Thermodynamics of ionization of Hammett bases in concentrated aqueous solutions of perchloric acid

Abstract

The Hammett acidity function H₀ for aqueous perchloric acid has been measured as a function of temperature. The data are analysed in terms of the enthalpies of protonation of primary amines in aqueous perchloric acid solutions. The enthalpy changes which occur when the species involved in the protonation equilibria of primary anilines are transferred from water to aqueous perchloric acid are in accord with solvation theories of acidity function behaviour. Thus, by analogy to equations relating H₀ to water activity, correlations between the enthalpy data and relative partial molal enthalpies of water in the acid solutions are consistent with proposals that the high acidity of concentrated solutions of strong acids may be largely explained in terms of the release of water molecules of hydration when a weak base B reacts with a solvated proton to give BH⁺.