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 incre asing 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 D elta E(0)(e) by 30 mV. The stability of Am(V) in aqueous and mixed aqu eous-acetonitrile solutions of KPW has been studied. Conversion of Am( V) in aqueous acetonitrile considerably increases as compared to aqueo us solutions of KPW and proceeds according to the rate law -d[An(V)]/d t = 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).