Studies on the Solubility and Dissociation Constants of Oxalic Acid in Aquo + Ethanolic Mixtures and Determination of Single-Ion Gibbs Energy of Transfer from Aqueous to Aquo + Ethanolic Mixtures

The dissociation constants of the simplest dibasic organic acid, oxalic acid were determined conductometrically and pH metrically in ethanol + water mixtures (0–87 wt% of ethanol). These data were coupled with solubility values of oxalic acid and potassium hydrogen oxalate to calculate the Gibbs energies of transfer of oxalic acid, bioxalate(HOX⁻), oxalate(OX²⁻) and potassium(K⁺) ions from water to aquo + ethanolic mixtures. The values of ΔG⁰_t(H₂OX), ΔG⁰_t(HOX⁻), ΔG⁰_t(OX²⁻), ΔG⁰_t(K⁺) represent the quantitative measure of solute or ion solvent interactions. pK₁ and pK₂ values and solubility values of oxalic acid (in general) increase continuously with increase in organic co-solvent though KHOX shows the reverse behaviour. ΔG⁰_t(H₂OX) and ΔG⁰_t(H⁺) are negative i.e. favourable for the dissociation process whereas ΔG⁰_t(HOX⁻), ΔG⁰_t(OX²⁻), and ΔG⁰_t(K⁺) (at above 16 wt% of alcohol) are positive and hence unfavourable. Though the increase in basicity is helpful for the dissociation process, the dielectric constant and H-bonding capability enhance the association process as the organic co-solvent increases.

The pK-values can be suitably utilised to prepare buffer solutions in aquo-ethanolic solutions. Collection of ΔG⁰_t of ions should give better idea on ion solvent interactions and may be used as a guide to calculate the solubilities of electrolytes in aquo + organic solvents.

  [H₂OX = Oxalic acid,    HOX⁻ = bioxalate ion]