In aqueous solution americium exists in the four oxidation states Ат(Ш), Am(IV), Am(V), and Am(VI). In the absence of complexing agents trivalent, pentavalent, and hexavalent americium exist as Am3*, Am02+, and Am022+, usually in hydrated form. In aqueous solution tetravalent americium rapidly disproportionates, except in concentrated fluoride and phosphate solutions.

Trivalent americium is the most stable state in solution. As shown by the oxidation-reduction potentials of Table 9.6, the higher oxidation states of americium are strong oxidizing agents, so they exist only in solutions that contain no oxidizable species.

Trivalent americium forms relatively unstable complexes with Cl” and N03” and more stable complexes with the thiocyanate ion CNS”. These americium complexes are more stable than those of the corresponding lanthanide compounds, so that americium can be separated from trivalent lanthanides by anion exchange with concentrated solutions of LiCl, IiN03, or NH4CNS. Tri­valent americium can be extracted with TBP from a concentrated nitrate solution. It can also be extracted with TBP from a molten LiN03 — KN03 eutectic at 150°C, with much higher distribution coefficients than in extraction from aqueous solutions. Americium is more readily extracted by this process than is trivalent curium [K2].