Thermodynamics of U(VI) complexation by succinate at variable temperatures

Complexation of U(VI) by succinate has been studied at various temperatures in the range of (298 to 338) K by potentiometry and isothermal titration calorimetry at constant ionic strength (1.0 M). The potentiometric titrations revealed the formation of 1:1 uranyl succinate complex in the pH range of 1.5 to 4.5. The stability constant of uranyl succinate complex was found to increase with temperature. Similar trend was observed in the case of enthalpy of complex formation. However, the increase in entropy with temperature over-compensated the increase in enthalpy, thereby favouring the complexation reaction at higher temperatures. The linear increase of enthalpy of complexation with temperature indicates constancy of the change in heat capacity during complexation. The temperature dependence of stability constant data was well explained with the help of Born equation for electrostatic interaction between the metal ion and the ligand. The data have been compared with those for uranyl complexes with malonate and oxalate to study the effect of ligand size and hydrophobicity on the temperature dependence of thermodynamic quantities.

Research highlights
► lg β and ΔHC for U(VI)-succinate determined at variable temperatures.
► Increase in lg β with temperature well explained by Born equation.
► ΔSC plays the dominant role in variation of ΔGC with temperature.
► ΔHC for U(VI)-succinate increases linearly with temperature.
► ΔCP of U(VI)-succinate is higher than that of oxalate and malonate complexes.