Determination of the Stability Constants of the Acetate Complexes of the Actinides Am(III), Th(IV), Np(V), and U(VI) Using Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry

Capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS) was used to determine the stability constants of the actinides Am(III), Th(IV), Np(V), and U(VI) at an ionic strength of I = 0.3 M. The obtained stability constants were extrapolated to zero ionic strength by means of the Davies equation. For both U(VI) and Am(III), three consecutive acetate complexes with log(β10) = 3.01 ± 0.12, log(β20) = 5.27 ± 0.07, log(β30) = 6.82 ± 0.09, and log(β10) = 3.70 ± 0.09, log(β20) = 5.35 ± 0.08, log(β30) = 6.45 ± 0.09, respectively, could be identified. For Np(V), there was just one acetate complex, with log(β10) = 1.56 ± 0.03. In the case of Th(IV), five different complex species could be determined: log(β10) = 4.73 ± 0.16, log(β20) = 8.92 ± 0.09, log(β30) = 12.16 ± 0.11, log(β40) = 12.96 ± 0.87, and log(β50) = 14.39 ± 0.16. The actinides were selected with regard to their most stable oxidation state in aqueous solution so that four different oxidation states from +III to +VI could be inves...