Complexation of Th(IV) and Eu(III) by alpha-isosaccharinic acid under alkaline conditions

The complexation of Th(IV) and Eu(III) by alpha -isosaccharinic acid (ISA) has been studied in the pH range from 10.7 to 13.3 by batch sorption experi ments, and the influence of Ca on the complexation was investigated. Sixtee n data sets - each determined at variable ISA concentrations are used to de termine the stoichiometry of the complexation reactions and the stability c onstants. 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 reversible arrow (ThISA)(-4H) + 4H with log K = -10.1 at I = 0.3 M. In the presence of Ca, the sorption data can be in terpreted best by a mixed-metal complex, according to the complexation reac tion: Th + 2ISA + Ca reversible arrow (Th(ISA)(2)Ca)(-4H) + 4H with log K-C a = -3.6 at I = 0.3 M. There are no indications that Ca participates in com plexes of ISA with Eu. The sorption data suggest that 1:I Eu:ISA complexes are dominant in the pH range from 10.7 to 13.3 according to the complexatio n reaction: Eu + ISA reversible arrow (EuISA)(-4H) + 4H with log K = -30.6 at I = 0.3 M. It is important to note that the stoichiometric numbers and s tability constants proposed here are not independent of the hydrolysis reac tions of the two metal cations. Thus, the same hydrolysis data as used here have to be applied in speciation calculations with the ISA complexation da ta for Eu(III) and Th(IV).