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Sujit Chandra Lahiri, S. K. Gumtya
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
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 (0–87wt 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 ΔG0t of oxalic acid (H2OX), bioxalate (HOX−), oxalate (OX2−) 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. pK1, pK2 and the solubility values of oxalic acid increase continuously with increase in organic co-solvent. ΔG0t(H2OX) and ΔG0t(H+) are negative i.e. favourable but ΔG0t(H2OX), ΔG0t(OX2−) and ΔG0t(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 pK 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, Oldenbourg Wissenschaftsverlag
Print ISSN: 0942-9352
Volume: 217, 11/2003
Pages: 1341