The solution chemistry of the actinide elements has been investigated in both aqueous and selected organic solutions. The majority of the informa­tion available describes species found in aqueous solutions. Although

actinide cations can exist in a variety of oxidation states (2+ to 7+) in aqueous solution (Table 2.2), the most common for light actinides in aqueous acidic solutions are trivalent, tetravalent, pentavalent and hexavalent oxida­tion states. The stability of a particular oxidation state across the actinide series is quite variable, and for some actinides (Np, Pu) several oxidation states can coexist in the same solution. This is most evident for plutonium as there are small differences in the reduction potentials of Pu(III), Pu(IV), Pu(V), and Pu(VI) over a range of pH values (Choppin, Jensen, 2006).