Proton-ligand ionization equilibria of 2,3-dimethyl-1-phenylpyrazol-5-one in mixtures of water and dioxane: a thermodynamic study

Abstract

Thermodynamic proton-ligand ionization constants, Tp K a, of 2,3-dimethyl-1-phenyl-pyrazol-5-one (HDPP) have been determined pH-metrically in 0–40 vol.% dioxane-water mixtures at 25 and 35±0.1°C applying an empirical pH correction for mixed aquo-organic media. Both extrapolation and a least-squares method were used to obtain TpK a values in pure water (0%). HDPP is a very weak monoprotic acid having a TpK a value of between 11.4 and 13.0. Tp K a's do not vary linearly with the reciprocal of the dielectric constant of the medium; however, a plot of Tp 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) n = K a b7 have been evaluated by analysing the experimental data mathematically. Changes in the standard free energy, enthalpy and entropy associated with the proton-ligand ionization of HDPP have also been calculated. Temperature and medium effects are briefly discussed.