Deconvolution of composite fluorescence spectra and related decay curves for Uv' in the pH ranges &5 and 9-1 1 at different total U concentrations, yielded decay times and individual fluorescence spectra for various hydroxocomplexes (U02),(0H),c2" '-") , where (m, n ) = (1, 1); (1, 3); (2, 2) and (3, 5). Depending on the emission wavelength, the fluorescence efficiencies of (2, 2) and (3, 5) were found to be, respectively, 7 to 85, and 3 to 4 times higher than that of t h e uranyl ion, whereas the latter is ca. 3 times more fluorescent than t h e (1, 1) complex. Thus, relatively small concentrations of polynuclear species contribute appreciably to t h e overall fluorescence of aqueous Uv' solutions. The fairly large difference between decay times and the high sensitivity of t h e equipment used made it possible to detect even small amounts of t h e hydroxo complexes. In the alkaline pH range 10-12, t h e (1, 3) hydroxo-complex was found to predominate. In t h e presence of 0.05 mol dm-3 phosphate and in the same pH range, there was a pronounced change in the fluorescence spectrum indicating that the chemical speciation in the system had changed. These observations make it necessary to revise the current equilibrium data for uranium(v1) phosphate complexes at high pH.

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