Uranium(VI) complex / Europium(III) complex / Carboxylic acid / Stability constant / Enthalpy / EntropySummary. The thermodynamic quantities (∆G, ∆H and ∆S) 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−(CH 2 ) n −COO − ) showed an interesting tendency, that is, −∆H decreased and T ∆S increased systematically with increasing the length of the carbon chain from n = 2 to 6. The comparison of T ∆S values of 1 : 1 carboxylate formation with U(VI) and Eu(III) showed that T ∆S 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).

Potentiometric and microcalorimetric titration techniques were applied to determine the Gibbs free energies and enthalpies of the protonation of some carboxylic acids (acetic, glycolic, malonic and malic acids) and their complexation with Eu(III) in 1.0 M NaClOA few to several kJ/mol of enthalpies were determined for the reactions within the uncertainties of ±0.01 to ±0.4. These enthalpy values indicated that the protonation of these carboxylates were entropy-driven, that is, |−

No abstract