Pulse radiolysis actinides

Radiation chemistry also made it possible to prepare radicals and ions of interest and study their properties. With the advent of pulse radiolysis, it was possible to directly explore the reactivity of such intermediates. In fact, many reactions that were suggested to be of importance in solar energy conversion could be more cleanly studied using radiation chemistry. Similarly, questions about the mobility of actinide species in the biosphere often depended on the reactivity of different oxidation states of materials such as plutonium. Thus, it was possible to show that plutonium oxides were unlikely to move quickly through water in the earth, because the soluble oxides were very reactive and the equilibrirrm values were far to the side of the insoluble compounds. 

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. 

In general, lower oxidation states are stabilized by acidic conditions, while higher oxidation states are favored by basic media. A point of difference between divalent lanthanides and actinides is that Eu (4f is more stable [the standard potential of the Eu /Eu coupleis -0.35 V(Rard 1985)] than Yb (4f )[JE (Yb +A b ) = -1.05 V (Morss 1994)] whereas Am (4f ) is less stable than No " " (41 ) (Penneman and Mann 1972, Penneman et al. 1971). In fact, Am " does not exist as a stable species in aqueous solution in spite of its half-filled structure. Pulse-radiolysis measurements indicate that its half life in solution is about 5 ms (Sullivan et al. 1976). 

The redox chemistry of Am(II), Cm(II) and Cf(II) has been studied by pulse-radiolysis techniques. The relative stability of these divalent actinides is Cf(II)> Am(II)>Cm(II). There is no obvious correlation in this order and the electronic configurations or the ionic radii of these elements (Nash and Sullivan 1986). 

Two accounts have been presented of the mechanisms of chemical oscillators. The cerium(iv)-catalysed oxidation of malonic acid by bromate serves as a model for a conceptual approach and in the second article other examples involving both homogeneous and heterogeneous processes are described. Two reviews have been published of radiation chemistry of metal ions in aqueous solution. - In one article, details are presented of reactions of main-group and first-, second-, and third-row transition metals and lanthanides and actinides. Meyerstein covers somewhat similar ground but deals with complexes in low, intermediate, and high oxidation states. The pulse radiolysis technique has recently been used to provide... 

In Table 14.4, the most stable states are shown in bold type and the most unstable states are indicated by parentheses. (Oxidation states that have been claimed to exist, but not independently substantiated, are indicated with question marks.) The most unstable oxidation states have only been observed in solid compounds, or produced as transient species in solution by pulse radiolysis [17-20]. In this very interesting technique, a beam of electrons is injected into an aqueous solution of the ion under investigation. These have been mainly the 3 + actinide ions. When N2O is present in the reaction mixture, the hydrated electrons formed by the injection of the electrons into water are converted into OH radicals, which are strong oxidants. If t-butanol is present in place of nitrous oxide, the OH radicals are scavenged, and only the hydrated electron, e (aq), a powerful reducing agent, is formed. The reactions of these reagents with actinide ions is followed spectrophotometrically. Reaction of the in ions in 0.1 m perchloric add with e (aq) forms Am(ii), Cm(ii), and Cf(ii). When OH radicals react, Am(iv) and Cm(iv), but no Cf(iv), are produced. All of the 2+ and 4 + spedes are transient. The ii spedes disappear with rate constants of about 10 s by what appears to be a first-order process. Am(ii), Cm(ii), and Cf(ii) have half-lives of the order of 5-20 /is Am(iv) appears to be appredably more stable and... 

Gordon, S., J.C. Sullivan, W.A. Mulac, D. Cohen, and K.H. Schmidt, 1976, Pulse Radiolysis of the Lanthanide and Actinide Elements, in Proc. Fourth Symposium on Radiation Chemistry, Keszthely, Hungary. Gruber, J.B. and J.G. Conway, 1960, J. Inorg. Nucl. Chem. 14, 303. 

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