COMPLETE RESOLUTION OF THE IONIZATION EQUILIBRIA OF 5-DEOXYPYRIDOXAL IN WATER-DIOXANE MIXTURES

We have used a potentiometric method to determine the thermodynamic eq uilibrium constants for the macroscopic ionization processes of 5-deox ypyridoxal (DPL) in water-dioxane mixtures (0-70% weight fraction in d ioxane) at temperatures ranging from 10 degrees C to 50 degrees C. The se data, together with previously published equilibrium constants for the tautomerism and hydration processes, have allowed us to resolve th e complete microconstant system. We have also calculated the microscop ic ionization equilibrium constants under all the experimental conditi ons. The changes of standard thermodynamic function for the macroscopi c and microscopic ionization processes were obtained in various water- dioxane mixtures at 25 degrees C. The values of a given microscopic pK with different solvents and temperatures fit very well to an equation which relates this magnitude with the thermodynamic parameters, the s olvation of the components of the reaction, and a solvent parameter. W e have obtained an interesting linear correlation between the thermody namic parameters corresponding to all the microscopic ionizations of D PL and the net change of the solvation during the process: enthalpies correlate linearly for all the microscopic ionizations, while entropie s do so for the phenols and pyridinium ions separately.