Protonation and complexation of isosaccharinic acid with U(VI) and Fe(III) in acidic solutions: potentiometric and calorimetric studies

Protonation and complexation of α-isosaccharinic acid with U(VI) and Fe(III) have been studied in acidic solutions at t = 25 C and I = 1.0 mol dm-3 NaClO4. From the potentiometric titrations, the protonation constant of the carboxylate group is calculated to be 3.65 ± 0.05 and the data are consistent with the presence of three and four successive mononuclear complexes for U(VI) and Fe(III), respectively. The formation constants of the complexes, logβj for the reactions of M + L = MLj where j = 1-3 for U(VI), j = 1-4 for Fe(III) and L stands for isosaccharinate, are determined to be 2.91 ± 0.15 (UO2L), 5.37 ± 0.07 (UO2L2), 7.25 ± 0.18 (UO2L3), 5.06 ± 0.17 (FeL), 8.51 ± 0.15 (FeL2), 11.00 ± 0.16 (FeL3), and 12.99 ± 0.17 (FeL4). From the calorimetric titrations, the enthalpy of protonation of the carboxylate group is determined to be -(7.94 ± 0.03) kJ mol-1, similar to that of other α-hydroxycarboxylates. The enthalpies of complexation between U(VI) and isosaccharinate are quite small: ΔH1 = -(1.0 ± 1.0) kJ mol-1, ΔH2 = 1.4 ± 1.8 kJ mol-1 and ΔH3 = -(6.2 ± 3.0) kJ mol-1, typical of the interactions between carboxylates and hard-acid cations. The complexation between U(VI) and isosaccharinate is mainly entropy-driven. In comparison, the enthalpies of complexation for FeL3 and FeL4 are large and exothermic, contributing significantly to the stability of the complexes. (orig.)