Complexation of uranium(VI) with aromatic acids in aqueous solution – A comparison of hydroxamic acids and benzoic acid

The complex formation of uranium(VI) with salicylhydroxamic, benzohydroxamic, and benzoic acid in 0.1 M NaClO4 was studied by UV-vis spectroscopy at pH 3 and 4. Uranium(VI) species of the type MpLqHr were identified from the UV-vis spectra in all three systems. An increase in the absorption combined with a blue shift of the absorption maxima in comparison to the bands of the free uranyl ion of 22.5 ± 2 nm was observed in the uranium (VI)-salicylhydroxamic acid-system. Besides indications for a 1:2 complex, the formation of a 1:1 complex with a stability constant of log β111= 17.12 ± 0.10 could be demonstrated by its individual absorption spectrum and molar absorption coefficient. Also in the uranium(VI)-benzohydroxamic acid-system a blue shift of the absorption maxima in comparison to the bands of the free uranyl ion of 27 ± 1.4 nm indicate the complex formation. The stability constants are log β110 = 7.96 ± 0.05 for UO2[C6H4CONHO]+ and log β120 = 15.25 ± 0.11 for UO2[C6H4CONHO]2. In contrast to the hydroxamic acids, benzoic acid shows a red shift of the absorption maxima of 2.5 ± 2 nm. Only the 1:1 complex UO2[C6H4COO]+ with a stability constant of log β110 = 3.37 ± 0.14 is existent. An estimate is made in order to discuss the dependencies observed in the absorption spectra in relation to possible coordination modes of uranium(VI). The strength of the complex formation between uranyl and the three aromatic acids is discussed.