Thermodynamics of the solution equilibria of 3-hydroxypyridine and pyridoxine in water-dioxane mixtures

The thermodynamics of tautomeric and microscopic ionization equilibrium constants of 3-hydroxypyridine and pyridoxine have been determined in water-dioxane mixtures (0-70% weight fraction in dioxane) ranging from 10 degrees C to 50 degrees C. Generally, for both types of processes, the entropic contributions to the Gibbs energy predominate and this tendency increase concomitantly with the dioxane content in the media. The thermodynamic parameters are consistent with the corresponding values obtained from macroscopic ionization processes. An equation for the equilibrium constant K as a function of temperature and a solvent characteristic parameter is proposed to predict the K values throughout the whole range of the temperature and solvent compositions studied.