Intramolecular hydrogen bonding and the dissociation constant of salicylic acid in H2O + D2O mixtures

Abstract

The dissociation constant of salicylic acid in H₂O + D₂O solvent mixtures at 25°C has been determined from conductance measurements and from e.m.f. measurements with glass and silver–silver chloride electrodes in a cell without liquid junctions. The isotope effects ΔpK_n= pK_n–pK_H obtained by the two methods are in good agreement and, provided the fractionation factor ₁ for the carboxyl group is assumed to be in the range 0.90 < ₁ < 1.05, they can only be represented satisfactorily by the equilibrium theory of solvent isotope effects if fractionation at the phenolic groups in the acid and base species is taken into account. It is suggested that this is consistent with intramolecular hydrogen bonding in either or both species as is the abnormally large value (0.523) of the isotope effect ΔpK_D.