Np(V)/Np(IV) in concentrated carbonate/bicarbonate solutions

A 1.5 M Na2CO3 solution of Np(V) is electrolysed to Np(IV) at -2.0 V/SHE. -1g(H+) is decreased from 10.4 to 7.2 by bubbling CO2 in these solutions, where Np(IV) spectra can be interpreted with the only lost of one CO32- anion from the Np(IV) limiting complex. From these spectral changes, the following parameters are fitted: 20.5 ± 2.1, 8.44 ± 0.9 and 28.9 ± 2.9 l/mol./cm for the Np(CO3)56- molar absorptivity at 823, 990 and 1013 nm respectively, and 54.5 ± 5.5, 40.6 ± 4.1 and 8.53 ± 0.9 for the Np(CO3)44- ones, and log((Np(CO3)56-) / ((Np(CO3)44-)(CO32-))) = 1.47 ± 0.08, 1.63 ± 0.05, 1.80 ± 0.04, 1.79 ± 0.10 and 2.21 ± 0.03 at the half point reaction in initially 0.2, 0.3, 0.4, 0.5 and 0.6 M Na2CO3 solutions. These values are extrapolated to 0 ionic strength by using the Specific Interaction Theory (SIT). The redox potential of 0.3, 0.6, 1 and 1.5 M Na2CO3 solutions of Np(V) and Np(IV) mixtures, is stable usually after three hours at T from 5 to 60 deg C, and then for up to three weeks at 21.5 deg C. At 25 deg C, its values are 0.247, 0.234, 0.244 and 0.228 V/SEH in 0.3, 0.6, 1 and 1.5 M Na2CO3 solutions. When the tonic strength is equal to 0: E = 0.52 ± 0.1 V/SEH and ΔS/F = -1.1 ± 0.7 mV. deg C-1. Assuming this potential is controlled by the NpO2(CO3)35- + 2 CO2 + e- ↔ Np(CO3)56- equilibrium, the formation constant of the limiting complex is deduced by using published values of the other needed equilibria: log Β5deg = 38 ± 4. Qualitative results on the preparation and on the spectra of Np(IV) are used to explain the apparent contradictions between some published results. (authors). 39 refs., figs., tabs