Kinetics of neptunium (5) disproprotionation in nitric acid solution

The kinetics of Np(5) disproportionation in HNO3+NaNO3 solutions at the ionic strength μ=4 is studied by spectrophotometric method in the region of [HNO3] = 2.1-4 M at 61-96 deg C. To shift an equilibrium of reaction and to exclude the reverse reaction of Np(6) and Np(4), 0.02 M of hydrazine was added to the solution which rapidly reduced Np(6) to Np(5). Under these conditions no further reduction of Np(5) into Np(4) occurs directly with hydrazine. The disproportionation rate of NpO2+ ions obeys the equation: -d[Np(5)]dt = 2k[Np(5)]2H+2, where k = 0.113 + 0.005M-3min-1 at 90 deg C and μ = 4. Temperature dependence of k enabled to compute the energy (E=16.4 kcal/mole), enthalpy (ΔHsup(*) = 15.8 kcal/mole) free energy (ΔFsup(*) = 24.0 kcal/mole) and entropy (ΔS=-27 eu) of the reaction activation. The reaction mechanism includes a slow step of charge transfer between two protonated NpO2H2+ ions. The data obtaned are compared to the results of Np(5) diproportionation in HClO4 medium