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Linfeng Rao, Alexander Yu. Garnov, Dhanpat Rai, Yuanxian Xia, Robert C. Moore

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, log β_j for the reactions of M+L=ML_j 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: Δ H₁ = -(1.0±1.0)kJ mol^-1, Δ H₂=1.4±1.8 kJ mol^-1 and Δ H₃=-(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.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 92, 09/2004
Pages: 575 - 581

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