Thermodynamic study of complex formation between 3,5-di iodo-hydroxy quinoline and Zn^2+, Ni^2+ and Co^2+ cations in some binary solvents using a conductometric method

The complexation reactions between 3,5-di iodo-8-hydroxy quinoline (IQN) and Zn^2+, Ni^2+ and Co^2+ were studied in dimethylformamide (DMF)-ethanol (EtOH) binary solvent systems at different temperatures using a conductometric method. The stoichiometry of complexes was established 1:1. It was found that the stability of complexes formed between cations and this ligand increases with increases temperature. The standard enthalpies and standard entropies of complexes were obtained from the temperature dependence of stability constants. The results obtained by conductometric technique show that there is an inverse relationship between the formation constants of complexes and donor number of solvents. In all cases, the formation constants increased with increasing amounts of EtOH versus DMF in these binary systems. The standard enthalpy and standard entropies of complexes were obtained from the temperature dependence of stability constant. In all cases, ∆H° was positive and ∆S° was negative. The selectivity order for IQN complexes among various neat studied solvents varies in the order: Zn–IQN^2+— Ni–IQN^2+—Co–IQN^2+.