Figure 9.2 shows that the valence state of neptunium has a strong influence on the distribution coefficient. This is a general behaviour and elements with different valence states behave like different elements. In this context, it is useful to know the valence states of actinides in acid solutions.

Thorium. Thorium nitrate Th(NO3)4 is very soluble in water, thorium being in a tetravalent state. It can, rather easily, form complexes with TBP.

Uranium. In aqueous solution uranium can be found in trivalent U3+, quadrivalent U4+, pentavalent UVO+, and hexavalent UVIO2+ states. However the trivalent and pentavalent states are unstable so that, practically,
only the tetravalent and hexavalent states are important. The hexavalent state in the form of uranyl nitrate is highly soluble in TBP.

Neptunium. In acid aqueous solutions neptunium can be found in trivalent Np3+, quadrivalent Np4+, pentavalent NpVO+, and hexavalent NpVIO2+ states. Np4+ and NpVIO2+ are the more soluble states in TBP.

Plutonium. In aqueous solution plutonium can be found in trivalent Pu3+, quadrivalent Pu4+, pentavalent PuVO+, hexavalent PuVIO2+ and heptavalent PuVIIO2+ states. The heptavalent and pentavalent states are unstable. The tetravalent and hexavalent states can form complexes with TBP, while the trivalent state cannot and is not soluble in organic solvents.

Americium. In aqueous solution americium can be found in trivalent Am3+, pentavalent AmVO+ and hexavalent AmVIO2+ states. The tetravalent state is highly unstable. In the presence of oxidizable species, only Am3+ is of practical importance. It is moderately soluble in TBP. However, in the Purex process Am is not coextracted with uranium and plutonium and follows the fission products.

Curium. Curium is only stable in the trivalent state and less soluble in TBP than americium.