The kinetics and mechanism of the reduction of neptunium (VI) ions by uranium (IV) ions in nitric acid

V. S. Koltunov, Robin J. Taylor, V. I. Marchenko, G.I. Zhuravleva, E.A. Mezhov, G. V. Koltunov,  and K. N. Dvoeglazov

Summary

The rapid reduction of NpO22+ ions to NpO2+ by U(IV) ions in an aqueous nitric acid solution has been studied and, in many ways, is similar in character to the same reaction in HClO4. The major difference is that in HNO3 the reaction proceeds via two parallel routes. The first is via the hydrolysed UOH3+ ion, as in the reaction in HClO4 and the second is via the non-hydrolysed U4+ ion. These parallel routes lead to the observed order of reaction with respect to H+ ions being reduced from −1 in HClO4 to −0.7 in HNO3. After accounting for the reaction mechanism the rate equation is described by:

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where k8 = 404 min−1, k19 = 275 M−1 min−1 and β = 0.009 M at 10 °C and μ = 2. The activation energy was 66.5 ± 4.9 kJ mol−1. Nitrate ions had no effect on the reaction rate but sulphamic acid increased the observed rate, probably through catalysis by sulphate ions arising from sulphamic acid reaction with HNO2 and hydrolysis.

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Radiochimica Acta publishes original papers, review articles, and “rapid communications” (short articles of a more timely interest) on all chemical aspects of nuclear science and technology. The journal is geared toward scientists who are actively engaged in research work.

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