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Pure Appl. Chem., Vol. 71, No. 9, pp. 1771-1807, 1999



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


Sorin Kihara, Zenko Yoshida, Hisao Aoyagi, Kohji Maeda, Osamu Shirai, Yoshihiro Kitatsuji and Yumi Yoshida

Abstract: Standard redox potentials, E0s, and redox processes of U, Np and Pu ions in acidic aqueous solutions are reviewed and evaluated critically. The E0s of 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+.


1. Introduction

2. Standard redox potentials for uranium, neptunium and plutonium ions in acidic aqueous solutions

2.1 Evaluation of E0 from E0' determined by electrochemical measurements
2.2 Temperature dependence of E0

3. Redox reactions of uranium, neptunium and plutonium in acidic aqueous solutions investigated by polarography or voltammetry

3.1 Uranium
3.2 Neptunium
3.3 Plutonium
3.4 Disproportionation of NpO2+, PuO2+, Np4+ and Pu4+
3.5 Reduction of MO2+ and reduction intermediates

4. Redox reactions of uranium, neptunium and plutonium in acidic aqueous solutions investigated by flow coulometry

4.1 Electrode processes of the uranium, neptunium and plutonium ions investigated by flow coulometry at the column electrode at the column electrode

4.2 Disproportionation of MO2+ during the electrolysis by flow coulometry
4.3 Reduction mechanisms of MO2+ (M = Np or Pu) and reduction intermediates investigated by flow coulometry

5. Conclusions

6. List of abbreviations

7. Appendix

8. References

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