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 ³⁻₄ + UO₂Cl ²⁻₄ ⇆ {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.