Formation-decomposition equilibrium of the NpO 2 + ·UO 2 2+ complex in chloride melts

Abstract

The absorption spectra of the NpO 2 + (5f 2) ion were examined in the region of the 3H 5 ← 3 H 4 magnetic dipole transition (1530–1760 nm) for series of melts with the UO 2 2+ concentration varied in the opposite directions: (1) NaCl-2CsCl eutectic melt with growing additions of the Cs2UO2Cl4 complex salt and (2) Cs2UO2Cl4 melt with growing additions of the NaCl-2CsCl mixture. Measurements of the integrated intensities of the bands belonging to the NpO 2 + ·UO 2 2+ complex and unbound NpO 2 + throughout the UO 2 2+ concentration range examined (up to 4.4 M in neat Cs2UO2Cl4 melt) and processing of the data obtained in terms of the mass action law showed that the formation-decomposition reaction of the cation-cation complex can be described adequately only using the equation of reaction in the form NpO2Cl 4 3− + UO2Cl 4 2− ⇆ {Cl4ONpO⋯UO2Cl3}4− = Cl− (with the equilibrium constant of 1.3±0.1). Thus, the formation of the cationcation complex should be treated as replacement of chloride ion in the equatorial plane of uranyl(VI) by neptunyl(V), rather than as simple addition of UO 2 2+ to NpO 2 + . The reverse reaction, decomposition of the cation-cation complex, consists essentially in replacement of neptunyl(V) by chloride ion.