COMPLEX-FORMATION IN THE U(VI)-OH(-)-F(-) SYSTEM

Complex formation in the ternary system U(VI)-OH--F- has been studied in a 3.00 M NaClO4 ionic medium. Only binary complexes, (UO2)2 (OH)22 (log beta2,2 = -5.98 +/- 0.02); (UO2)3 (OH)5+ (log beta3,5* = -16.23 +/- 0.05); UO2Fn2-n (log beta(n) = 4.86 +/- 0.02, 8.62 +/- 0.04, 11.7 1 +/- 0.06 and +/- 13.78 +/- 0.08, respectively, for n = 1-4), were fo rmed. The equilibrium constants refer to 25-degrees-C. By combining th e equilibrium data in 3.00 M NaClO4 with previously reported data obta ined in 1.00 M NaClO4, the medium dependence of the equilibrium consta nts has been estimated using the specific ion interaction model of Bro nsted, Guggenheim and Scatchard. By using F-19 NMR spectroscopy it was possible to demonstrate the formation of UO2F53-, with the formation constant K5 almost-equal-to 3 M-1, at -4-degrees-C for the reaction UO 2F42- + F half arrow right over half arrow left UO2F53-. The chemical shifts at -4-degrees-C for the various UO2F(n)2-n complexes were 165, 143, 123, 104.5 and 83 ppm for n = 1-5, respectively (referred to 0.1 M NaF in 3.00 M NaClO4 at 25-degrees-C). At higher temperatures there was a significant exchange broadening of the spectra. There was no evi dence for the formation of ternary complexes (UO2)p (OH)q F(r) in the concentration available for experimental study [this was limited owing to precipitation of UO2(OH)2(s)]. From the NMR data it is possible to estimate an upper limit of the equilibrium constants of (UO2)2 (OH)2F + and (UO2)3(OH)5 F, if such complexes are formed.