion have been determined at 25° in the ‘isodielectric’ methanol–propylene glycol solvent system by e.m.f. measurements of suitable galvanic cells in the first two cases, and by spectrophotometric measurements in the case of the latter. The solubilities of the uncharged species, benzoic acid, Tris, and p-nitroaniline, have also been determined at 25°. The solvent effects on the proton transfer processes of these acids have been discussed in terms of the free energies of transfer of the different species concerned from methanol to the other solvents. ‘Electrostatic effect’ differences being negligibly small between these isodielectric media, specific solute–solvent interactions dictate the overall proton transfer behaviour of these acids.