Abstract
A potentiometric method has been used to determine the thermodynamicequilibrium constants for the macroscopic ionization processes of pyridoxal inwater—1,4-dioxane mixtures (0–70% weight fraction dioxane)at temperatures ranging from 10 to 50°C. These data, combined with the equilibrium constants for the tautomericand hemiacetalization processes, allow complete resolution of the microconstantsystem and calculation of the microscopic ionization equilibrium constants underall our experimental conditions. The standard thermodynamic function changesfor the macroscopic and microscopic ionization processes were obtained in variouswater—1,4-dioxane mixtures at 25°C. The values of any given microscopic pKfor the different solvents and temperatures fit very well to a single equation. Thefree energy, enthalpy, and entropy obtained for the different ionization processesin water—dioxane mixtures correlate with Kamlet and Taft's solvatochromicparameters π* and α, which are a measure of the dipolarity/polarizability andhydrogen-bonding capacity of the solvent, respectively. These correlations explainmore fully the mutual compensation between the contributions of enthalpy andentropy and the origin of the solvent effect on the pK.
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Llor, J., Ros, M.P. & Asensio, S.B. Complete Resolution and Thermodynamic Parameters of the Ionization Equilibria of Pyridoxal in Water—Dioxane Mixtures. Journal of Solution Chemistry 29, 1123–1141 (2000). https://doi.org/10.1023/A:1005143217105
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DOI: https://doi.org/10.1023/A:1005143217105