Thermodynamic study of complexation of Eu(III) with carboxylates by potentiometry and calorimetry

Complexation of Eu(III), a chemical analogue of trivalent actinides, by various carboxylate anions, namely, acetate, 2-hydroxy-2-methylpropanoate (HMP), succinate and phthalate, have been studied at I = 1.0 M and 25 °C by potentiometry and isothermal titration calorimetry. The enthalpy of formation of all carboxylate complexes was found out to be endothermic except for HMP and for ML2 with phthalate. The complexation reaction was found to be controlled by entropy. The dicarboxylic anions (succinate and phthalate) were found to form stronger complexes than the monocarboxylates (acetate and HMP). However, the presence of hydroxyl group in HMP increased the stability constant of the complex. The data in conjunction with the time resolved fluorescence spectroscopy measurements performed previously have been used to infer about the structure of the complexes. The thermodynamic data have been explained in terms of hard acid–hard base interaction between Eu(III) and carboxylate anions.