Sequestering ability of dicarboxylic ligands towards dioxouranium(VI) in NaCl and KNO3 aqueous solutions at T = 298.15 K

The formation constants of dioxouranium(VI)-2,2-oxydiacetic acid (diglycolic acid, ODA) and 3,6,9-trioxaundecanedioic acid (diethylenetrioxydiacetic acid, TODA) complexes were determined in NaCl (0.1 ≤ I ≤ 1.0 mol·L−1) and KNO3 (I = 0.1 mol·L−1) aqueous solutions at T = 298.15 K by ISE-[H+] glass electrode potentiometry and visible spectrophotometry. Quite different speciation models were obtained for the systems investigated, namely: ML, MLOH, ML2, M2L2(OH), and M2L2(OH)2 , for the dioxouranium(VI)–ODA system, andML, MLH, and MLOH for the dioxouranium(VI)–TODA system (M = UO2+ 2 and L = ODA or TODA), respectively. The dependence on ionic strength of the protonation constants of ODA and TODA and of both metal-ligand complexes was investigated using the SIT (Specific Ion Interaction Theory) approach. Formation constants at infinite dilution are [for the generic equilibrium pM +qL+rH+ = (M)p(L)qHr ;βpqr ]: logβ110 = 6.146, logβ11−1 = 0.196, logβ120 = 8.360, logβ22−1 = 8.966, logβ22−2 = 3.529, for the dioxouranium(VI)–ODA system and logβ110 = 3.636, logβ111 = 6.650, logβ11−1 = −1.242 for dioxouranium(VI)–TODA system. The influence of etheric oxygen(s) on the interaction towards the metal ion was discussed, and this effect was quantified by means of a sigmoid Boltzman type equation that allows definition of a quantitative parameter (pL50) that expresses the sequestering capacity of ODA and TODA towards uranyl ion ; a comparison with other dicarboxylates was made. A visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to better characterize the compounds found by pH-metric refinement.