Behaviour of americium(IV) in phosphoric acid solutions

Abstract

The completeness of Am(III) electrochemical oxidation in 3–15 M H 3PO 4 depending on phosphoric acid and americium concentration, as well as the influence of H 3PO 4, americium concentration and temperature on Am(IV) stability in phosphoric acid solutions has been investigated. It has been found that in 3–8 M H 3PO 4 an Am(IV) disproportionation occurs according to the scheme: 3Am(IV) → 2Am(III)+Am(VI), and in solutions with H 3PO 4 concentration greater than 10 M—an Am(IV) reduction to Am(III). The apparent rate constant of Am(IV) disproportionation increases with H 3PO 4 concentration decrease; the rate constant of Am(IV) reduction increases with the decrease of americium concentration, phosphoric acid concentration (15-10M), and with the increase of temperature. The activation energy for the reaction Am(IV) → Am(III) in 12 M H 3PO 4 is 15·6 ± 1·1 kcal. mole −1. The mechanism of Am(IV) And Am(VI) formation during electrochemical oxidation of Am(III) is discussed.