The thermodynamics of the ionization of 3-hydroxypyridine and pyridoxine in water–dioxane mixtures

Abstract

A potentiometric method has been used to determine the thermodynamic equilibrium constants for the macroscopic ionization processes of 3-hydroxypyridine and pyridoxine in water–dioxane mixtures (0–70% weight fraction in dioxane) at temperatures from 10 to 50 °C. From these data the reaction standard thermodynamic function changes were obtained for the processes in various water–dioxane mixtures at 25 °C. It was found, in both molecules, that during the first ionization process the contributions of entropy and enthalpy to free energy are similar but that during the second ionization the main contribution comes from entropy, especially in mixtures with a high dioxane content. The effect of the solvent on the tautomeric equilibrium in the net zero charge specie could explain this behaviour.