Uranium potential diagram

The release of uranium and thorium from minerals into natural waters will depend upon the formation of stable soluble complexes. In aqueous media only Th is known but uranium may exist in one of several oxidation states. The standard potential for the oxidation of U in water according to equation (2) has been re-evaluated as E° - 0.273 0.005 V and a potential diagram for uranium in water at pH 8 is given in Scheme 3. This indicates that will reduce water, while U is unstable with respect to disproportionation to U and U Since the Earth s atmosphere prior to about 2 x 10 y ago was anoxic, and mildly reducing, U " would remain the preferred oxidation state in natural waters at this time. A consequence of this was that uranium and thorium would have exhibited similar chemistry in natural waters, and have been subject to broadly similar redistribution processes early in the Earth s history. Both U " and Th are readily hydrolyzed in aqueous solutions of low acidity. A semiquantitative summary of the equilibrium constants for the hydrolysis of actinide ions in dilute solutions of zero ionic strength has been... 

The following potential diagram summarizes the results of electrochemical studies of the aqueous solution (pH 0) chemistry of uranium ... 

In the remaining sections, we focus on the chemistries of thorium and uranium (the actinoids for which the most extensive chemistries have been developed) and plutonium. Potential diagrams for Np, Pu and Am are included in Figure 24.6. 

The incorporation of anions, as for example, S04 , CO2-, etc., makes leaching possible through the formation of stable uranyl (VI) oxyanions. In sulfate leaching, an observation of the potential-pH diagram for the uranium system reveals that uranium species in solution may be in the form of cations U02+, neutral species U02(S04)2 or anions U02(S04)4-. The oxidation of uraninite, U02, in acid solutions, transforming U(IV) to U(VI), yields soluble uranyl sulfate through the reaction as shown below ... 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

Figure 25.7 Potential-pH diagram for the uranium system in the pH region 0-4. [Adapted from Ref. 2, p. 585.]...

Fig. 2 Potential-pH diagram of the uranium-water system at 298 K with SnC>2 conduction band edge overlaid. Dissolved uranium activity= 0.01SCE Saturated calomel electrode...

From the Frost diagram in Figure 23.15, it can be seen that the most stable oxidation state of uranium in aqueous acid is U (i.e., it has the most negative free energy of formation, the quantity plotted on the y axis). However, the reduction potentials for the U02 /U02 and U02 /U couples are quite small, +0.170 and +0.38 V, respectively (hence, the U02 /U potential is +0.275 V). Therefore, b ause the O2/H2O reduction pwtential is 1.229 V, the most stable uranium ion is U02 if sufficient oxygen is present ... 

A somewhat similar approach can also be used for the mixed self energy - vacuum polarization diagrams of Fig. 13. The detailed evaluation of these graphs is presented by Lindgren et al. [59] and we report only the result of their calculation here, which for the lsi/2-state of uranium yields 1.12 eV in the Uehling approximation (no Wichmann-Kroll vacuum polarization potential included in the Dirac equation) and 1.14 eV by taking into account the Wichmann-Kroll potential also [7]. 

Other analytical techniques. Electroanalytical methods can also be used to differentiate between ionic species (based on valence state) of the same element by selective reduction or oxidization. In brief, the electroanalytical methods measure the effect of the presence of analyte ions on the potential or current in a cell containing electrodes. The three main types are potentiometry, where the voltage difference between two electrodes is determined, coulometry, which measures the current in the cell over time, and voltammetry, which shows the changes in the cell current when the electric potential is varied (current-voltage diagrams). In a recent review article, 43 different EA methods for measuring uranium were mentioned and that literature survey found 28 voltammetric, 25 potentiometric, 5 capillary electrophoresis, and 3 polarographic methods (Shrivastava et al. 2013). Some specific methods will be discussed in detail in the relevant chapters of this tome. 

Mikheev (1988,1989,1992) has obtained extensive evidence through cocrystallization that almost all the tripositive lanthanide (except for Sm, Eu, Tm and Yb) and actinide ions (U, Np, Pu, Cm, Bk) can be reduced and have (M /M ) in the neighborhood of — 2.5 to — 2.9 V. The lanthanide potentials are not consistent with the experimentally confirmed generalized f electron energetics scheme developed by Nugent (1975). The potentials are not consistent with potentials inferred from pulse-radiolysis studies (Sullivan et al. 1976,1983,1988). If the potentials (M /M ) proposed by Mikheev for uranium, —2.54 V, and plutonium, —2.59 V, at macroscopic concentrations (Mikheev etal. 1991) were correct, phase diagram studies and electrochemistry ih molten salts should have revealed the ions and Pu ", but no evidence other than cocrystallization has been presented. In fact, the crystal chemistry of the reduced uranium halide NaUjCl (Schleid and Meyer 1989) is consistent with ions and metallic electrons. The cocrystallization model (Mikheev and Merts 1990) may not be transferable to aqueous solution and thus Mikheev s (M /M ) potentials are not cited in table 5. 

---