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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  NaClO 4 . 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 (UO 2 L), 5.37±0.07 (UO 2 L 2 ), 7.25±0.18 (UO 2 L 3 ), 5.06±0.17 (FeL), 8.51±0.15 (FeL 2 ), 11.00±0.16 (FeL 3 ), and 12.99±0.17 (FeL 4 ). 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 H 1  = -(1.0±1.0)kJ mol -1 , Δ H 2 =1.4±1.8 kJ mol -1  and Δ H 3 =-(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 FeL 3  and FeL 4  are large and exothermic, contributing significantly to the stability of the complexes.

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