Behaviour of the uranyl (UO 2 2+ ) ion in different strongly acidic media: Characterisation of UO 2 2+ in common acids by electronic absorption spectroscopy

Abstract

The linear uranyl (UO₂²⁺) ion has been characterised by electronic absorption spectroscopy in the media of four highly concentrated acids: sulfuric, nitric, acetic, and hydrochloric acids. In sulfuric acid, the magnitude of splitting (cm¹) of the vibronic peaks in the UO₂²⁺ spectra increased linearly as molar concentration of the acid increased from zero to 18 M. Such an unprecedented spectral progression has been attributed to the high concentrations of sulfuric acid reducing the extent of coordination of water molecules to the central uranium atom of UO₂²⁺. This is believed to reduce the electron density in the uranium valence shell and decrease the electrostatic repulsion between the uranium and oxygen valence shells, making the U=O bonds stronger. It results in the observed increase in splitting of the vibronic peaks. In nitric acid, a completely different spectral feature of uranyl was observed. First, as molar concentration of nitric acid increased, the vibronic structure in the UO₂²⁺ spectra disappeared gradually and the absorption band became continuous at 6 M. As the acid concentration further increased, vibronic lines were resolved in the lower-energy side of the spectra. The higher-energy side remained continuous with no vibronic structure observed. The convergence limit (cm¹), the borderline between vibronic peaks in the lower-energy side and a continuous band in the higher-energy side, decreased linearly as the molarity of nitric acid increased from 8 M to 16 M. All this shows that nitric acid, with involvement of its conjugate nitrate anion, has made UO₂²⁺ dissociative beyond the convergence limit of its excited state. In acetic acid, as the molar concentration of the acid increased from zero to 17.5 M, the UO₂²⁺ spectra exhibited three-stages of progression due to a predominating coordination of a carboxyl group to UO₂²⁺ ([HOAc] = 0–9 M), dissociation of UO₂²⁺ at the convergence limit of its excited state ([HOAc] = 10–14 M), and formation of an undissociative excited state for UO₂²⁺ ([HOAc] = 15–17.5 M). In hydrochloric acid, UO₂²⁺ exhibited a comparable spectral feature to that observed in nitric and acetic acids due to a predominating coordination of the chloride anion to UO₂²⁺.