Spectrophotometric and electrochemical study of neptunium ions in molten NaCl–CsCl eutectic

The chemical oxidation states of NpO2+, Np4+ and Np3+ in NaCl–CsCl eutectic were controlled by using Cl2, O2, H2 and Ar gas mixtures, the redox behavior and electronic absorption properties of their Np ions were studied. The Np4+ was prepared from NpO2Cl by bubbling Cl2 gas into the melt in the presence of carbon rod. Np3+ was quantitatively prepared by bubbling H2–Ar gas mixture. The molar absorptivities of NpO2+, Np4+ and Np3+ were determined in molten NaCl–CsCl eutectic at 923 K and hypersensitive transitions of Np4+ and Np3+ ions were assigned. Since the polarizing ability of the cations in the NaCl–CsCl eutectic is lower than that in some other melts, it has been shown that the coordination symmetry of the Np–Cl complex is higher.In the electrochemical measurement of Np4+, the cathodic current for the reduction of Np4+ was found to be controlled by the diffusion of Np4+. The temperature dependence of the diffusion coefficient between 823 and 923 K was formulated to be lnDNp4+=-4304/T-6.172lnDNp4+=-4304/T-6.172. The formal redox potential of the Np4+|Np3+ couple depended on the temperature, this dependence was formulated as ENp4+|Np3+∘′=-1.313+6.210×10-4TV (vs. Cl2|Cl−).