Electrochemical and thermodynamic studies of the ion-pair formation of chloropentamminecobalt(III) ion in ethyl alcohol-water media containing different dicarboxylate ligands

The ion-pair dissociation constants, K D, of the ion-pair formed between chloropentamminecobalt(III) ion (CpX 2+) and a variety of dicarboxylate ligands, have been determined from EMF measurements of a cell composed of glass and calomel electrodes. Measurements were made in water and in aqueous binary mixtures of ethyl alcohol, over a wide range of solvent composition (0–60 wt% ethyl alcohol), at six different temperatures (ranging from 30 to 55°C at intervals of 5°C). The thermodynamic parameters of association Δ G ass 0, Δ H ass 0 and Δ S ass 0 have been calculated and discussed. Δ H ass 0–Δ S ass 0, Δ S ass 0–Δ S 1(or 2) 0, Δ G ass 0– G 1(or 2) 0 and Δ H ass 0–Δ H 1(or 2) 0 correlations among different solvent media and different dicarboxylate ligands were examined (where 1 and 2 denote the first and the second dissociation reactions of the studied dicarboxylic acids). The p K D value has been correlated with the dielectric constant of the medium according to Born's equation.