The electrophoretic mobilities (μ e ) of the actinides Th and U−Am in different oxidation states (prepared in 1 M HCl and 1 M HClO 4 ) have been determined by capillary electrophoresis (CE)-inductively coupled plasma mass spectrometry (ICPMS) using 1 M acetic acid as the background electrolyte, which has proven to provide an excellent setup for trace analysis at environmentally relevant concentrations (1 × 10 −9 M). The values are independent of the respective acid solution. The μ e of the Pu oxidation states +III to +VI have been measured. They agree with both the available literature data and the redox-stable analogues (Eu(III), Th(IV), Np(V), U(VI)) that have also been investigated. The trend in the μ e for the actinides U−Pu was found to be An(III) > An(VI) > An(V) > An(IV). The μ e values of Am(III) (μ e (Am(III)) = 3.86 × 10 −4 cm 2 /(Vs)), U(IV) (μ e (U(IV)) = 0.34 × 10 −4 cm 2 /(Vs)), and U(VI) (μ e (U(VI)) = 1.51 × 10 −4 cm 2 /(Vs)) have been measured for the first time under these experimental conditions. Furthermore, the measured μ e values show systematic trends that can be rationalized on the basis of the calculated species distribution of the actinides in 1 M acetic acid and the corresponding average effective charges (q eff ).

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(β1 0) = 3.01 ± 0.12, log­(β2 0) = 5.27 ± 0.07, log­(β3 0) = 6.82 ± 0.09, and log­(β1 0) = 3.70 ± 0.09, log­(β2 0) = 5.35 ± 0.08, log­(β3 0) = 6.45 ± 0.09, respectively, could be identified. For Np­(V), there was just one acetate complex, with log­(β1 0) = 1.56 ± 0.03. In the case of Th­(IV), five different complex species could be determined: log­(β1 0) = 4.73 ± 0.16, log­(β2 0) = 8.92 ± 0.09, log­(β3 0) = 12.16 ± 0.11, log­(β4 0) = 12.96 ± 0.87, and log­(β5 0) = 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 investigated. A great benefit of CE-ICP-MS is the opportunity to measure at significantly lower concentrations compared to the available literature, allowing the study of actinide complexation in environmentally relevant concentration ranges. Furthermore, it is possible to analyze all four actinides simultaneously in one and the same sample.

DOI: https://doi.org/10.1002/elps.201800318 The cover picture shows a schematic sketch of the general procedure for the determination of kinetic parameters of a redox reaction using CE‐ICP‐MS. It was applied to the reduction of Np(V) to Np(IV) by hydroxylamine hydrochloride (HAHCl) in aqueous solution. The rate constants k and the energy of activation EA were derived from a series of electropherograms measured at different reaction times and temperatures. This proof‐of‐principle investigation shows that the coupling of CE to ICP‐MS is a powerful tool to investigate the kinetics of redox reactions of actinides at low concentrations.

The rate constants k of the reduction of 5 × 10 M Np(V) to Np(IV) by hydroxylamine hydrochloride (HAHCl) in 1 M HCl have been determined by CE-ICP-MS in the temperature range of ϑ = 30-70°C and with varying concentrations of HAHCl from 1 to 7.2 M. The reaction was found to have (pseudo)first order kinetics with respect to HAHCl. The experimental results for k ranged from 0.0029(1) min (ϑ = 40°C, c(HAHCl) = 3 M) to 0.039(7) min (ϑ = 60°C, c(HAHCl) = 7.2 M). The activation energy of the reaction was determined as E = (72 ± 10) kJ/mol. These results and a comparison with literature data show that the coupling of CE to ICP-MS provides a powerful analytical tool for the investigation of the kinetic aspects of redox reactions of actinides at low concentrations. On the basis of this proof-of-principle study, the method presented here can be extended to the investigation of the kinetic parameters of other redox systems containing different actinides (or transition metals) and oxidants/reductants.