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The complexation of Th(IV) and Eu(III) by α-isosaccharinic acid (ISA) has been studied in the pH range from 10.7 to 13.3 by batch sorption experiments, and the influence of Ca on the complexation was investigated. Sixteen data sets -each determined at variable ISA concentrationsare used to determine the stoichiometry of the complexation reactions and the stability constants. Based on best-fit analysis of the sorption data, it is postulated that 1:1 Th:ISA complexes are formed in the absence of Ca according to the complexation reaction: Th + ISA (ThISA) −4H + 4H with log K = −10.1 at I = 0.3 M. In the presence of Ca, the sorption data can be interpreted best by a mixed-metal complex, according to the complexation reaction: Th + 2ISA + Ca (Th(ISA) 2 Ca) −4H + 4H with log K Ca = −3.6 at I = 0.3 M. There are no indications that Ca participates in complexes of ISA with Eu. The sorption data suggest that 1:1 Eu:ISA complexes are dominant in the pH range from 10.7 to 13.3 according to the complexation reaction: Eu + ISA (EuISA) −4H + 4H with log K = −30.6 at I = 0.3 M. It is important to note that the stoichiometric numbers and stability constants proposed here are not independent of the hydrolysis reactions of the two metal cations. Thus, the same hydrolysis data as used here have to be applied in speciation calculations with the ISA complexation data for Eu(III) and Th(IV).

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Ion-pair formation between Ca 2+ and α-isosaccharinate, Ca 2+ + ISA − → ← CaISA + , was studied by two independent methods: an ion-exchange and a potentiometric method (Ca-selective electrode). The two methods gave similar values for the complexation constant, log K o CaISA + at I = 0, (22 ± 1) • C. The ion-exchange method gave a value of log K o CaISA + = (1.8 ± 0.1) and the potentiometric method resulted in logK oCaISA + = (1.78 ± 0.04). These values are in good agreement with the estimated value, log K o CaISA + = 1.7, based on the formation of a Ca-gluconate ion pair.

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