Ions of different valences of a metal behave like different elements with respect to extract — ability. The difference between Ce3+ and Ce4+ in Table 4.2 is one example. Another is afforded by Pu4+ and Puvi022+, which are readily extracted by TBP in kerosene, whereas Pu3+ has a very low distribution coefficient [G3]. Consequently, by adjusting the oxidation — reduction potential of the aqueous phase to control the proportion of an element in different valence states, it is possible to vary its distribution coefficient between wide limits. This is the means by which plutonium is stripped from aqueous solutions containing plutonium and uranium in sections C and D of Fig. 4.5 illustrating the Purex process. Addition of a reducing

agent such as a ferrous salt to the aqueous stripping solution entering section C reduces plutonium to Pu3+ and renders it readily back extracted into water. Uranium remains as U0)3+ in the organic phase.