THERMODYNAMICS OF THE SOLUTION EQUILIBRIA OF 3-HYDROXYPYRIDINE AND PYRIDOXINE IN WATER-DIOXANE MIXTURES

The thermodynamics of tautomeric and microscopic ionization equilibriu m constants of 3-hydroxypyridine and pyridoxine have been determined i n water-dioxane mixtures (0-70% weight fraction in dioxane) ranging fr om 10 degrees C to 50 degrees C. Generally, for both types of processe s, 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 v alues 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 throughou t the whole range of the temperature and solvent compositions studied.