Infrared spectroscopic studies of aqueous solutions of dioxouranium(VI) and its hydrolysed products and of in situ electro-generated dioxouranium(V)

Aqueous solutions of dioxouranium(VI) (pH range 0 to 4) give rise to bands at 954 and 938 cm⁻¹ attributable to the v₃(MO₂) stretching modes of the UO₂²⁺ and (UO₂)₂(OH)_2²⁺ cations, respectively. A shoulder at 916 cm⁻¹ is assigned to the v₃(MO₂) mode of hydrolysed dioxouranium(VI) species of higher nuclearity. Infrared spectro-electrochemical studies using a thin-layer reflection-absorption cell have facilitated the study of the reduction of aqueous solutions of dioxouranium(VI) to yield dioxouranium(V) which may be further reduced to uranium(IV). The electrogeneration of dioxouranium(V) is monitored by following the increase in intensity of a band at 914 cm⁻¹ which is present in the spectra at potentials between −0.2 and −0.8 V. The dioxouranium(V) species is predominantly in the form UO₂⁺, which may be in solution or incorporated into an insoluble phase of uranium oxides which deposit onto the working electrode. The U^VO bond length is estimated to be 1.76 Å, 0.03 Å longer than the U^VIO bond in aqueous solution. The maximum concentration of UO₂⁺able to be achieved is highly dependent on the pH and is optimum at pH 3.4. Changes in the pH of the solution under study can be monitored by infrared spectroscopy during the course of the reduction by determining the relative concentrations of hydrolysed dioxouranium(VI) species.