DETERMINATION OF E0 FOR THE UO2(2+)/U4+ COUPLE FROM MEASUREMENT OF THE EQUILIBRIUM - UO2(2+)+CU(S)+4H+=U4++CU2++2H2O AT 25-DEGREES-C AND SOME GEOCHEMICAL IMPLICATIONS

The standard potential (E0) of the UO2(2+)/U4+ couple was obtained from the equilibrium constant (K(eq)) for the reaction UO2(2+) + Cu(s) + 4H+ = U4+ + Cu2+ + 2H2O, measured in 0.3-2.6 m HClO4-NaClO4 media at 25-degrees-C. K(eq) was computed from gravimetric and titrimetric solution analyses, avoiding the uncertainties associated with liquid junction corrections to measured potential and pH measurements in these high ionic strength, acid solutions. Bronsted-Guggenheim-Scatchard specific ion interaction (SIT) theory [1] was used to correct all measurements and calculations made at elevated ionic strengths to infinite dilution. The relative importance of the UOH3+ complex in the computation of E0 depends upon the choice of the equilibrium constant, K11, for the reaction U4+ + H2O = UOH3+ + H+, and of the specific ion interaction (epsilon) parameter for U4+ with perchlorate ion used in SIT calculations. The constant, K11 = 10(-0.65 +/- 0.04), proposed by Baes and Mesmer [2] was confirmed by reanalysis of published data for the hydrolysis reaction, measured in below 0.55 m ionic strength perchlorate media. The epsilon-(U4+, ClO4-) value most consistent with our study results is 1.42, assuming which mUOH3+ = 8-51% of total U(IV) in our experiments. The corresponding corrected and preferred value of E0 is 0.263 +/- 0.004 V, where the +/- denotes the standard deviation of the results. This compares with E0 = 0.2674 +/- 0.0015 V reported by Bruno et al. [3] based upon their work in perchlorate media, and the work of others in perchlorate, chloride and sulfate media, all corrected for hydrolysis, however assuming K11 = 10(-0.54 +/- 0.20), and epsilon-(U4+, ClO4-) = 0.76 +/- 0.03. These results differ from E0 = 0.273 +/- 0.005 V proposed earlier by Fuger and Oetting [4] and adapted by Langmuir [5]. Assuming that E0 = 0.263 +/- 0.004 V for the UO2(2+)/U4+ couple, and for the UO2(2+)/UO2+ couple, that E0 = 0.0879 +/- 0.0013 V based on the reevaluation of Grenthe et al. [1], we conclude that the 4+ and 5+ oxidation states of uranium are less important in geological environments than previously thought, relative to more mobile UO2(2+) species.