Potentials of Np(VI)/Np(V) and Pu(VI)/Pu(V) Couples in Mixed Solvents.

The formal potentials of Np(VI)/Np(V) and Pu(VI)/Pu(V) couples in acetonitrile–water and ethanol–water solutions containing 0.01 M HClO4 at 25°С were measured by the potentiometric method. In acetonitrile–water solutions, the potentials of both couples decrease with decreasing water content from 55.55 to 30 M. This is caused by the hydrolysis of NpO22+ and PuO22+ ions. With a further decrease in the proportion of water, the potentials of the couples do not change. This indicates that the hydration numbers of An(VI) and An(V) are the same and equal to 5. Similar results were obtained in ethanol–water solutions. [ABSTRACT FROM AUTHOR]