Solvent effects and ion-solvent interactions from the studies of the solubility and dissociation constants of oxalic acid in aquo + 2-propanol mixtures and determination of single-ion Gibbs energies of transfer from water to 2-propanol + H2O mixtures

Abstract

The solvent effects on the dissociation of the simplest dibasic organic acid were examined from the determination of the first and second dissociation constants ot oxalic acid conductometrically ana pH-metrically in water + 2-propanol mixtures (0-87 wt% of 2-propanol). The solubility values of oxalic acid and potassium hydrogen oxalate were also determined. The dissociation constants and the solubility values were used to calculate the Gibbs energies of transfer ΔG 0 t of oxalic acid (H 2 OX), bioxalate (HOX-), oxalate (OX 2- ) and potassium (K + ) ions from water to aquo-propanolic mixtures. These represent the 'medium effects' or quantitative measures of solute or ion-solvent interactions of the respective species in going from water to 2-propanol + water mixtures. pK 1 , pK 2 and the solubility values of oxalic acid increase continuously with increase in organic co-solvent. AG 0 t (H 2 OX) and AG 0 t (H + ) are negative i.e. favourable but AG 0 t (H 2 OX), ΔG 0 t (OX 2- ) and AG 0 t (K + ) are in general positive and hence unfavourable for the dissociation processes. The increase in basicity facilitated the dissociation processes but decrease in dielectric constant and H-bonding capability enhances the association processes as the organic co-solvent increases. The pK-values can be suitably utilised to prepare buffer solutions in 2-propanol + water mixtures. The solvent effect and solvation phenomena can be better understood from the collection of single-ion values of transfer. These may be utilised to calculate the solubilities of electrolytes in 2-propanol + water mixtures. Attempts have been made to understand the solvation processes in terms of the structure of the aquo-organic mixtures and Walden products.