Abstract
The thermodynamic macroscopic ionization, tautomeric and hydration constants of several vitamin B6, molecules have been measured as a function of the temperature and solvent composition in water-dioxane mixtures. From these values the microscopic equilibrium constants for the ionization of the 3-hydroxy (pKOH) and nitrogen pyridinium (pKNH) groups of these molecules have been obtained, as well as their ΔH and ΔS° values. The AH values are practically independent of the solvent composition, while the AS values clearly depend on this composition. The solvent influence on the ΔG, or pK values, is thus of an entropic origin, probably arising from changes in the order of the water solvation molecules.
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