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The solvent effects on the dissociation of the simplest dibasic organic acid were examined from the determination of the first and second dissociation constants of oxalic acid conductometrically and pH-metrically in water + methanol mixtures (0–87 wt% of methanol). The solubility values of oxalic acid and potassium hydrogen oxalate were also determined in water + methanol mixtures. The dissociation constant and the solubility values were used to calculate the Gibbs energies of transfer Δ G  0   t   of oxalic acid (H 2 OX), bioxalate (HOX − ), oxalate (OX 2− ) and potassium (K + ) ions from water to aquo-methanolic mixtures. These represent the quantitative measures of solute or ionsolvent interactions of the respective species in going from water to methanol + water mixtures. p K  1 , p K  2  and the solubility values of oxalic acid increase continuously with increase in organic co-solvent. Δ G  0   t  (H 2 OX) and Δ G  0   t  (H + ) are negative  i.e.  favourable but Δ G  0   t  (H 2 OX), Δ G  0   t  (OX 2− ) and Δ G  0   t  (K + ) are in general positive and hence unfavourable for the dissociation processes. The increase in basicity fecilitated the dissociation processes but decrease in dielectric constant and H-bonding capability enhance the association processes as the organic co-solvent increases. Attempts have been made to understand the solvation processes in terms of the structure of the aquo + organic mixtures and Walden products. The p K  values can be suitably utilised to prepare buffer solutions in methanol + 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 methanol + water mixtures.

zeitschrift für physikalische chemie

Ion–Solvent Interactions and Determination of Single-Ion Gibbs Energies of Transfer from the Studies of the Solubility and Dissociation Constants of Oxalic Acid in Aquo + Methanolic Mixtures