Actinide elements reduction potentials

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. 

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

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 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... 

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

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). 

Reduction potentials for the actinide elements are given in Table IX. The and the MO2+/MOJ cou-... 

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 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... 

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... 

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]. 

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