A Critical Evaluation of the Redox Properties of Uranium, Neptunium and Plutonium Ions in Acidic Aqueous Solutions

Abstract

Standard redox potentials, E0s, and redox processes of U, Np and Pu ions in acidic aqueous solutions are reviewed and evaluated critically. The E0sof reversible redox processes, MO22+/MO2+ and M4+/M3+ (M: U, Np or Pu) adopted are those proposed mainly by Riglet et al. on the basis of the precise correction of formal potentials, E0's, according to the improved theoretical approach to estimate the activity coefficient. Electrode processes of the U, Np and Pu ions are discussed in terms of current-potential curves, measured so far by polarography, voltammetry or flow coulometry. Special attention is payed to the irreversible MO2+/M4+ reactions. Disproportionation reactions of MO2+ are also discussed. New substances are introduced as intermediates during reductions of MO2+ to M4+ or disproportionations of MO2+.CONTENTSIntroductionStandard redox potentials for uranium, neptunium and plutonium ions in acidic aqueous solutions2.1 Evaluation of E0 from E0' determined by electrochemical measurements2.2 Temperature dependence of E0Redox reactions of uranium, neptunium and plutonium in acidic aqueous solutions investigated by polarography or voltammetry3.1 Uranium3.2 Neptunium3.3 Plutonium3.4 Disproportionation of NpO2+, PuO2+, Np4+ and Pu4+3.5 Reduction of MO2+ and reduction intermediatesRedox reactions of uranium, neptunium and plutonium in acidic aqueous solutions investigated by flow coulometry4.1 Electrode processes of the uranium, neptunium and plutonium ions investigated by flow coulometry at the column electrode at the column electrode4.2 Disproportionation of MO2+ during the electrolysis by flow coulometry4.3 Reduction mechanisms of MO2+ (M = Np or Pu) and reduction intermediates investigated by flow coulometryConclusionsList of abbreviationsAppendixReferences

Rapid and Precise Coulometric Determination and Separation of Redox Inert Ions Based on Electrolysis for Ion Transfer at the Aqueous|Organic Solution Interface

“…Coulometry has been applied not only to the accurate and precise determinations, 3 but also to the identification of electrolysis products and elucidation of electrode processes. [4][5][6][7] Coulometry is considered to be especially useful for the absolute determination of compounds that decompose or explode easily by heating, since it is difficult to dry such compounds to constant weights and to determine the compounds by gravimetric analysis.…”

“…Though electrochemical principles applied to coulometric works performed so far have been redox reactions [1][2][3][4][5][6][7] except for the works introduced in this review, [8][9][10][11][12][13][14][15] the ion transfer reactions at the aqueous | organic solution, W | O, interface, [16][17][18] which have been investigated extensively during the last few decades as a field of electrochemistry, are expected to provide another principle applicable to coulometry. If coulometry expands its field to the interfacial ion transfers, the precise and absolute determination becomes possible not only for species that are easily oxidized or reduced in solutions, but also for those that are not easily oxidized or reduced.…”

“…The quantitative electrolysis can be attained within a few minutes when an appropriate potential is applied to the column electrode. 6 For example, when a sample solution containing U, Np and Pu ions was introduced to a two-step column electrodes system, the oxidation states of U, Np and Pu ions were adjusted to U 4+ , Np 3+ and Pu 3+ , respectively, by the electrolysis at the 1st column electrode of a potential more negative than -0.3 V versus SSE. Then, the U 4+ and Np 3+ that entered into the 2nd column electrode of potential between +0.35 and +0.6 V versus SSE were converted to UO2 2+ and Np 4+ , respectively, leaving Pu 3+ as it was.…”

Rapid and coulometric electrolysis for ion transfer at the aqueous|organic solution interface

“…Coulometry is one of very important branches of analytical chemistry because of its unique advantage as an absolute method [1,2], and has been applied not only to the accurate and precise determination [3], but also to the identification of electrolysis products and elucidation of electrode processes [4][5][6][7]. Though electrochemical principles applied to coulometric works performed so far have been redox reactions [1][2][3][4][5][6][7] except for a few works described later [8,9], the ion transfer reaction at the aqueous|organic solution, W|O, interface [10][11][12], which has been investigated extensively for the last a few decades as a field of electrochemistry, is expected to be another principle applicable to coulometry.…”

“…Though electrochemical principles applied to coulometric works performed so far have been redox reactions [1][2][3][4][5][6][7] except for a few works described later [8,9], the ion transfer reaction at the aqueous|organic solution, W|O, interface [10][11][12], which has been investigated extensively for the last a few decades as a field of electrochemistry, is expected to be another principle applicable to coulometry. If coulometry expands its field to the interfacial ion transfer, the precise and absolute determination becomes possible for not only species which are easily oxidized or reduced in solutions, but also those which are not easily oxidized or reduced.…”