OXIDATION POTENTIALS OF AM(VI)/AM(V) COUPLE AND STABILITY OF AM(V) IONS IN AQUEOUS AND AQUEOUS-ORGANIC SOLUTIONS OF POTASSIUM PHOSPHOTUNGSTATE

To estimate the thermodynamic stability of Am(V), we have measured the oxidation potentials (E(0)(e)) of Am(VI)/Am(V) couple in aqueous and aqueous-acetonitrile (AN) solutions of potassium phosphotungstate (KPW) over a wide range of concentrations of HNO3, KPW, and AN. With increasing pH, E(0)(e) decreases from 1.60 V (pH 0) to 1.31 V (pH 6.0). The greatest shift of potential (Delta E(0)(e)) is 290 mV as compared to its standard value. Introduction of acetonitrile additionally shifts Delta E(0)(e) by 30 mV. The stability of Am(V) in aqueous and mixed aqueous-acetonitrile solutions of KPW has been studied. Conversion of Am(V) in aqueous acetonitrile considerably increases as compared to aqueous solutions of KPW and proceeds according to the rate law -d[An(V)]/dt = k(1)[Am(V)] + k(2)[Am(V)](2). Complex mechabism of the conversion may be due to the existence of various complex forms of Am(V).