Thermodynamic studies on the formation of ion-pair cobalt complexes in acetone–water mixtures

Dicarboxylic acids are stronoger acids than moncraboxylic acids and more soluble in water due to the presence of two carboxylic (–COOH) groups which are industrially important in producing polyester, polyamides, and polyols and as a pioneer to pharmaceutical additives. Succinic acid is one of the most prosperous chemicals that can be generated from renewable resources which can be used in the industries of antiseptics, cosmetics, sutures, drugs, solvents, detergents, herbicides, fungicides, synthetic resins, and inks. The ion–pair formation is very important in diferent applications, used widely for inorganic analysis by liquid chromatography and used for the heavy metal removal from waste water, so studing the ion-pair association constant (K_A) formed between chloropentamminecobalt(III) cation [CpX²⁺] and a different ligands of dicarboxylate anions (AC^2-), have been determined by EMF technique in different compositions of acetone aqueous media (0–44.20 wt% acetone) for succinic acid and for the other dicarboxylic acids (Maleic, Malonic and Malic) in the same solvent composition (40 wt% acetone) with in temperature range (30 – 60 °C). The thermodynamic association properties (${\Delta G}_A^{°}$, ${\Delta H}_A^{°}$ and ${\Delta S}_A^{°}$) have been estimated and discussed. Extra thermodynamic correlations like ${\Delta H}_A^{°}$- ${\Delta S}_A^{°}$ and ${\Delta G}_A^{°}$- ${\Delta G}_{I\left(orII\right)}^{°}$ (where I and II are mentioned for the first and second dissociation constant of the dicarboxylic acids) in different compositions of acetone aqueous media for succinic acid and in fixed solvent composition for the other dicarboxylic acids were examined in order to give more information about the behavior of solvent molecules and its effect on formation of the ion-pair molecules. In addition, The Gibbs free energy of transfer for the association process ${\Delta G}_A^{°}$(t) from water to acetone mixed solvents were calculated for succinic acid in order to indicate the spontaneous nature and the stabilization of the ion-pair by increasing the acetone weight percent in the mixed acetone – aqueous media. The linear correlation between pK_D values and the relative permittivity of acetone aqueous medium is in accordance to Born’s equation.