Thermodynamic study on the U(VI) complexation with dicarboxylates by calorimetry

Abstract

The thermodynamic quantities (ΔG, ΔH and ΔS) of U(VI) complexation with oxalate, malonate, succinate, glutarate, adipate, phthalate, tartronate and malate, and of Eu(III) complexation with succinate, glutarate and adipate were determined by potentiometric and calorimetric titration techniques. The obtained thermodynamic quantities indicated that these complexation reactions are mainly driven by the entropy changes while the enthalpy changes are nearly zero or preventing the complexation. The thermodynamic quantities of 1:1 U(VI) complexes with 5 dicarboxylates having different carbon chain lengths (⁻OOC−(CH₂)_n−COO⁻) showed an interesting tendency, that is, −ΔH decreased and TΔS increased systematically with increasing the length of the carbon chain from n = 2 to 6. The comparison of TΔS values of 1:1 carboxylate formation with U(VI) and Eu(III) showed that TΔS of U(VI) were always lager than those of Eu(III). The entropies of 1:1 complex formation of U(VI) with hydroxycarboxylates were always smaller than those with carboxylates of the same carbon chain length, while the enthalpies of uranyl hydroxycarboxylates were always larger. This tendency was also observed in the protonation and the complex formation of Eu(III).