Complexation of U(VI) with picolinic acid in aqueous solution at variable temperatures: Potentiometric, spectrophotometric and calorimetric studies

Thermodynamics of the U(VI) complexation with picolinic acid (HL) was investigated under constant ionic strength (1.05 mol.kg(-1) NaClO4) with multiple techniques, including potentiometry, spectrophotometry and calorimetry. Three stepwise complexes, UO2L+, UO2L2(aq) and UO2L3-, were identified, and their formation constants were determined by potentiometric titrations at 283-313 K. The spectrophotometric work at 298 K further demonstrated the formation of these three complexes. By calorimetric titrations, stepwise enthalpies of the complexation at 298 K were determined to be Delta H-11 = -(3.43 +/- 0.24) kJ.mol(-1) , Delta H-12 = -(3.3 +/- 0.6) kJ.mol(-1), and Delta H-13 = 9.4 +/- 0.6 kJ.mol(-1). A formation of the 1:1 and 1:2 complexes is driven by both enthalpy and entropy, but the formation of the 1:3 complex by entropy only. The thermodynamic parameters also suggest that in the 1:1 and 1:2 complexes, picolinate coordinates to U (VI) through the chelation mode, i.e., one carboxylate oxygen and an aromatic nitrogen bind to the U(VI) atom. (C) 2017 Elsevier Ltd.