Rapid and quantitative electrolytic preparation and speciation of neptunium ions of various oxidation states using multi-step column electrodes

Redox behavior of Np(III)-Np(VI) ions in aqueous perchloric, nitric and sulphuric acid solutions was elucidated by using column electrodes connected in series in a flow system (multi-step column electrode system). Using a glassy carbon fiber working electrode as the column electrode was found to be very useful for the investigation of current-potential relations of not only reversible redox processes, such as Np(VI)/Np(V) or Np(IV)/Np(III) couple but also irreversible processes, such as Np(V)/Np(IV) or Np(V)/Np(III) couple, the latter not observed by conventional voltammetry which uses a glassy carbon or platinum electrode. Quantitative (coulometric) electrolysis could be executed even if the redox process was completely irreversible by the use of the column electrodes. By taking advantage of column electrode electrolysis, a novel method was developed for the rapid preparation of a neptunium species of a desired oxidation state, including unstable species. The multi-step column electrode system was demonstrated to be useful for the coulometric determination and speciation of Np(IV)-Np(VI) in aqueous HNO3 solution as an example.