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

Abstract The chemical oxidation states of Np O 2 + , Np 4+ and Np 3+ in NaCl–CsCl eutectic were controlled by using Cl 2 , O 2 , H 2 and Ar gas mixtures, the redox behavior and electronic absorption properties of their Np ions were studied. The Np 4+ was prepared from NpO 2 Cl by bubbling Cl 2 gas into the melt in the presence of carbon rod. Np 3+ was quantitatively prepared by bubbling H 2 –Ar gas mixture. The molar absorptivities of NpO 2 + , Np 4+ and Np 3+ were determined in molten NaCl–CsCl eutectic at 923 K and hypersensitive transitions of Np 4+ and Np 3+ 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 Np 4+ , the cathodic current for the reduction of Np 4+ was found to be controlled by the diffusion of Np 4+ . The temperature dependence of the diffusion coefficient between 823 and 923 K was formulated to be ln D Np 4 + = - 4304 / T - 6.172 . The formal redox potential of the Np 4+ |Np 3+ couple depended on the temperature, this dependence was formulated as E Np 4 + | Np 3 + ∘ ′ = - 1.313 + 6.210 × 10 - 4 T V (vs. Cl 2 |Cl − ).