Thermodynamics of acid proton dissociation of 5-hydroxy-1,4-naphthoquinone in dioxane-water mixtures

Abstract

Thermodynamic proton-ligand dissociation constants T p K a of 5-hydroxy-1,4-naphthoquinone (HHNQ) have been determined pH-metrically in 40–70 vol.% dioxane-water mixtures at 25 and 35 ± 0.1°C applying an empirical pH correction for mixed hydro-organic media. HHNQ is a weak monoprotic acid with T p K a between 9.7 and 11.8. Values of T p K a do not vary linearly with the reciprocal of the dielectric constant of the medium, but a plot of T p K a versus the mole fraction of dioxane ( n 2) is linear at a given temperature. The “true constant” K' a and solvation number n in the expression (H +) (A −)/(HA)= K' a( S)'= K a ∗, have been evaluated by analysing the experimental data mathematically. Values of the changes in the standard free energy, enthalpy and entropy associated with its proton-ligand ionization have also been calculated. Positive values of Δ H XXX found in aqueous-dioxane media for HHNQ indicate the endothermicity of the proton ionization reactions. Temperature and media effects are briefly discussed.