Uranium, standard reduction potentials

There are very few reports in the literature concerning heterogeneous photocatalysis for uranium treatment in water. In our previous review, only one case of photocatalytic reaction on uranium salts was reported (Amadelli et al., 1991). Taking into account the standard reduction potentials, U(VI) can be photocatalytically reduced by Ti02 conduction band electrons to U(V) and then to U(IV) (E° = +0.16 V and +0.58 V, respectively, Bard et al., 1985). However, more reduced U(III) and U(0) forms cannot be generated because of very negative redox potentials (Bard et al., 1985). In addition, U(V) rapidly disproportionates to U(VI) and U(IV), and its chemistry is very complex (Selbin and Ortego, 1969). For example, uranyl... 

In comparison with uranium, the (-1-3) state has become much more stable (Table 9.5), witness the standard reduction potential of -1-0.98 V for Pu" + -I- e -> Pu +, the comparative value for uranium being —0.63V. This means that quite strong oxidizing agents such as manganate(vii) are needed to effect this oxidation in the case of plutonium. 

The standard reduction potentials are -1.798 for the uranium half-reaction and —2.714 for the magnesium half-reaction, (a) Will this semp work spontaneously (b) Calculate the voltage produced by this cell as written. 

Since only a few standard aqueous electrode potentials °(Ln /Ln ) have been determined electrochemically, others have been estimated [e.g., (Dy /Dy ) (Morss and Fahey 1976, Morss and Spence 1992)] from calorimetric measurements on their dichlorides or from spectroscopic correlations. The only experimental values of °(An /An " ) are from radioelectrochemical measurements on Fm-No (David 1986a) and coprecipitation studies (Mikheev 1988,1992). Spectroscopic correlations of (An /An ) have also been made (section 3.4). These reduction potentials have been assessed for the rare earths (Morss 1985), for the lighter actinides (Martinot and Fuger 1985), and for the heavier actinides (David 1986a, Morss 1986). With the exception of uranium, neptunium and plutonium species [(Fuger 1992, table 1), discussed in section 2.4.1] and No + (discussed in section 2.4.1), these potentials are still valid. We note that the (An /An) values depend on the S°[An (aq)] which are referenced to a value for Pu (aq) that is based in part on experiment but also required estimation of S°[PuClj 6H20(s)]. 

Uranium tetrafluoride is a key intermediate in the production of thermal reactor fuels. It may be prepared directly from uranyl solutions by reduction of the U to U and addition of HF to precipitate UF4. A number of processes have been developed to produce UF4 by this wet route, which may be used to produce UF4 at the ore processing site. These employ iron, S02/Cu or electrolysis for the reduction step, the latter being preferred since it introduces no contaminants into the solution. The reduction of u in various media has been studied to assess the effect of complexation on the reduction reaction. The standard potential for the reduction of U02 to in 1 M HCIO4 has been given as +0.32 V. " The overall formation constants of fluoride complexes in 1 M NaCl were found to be log 82= 13.12, logj83 = 17.46 and log/84 = 21.8. Although wet processes have been developed as a short cut to UF4, the most widely used process at present involves dry processing. 

In general, formal potentials are reported with reference to the standard hydrogen electrode, cf. Section 11.1.6.5, as exemplified in Tables V.2 and V.3 of the uranium NEA review [92GRE/FUG]. In that case, the appearing in the reduction reaction is already at standard conditions. For example, experimental data are available on the formal potentials for reactions ... 

The standard red-ox potential of tellurium in alkali chloride melts is more positive than that of palladium. Addition of elemental tellurium to NaCl-CsCl-U02Cl2 melt at 550 °C resulted in a quite slow uranium reduction reaction. After 180 min the oxidation state of uranium decreased to 5.92 and the concentration of uranium in the melt from 0.85 to 0.79 wt%. The absorption spectra contained two well pronounced bands corresponding to the uranyl(V) complex, U02Cl4 . 

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