Electrochemical oxidation of americium and californium in carbonate solutions

Oxidation of californium-249 was carried out in 2 M K2CO3 solution at pH 13.2 at a platinum anode potential of +1.05 V versus normal hydrogen electrode (NHE). During electrolysis a broad-band absorption was observed to appear in a region of λ<500 nm (yellow colouration of a solution), and the absorption bands of californium(III) somewhat decreased. It has been determined by titration of this solution with K4Fe(CN)6 titrant that the oxidation extent of californium(III) was about 20% (if californium(IV) is the supposed oxidation product). Under these conditions partial oxidations of terbium(III) and praseodymium(III) to the tetravalent state were also observed. The results of electrochemical oxidation of 243Am(III) in 3 M KHCO3-K2CO3 solutions have been found to depend on both the pH of a solution and an anode potential. At a potential of +1.25 V versus NHE, stable americium(IV) was obtained at pH 8.2-9.7, americium(VI) at pH 11-12.5, and americium(V) at pH>13.5. Formal potentials have been measured for the following redox couples: Am(IV)-Am(III) (Ef=0.870±0.002 V at pH 8.2) and Am(VI)-Am(V) (Ef=0.910±0.003 V at pH 11.3). The mechanism of americium(III) oxidation to americium(VI) in 3 M K2CO3 solution at pH 11.3 has been studied. (orig.)