Kinetics and mechanism of the oxidation of neptunium(IV) by nitric acid in tributyl phosphate solution

Np(IV) oxidation by nitric acid in tributyl phosphate solutions is negligible at ambient temperatures whilst at an elevated temperature it proceeds according to the stoichiometric equation (without accounting for TBP complexes): Np(NO3)(4) + HNO3 + H2O = NpO2(NO3)(2) + HNO2 + 2 HNO3. The reaction is an autocatalytic reaction that is due to the formation of more reactive (compared to HNO3) oxidants, i.e. HNO2 and Np(VI). The main contribution is made by the Np(V) reproportionation reaction. Hence Np(TV) oxidation follows the scheme: Np(IV) + HNO3 = 2 Np(V) + HNO2 (slow) Np(V) + HNO2 = Np(VI) + NO (quick) Np(IV) + Np(VI)= 2 Np(V) (slow) 2 NO + HNO3 + H2O = 3 HNO2 (quick) The reaction rate is described by the equation: - d[Np(IV)]/dt = k1 ''[Np(IV)]/[H2O](10) + k(4)[Np(IV)][Np(VI)][H2O](3)/[HNO3](4) where k(4) = (9.5 +/- 0.8) X 10(-4) min(-1) at 60.2 degrees C and k(1) '' is an extremal function of the HNO3 concentration in TBP solution with the maximum at approximate to 0.4 M HNO3; at [HNO3] = 0.2 M and T = 60.2 degrees C, k(1) '' = (0.70 + 0.43) mol(10) l(-10) min(-1). The activation energy of the reaction represented by k(4) is 90 +/- 2 kJ/mol.