Complexation of uranium(VI) with thiodiacetic acid in solution at 10-85 °C

The protonation of thiodiacetate and its complexation with uranium(VI) in 1.05 mol kg(-1) NaClO4 are studied at variable temperatures (10-85 degrees C). Three U-VI complexes (UO2L, UO2HL+, and UO2HL2-, where L is thiodiacetate) are identified in this temperature range. The formation constants and the enthalpies of complexation are determined by potentiometry and calorimetry. The complexation of uranium(VI) with thiodiacetate becomes more endothermic at higher temperatures. However, the complexes become stronger due to increasingly more positive entropies of complexation at higher temperatures, which exceeds the increase in the unfavorable enthalpy of complexation. The values of the heat capacity of complexation (Delta C-p degrees) are 122 +/- 16, 302 +/- 26, and 242 +/- 23 JK(-1) mol(-1) f or UO2L, UO2HL+, and UO2HL2-, respectively. The effect of temperature on the thermodynamics of the complexation is discussed in terms of the electrostatic model and the change in the solvent structure.