The Determination of Formation Constants for Uranyl(VI) Complexes in Aqueous Solution by Spectroscopic Techniques

10:00 AM - 11:00 AM
Robert D. Hancock, The University of North Carolina, Wilmington
Chemical Sciences Division Seminar
Chemical and Materials Sciences Building (4100), Room C-201
Email: Sheng Dai

The aqueous solution chemistry of the UO22+ cation is discussed briefly, with regard to its hydrolysis behavior, and its general ligand affinities. Some personal studies of the UO22+ cation, and the stability of some of its complexes are discussed, with particular attention to the role of preorganization in complex stability and metal ion selectivity. The determination of formation constants for UO22+ complexes in aqueous solution can be approached spectroscopically by absorbance, fluorescence, and NMR spectroscopy. Absorbance is particularly useful for ligands with aromatic groups such as benzamidoxime (Bzam) that have an intense absorbance spectrum with bands in the 200-350 nm region. The variation of band intensity of Bzam as a function of pH at 2 x 10-5 M is used to determine its pKa values, and also to study its interaction with UO22+, VO2+, and VO3-. The changes in the absorbance spectra on complexation  with Bzam are quite small, and so to gain further information, the d-d spectra of a 1:1 VO2+/bzam solution were analyzed to confirm the results from the absorbance studies. The UO22+ cation has fluorescence bands in the 450-600 nm region, which become particularly intense for the UO2(OH)+ species. It is found that binding of Bzam to UO22+ greatly quenches the fluorescence, but the pH region where this occurs confirms the log K1 value for the UO2/Bzam complex measured  by absorbance. Ligands such as acetamidoxime (AAO) and carboxypropyl amidoxime (CPA) have no aromatic groups to provide absorbance or fluorescence spectra. An excellent technique for studying their protonation constants and formation constants with UO22+ is NMR, which is discussed. The implications of the formation constants of these amidoxime ligands for recovering uranium from seawater is discussed, as are some possible further directions in ligand design for the selective recovery of uranium.


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