Reduction Potentials of the Actinides

Although a full range of ionization energies is not available throughout the actinide series and thus cannot be used predictively, as for the lanthanides (Table 2.2), electrode potentials are more generally available (Table 9.5) and thus can be so used, as follows  

The negative reduction potentials for the M +/M potentials indicate that the process  

The large negative values of for M +/M + for Th and Pa indicate that reduction to form (+3) species for these metals will be difficult, whilst for U, Np, and Pu the smaller E° values indicate that both the +3 and +4 states will have reasonable stability. However, from Am onwards, E° 2V for all these elements (except Bk), suggesting that for all these metals the (+3) state will be more favoured (as observed). The tendency for the EE values for both M +/M +and M +/M + to become more positive, and for the reduction to be more favoured, on crossing the series from left to right, shows that overall the lower oxidation states become more stable the higher the atomic number. 

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

Formal Reduction Potentials of the Actinides for IM Perchloric Acid Solutions at 25° (in volts brackets [ ] indicate estimate)... 

The reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

Comparison of the tetravalent states of the two families is more limited. Tetravalent cations can be obtained for a number of the actinide elements, although the (IV) state is quite unstable for the heaviest members of the series (Sullivan et al. 1976, Propst and Hyder 1970). The chemistry of the An(IV) species shows a general resemblance to that of Ce(IV). The rate of reduction follows the order Bk thermodynamic stability as predicted from the reduction potentials of the couples (Nugent et al. 1971). 

Fig. 13.2 Correlations of the lll-ll reduction potentials for the actinides and lanthanides. The points are experimental values for the actinides.

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). 

Fig. 5a. Standard (or formal) reduction potentials of actinium and the actinide ions in acidic (pH 0) and basic (pH 14) aqueous solutions (values are in volts...

This is remarkable, since the reduction potential of Th(IV) to Th(III) recently has been estimated as —3.7 volts 73) and direct reduction of U(C5H5)4 and Pu(C5Hs)3 with potassium metal produces the actinide metals. The ei/z for naphthalene in acetonitrile is —2.63 V (nearly the same as the aLkaJi metals). Since this is much smaller than the Th(IV) to Th(III) reduction potential, it would seem to imply substantial stabilization of the +3 state by cyclopentadienide. The observed room temperature magnetic moment of Th(C 5115)3 (0.403 BM) is consistent with the Th(III) (5/ ) assignment. Thorium triscyclopentaxhenide is similar in behavior to U(C5H5)3, forms adducts with both THF and cyclohexyhso-nitrile and has been shown to be isostructural with the other tris (cyclopentadienyl) actinides and lanthanides. 

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. 

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)... 

The reduction of the potential long term hazard of radioactive wastes generated by the nuclear fuel cycle has been the objective of European and USA R D programmes. Several conceptual and experimental studies on the feasibility of the actinide separation from radioactive wastes (waste partitioning) have been developed at various national (USA, Sweden) laboratories (1—4). Experimental investigations in this field were also carried out until 1977 in France, at the CEA laboratories (5). 

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-... 

In view of the position of Cm in the actinide series, numerous experiments have been made to ascertain if Cm has only the +3 state in solution no evidence for a lower state has been found. Concerning the +4 state, the potential of the Cm4+/Cm3+ couple must be greater than that of Am44/ Am3+, which is 2.6 to 2.9 V, so that solutions of Cm4+ must be unstable. When CmF4, prepared by dry fluorination of CmF3, is treated with 15M CsF at 0°, a pale yellow solution is obtained which appears to contain Cm4+ as a fluoro complex. The solution exists for only an hour or so at 10° owing to reduction by the effects of a-radiation its spectrum resembles that of the iso-electronic Am34 ion. 

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. 

The divalent state is of major importance and has attracted the interest of many experimenters since 1967 when the appreciable stability of this state was first recognized in the actinides. The ni - ii reduction potential of einsteinium was first estimated to be - 1.6 V from the lowest-energy electron-transfer band [57]. A later estimate of - 1.21 V was given for chloroaluminate melts [58], as well as another estimate of the standard potential of —1.18 V [54]. Mikheev and co-workers identified Es(ii) from the co-crystallization of einsteinium tracer with SmCl in an ethanol solution [59]. Einsteinium was only partially reduced to the II state by SmCl2, which allowed them to conclude that the standard reduction potential ofEs " was close to that of Sm ", or —1.55 0.06 V [60]. An ionic radius of 0.105 nm was estimated from the radius of maximum electron density obtained in Hartree-Fock calculations, which was then corrected to obtain the crystalline radius by an empirical proportionality constant [61]. 

All actinides from thorium to californium form tetravalent oxidation states. For the three elements of highest atomic number, however, viz. americium, curium, and berkelium, the hydrated ions are too strongly oxidizing to be stable in aqueous solution [7,10]. Their rates of reduction nevertheless vary widely, in the order Bk + < Am < Cm + < Cf, with Bk" being by far the most resistant species. This is also the order of thermodynamic stability, as indicated by the oxidation potentials of the couples [11]. 

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... 

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