Actinides oxidation-reduction potentials

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

Oxidation-reduction potentials of the actinides. The formal potentials for transition between the valence states of the actinides are listed in Table 9.6. 

Table 9.7 Formal oxidation-reduction potentials for actinides and fission products in acid solutions ...

Reduction potential data for the actinide elements, including law-rencium, are given in Table 8.8 for the well-characterized oxidation states. 

The ability of the fight actinides to access multiple oxidation states leads to rich, and, sometimes, complex electrochemistry. The standard reduction potentials at pH = 0 for each of the... 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

The formal reduction potentials of actinide ions in aqueous solution are given in Table 28-6, from which it is clear that the electropositive character of the metals increases with increasing Z and that the stability of the higher oxidation states decreases. A comparison of various actinide ions is given in Table 28-7. It must be noted also that, for comparatively short-lived isotopes decaying by a-emission or spontaneous fission, heating and chemical effects due to the high level of radioactivity occur in both solids and aqueous solu-... 

R. E. Connick, "Oxidation States, Potentials, Eqxailibria, and Oxidation-Reduction Reactions of Plutonium."in The Actinide Elements, Natl. Nucl. Energy Series, Div. 

Radioelectrochemistry. Radioelectrochemistry has been used extensively to determine the oxidation-reduction behavior of heavy actinides. The earliest studies were those of Maly (1967), Maly and Cunningham (1967), Hulet etal. (1967), David (1970) and David etal. (1978). These pioneering studies have been summarized in papers by David (1986a) and Silva (1986). Most recently, radiocoulometry was used by David et al. (1990) to estimate the No /No electrode potential as — 2.49 0.06 V. 

At a time when little was known about ionization potentials of lanthanide ions as well as about thermochemistry of non-tripositive lanthanide speeies, Johnson (1969a) showed that differences in the third ionization potentials /j of the lanthanides are primarily responsible for many of their apparent oxidation-reduction anomalies. In a subsequent paper (Johnson 1969b) he compared and contrasted the relative stabilities of the di-, tri- and tetrapositive oxidation states of the lanthanides and actinides, pointing out how much less is the change in ionization potential for actinides than lanthanides at the half-filled shell (see fig. 4). He elaborated (Johnson 1974) on the first paper by systematizing the properties of the dipositive lanthanide ions in conjunction with those of the alkaline-earth metal ions. 

These trends become even more marked in the thermodynamic equilibrium parameters of the two types of reaction (Tables 21.21 and 21.22). The values of AS° are much the same for all oxidations, just as they are for all oxidations. But while the values of AS are very negative in the latter reactions, on account of the formation of strongly solvated ions, they are very positive in the former ones, on account of the disappearance of ions. These very favorable changes of AS° are strongly counteracted, however, by unfavorable changes of AH°. The oxidations are all exothermic, the M oxidations all endothermic. On balance, these conditions create a mixture of large and small, negative and positive, values of AG in both types of reactions. This of course illustrates the intricate oxidation-reduction pattern of the actinide elements, also reflected in their oxidation potentials (Chapter 17). 

Note that AG° q has a liquid-phase standard state of 1 moI/L, and we can use a gas-phase standard state of either 1 atm or 1 mol/L, as long as we use the same convention for the oxidized and reduced forms. Often the 1 mol/L standard state is used. There are several sources of uncertainties in the calculations of reduction potentials, and we will comment on them by scanning the published literature on actinide elements, for which the most studied redox systems are actinyl aqua ions, with the exception of one study on Pu(VII)/Pu(VIII) [172], The first comment is that redox potentials are defined with respect to the standard hydrogen electrode corresponding to the following half-equation... 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

The standard electrode potentials, E, for such reduction reactions are related to the free energy change for the process by equation 5.3. Since some elements may exist in a number of different oxidation states, it is possible to construct electrode potential diagrams, sometimes called Latimer diagrams, relating the various oxidation states by their redox potentials. Examples are shown in Figure 5.6 for aqueous solutions of some first-row d-block metals and for some actinides in 1 mol dm acid. In cases where the reduction involves oxide or hydroxide ions bound to... 

In addition to the aqueous raffinates from the solvent extraction cycles of the Purex process, an actinide bearing waste stream will arise from the washing of the TBP/OK solvent prior to its recycle to the first cycle. These wastes will typically contain actinides in a mixed NajCOs/NaNOs solution which also contains HjMBP and HDBP. The uranium present will form soluble U complexes with carbonate, as discussed in Section 65.2.2.l(i). Carbonate complexation of Pu also leads to solubility in alkaline solutions and in Na2C03 media precipitation did not occur below pH 11.4, although precipitates did form on reduction to Pu One Pu" species precipitated from carbonate media has been identified as Pu(0H)3-Pu2(C03)3 H20. In 2M Na2C03 media, Np is oxidized by air to Np above pH 11.7 while Np either precipitates or is reduced above pH 13. The potential of the Am /Am " couple, in common with those of other actinides, becomes more cathodic with increasing carbonate concentration. In the total bicarbonate plus carbonate concentration range 1.2-2.3 M all the americium oxidation states from (III) to (VI)... 

Polarographic studies gave no evidence for the existence of the bivalent oxidation states of selected actinides in acetonitrile solution. Only one wave corresponding to reduction of americium(iii) or curium(iii) to the zero-valent state was observed and experiments with berkelium(iii) and einsteinium(iii) failed to give conclusive results because of rapid radiolysis of the acetonitrile solution. A study of the electrochemical reduction of americium, thulium, erbium, samarium, and europium showed that the elements did assume the bivalent state with the actinide bivalent cations having a smaller stability than the lanthanides. The half-wave potential of nobelium was found to be —1.6 V versus the standard hydrogen electrode for the reaction... 

Diffusion of Tc and other fission products such as Cs and -Eu as well as the actinides Np, Am and natural uranium was studied in a sample of a sediment from an enclosed brackish water bay of the Baltic Sea. Under oxidizing conditions TcOq did not interact with the sediment to any large degree. Deeper laying sediments were depleted of oxygen and showed negative redox potentials. In this case the apparent diffusivity of Tc 0 =5-10 m -s was low compared to compacted clay with >a=8 -10 " m- s", indicating a reduction of TcOq [30]. 

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