Oxidation states: of actinide

The equilibria and variety of species discussed above would seem to make studying the specific oxidation states of actinides difficult. However, through careful control of the conditions, oxidation state pure solutions for all of the actinides can be obtained for synthetic, quantitative and/or qualitative studies. 

The formation and stabilization of various oxidation states of actinide positive ions in CaF crystals are described. Paramagnetic resonance and optical spectra are reported for divalent Am and trivalent Cm in these crystals. Tetravalent Cm and Pu, formed as a consequence of the intense alpha radiation, are identified by their optical spectra. 

A number of different investigations on the speciation of actinide ions have been performed by LPAS. However most of them are of qualitative nature as preliminary studies to speciate oxidation states of actinides and to identify their different complexes. The LPAS is used for an on-line measurement of redox reactions by connecting a circulation system between a sample cuvette and an electrochemical cell [27]. With this system different oxidation states of U, Np and Pu generated by the electrochemical cell have been investigated. Another redox reaction investigated is an autoradiolytic oxidation of Am(III) to Am(V) and of Pu(IV) to Pu(VI) in... 

Oxidation states of actinide elements the most stable oxidation state is bold font, while unstable oxidation states are shown in parentheses (Katz et al. 1986]... 

O Table 18.9 gives oxidation states of actinide elements (Katz et aL 1986). Actinium and transplutonium elements (from Am to Lr) take 3+ as the most stable oxidation state and they behave similar to the lanthanide elements, except element 102, No, which seems to prefer the 2+ state. Because of the itinerancy of the 5f electrons, the lighter actinide elements take broad range of oxidation states. The light transuranium elements, Np, Pu, and Am can behave as 3+ to 7+ cations and the most stable oxidation states of these three elements are 5+, 4+, and 3+, respectively. 

Evidence other than that of ion-exchange favours the view of the new elements as an inner transition series. The magnetic properties of their ions are very similar to those of the lanthanides whatever range of oxidation states the actinides display, they always have -1-3 as one of them. Moreover, in the lanthanides, the element gado-... 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Table 6 presents a summary of the oxidation—reduction characteristics of actinide ions (12—14,17,20). The disproportionation reactions of UO2, Pu , PUO2, and AmO are very compHcated and have been studied extensively. In the case of plutonium, the situation is especially complex four oxidation states of plutonium [(111), (IV), (V), and (VI) ] can exist together ia aqueous solution ia equiUbrium with each other at appreciable concentrations. 

Unsubstituted bisphthalocyanines 2 are formed in the presence of several elements which exist in a stable oxidation state of + III or +IV such as titanium, zirconium, hafnium, indium and most of the lanthanide and actinide elements. 

Unusual oxidation states of some actinide and lanthanide elements. L. B. Asprey and B. B. Cunningham, Prog. Inorg. Chem., 1960, 2, 267-302 (245). 

The quantum chemistry of unusual oxidation states of the lanthanides and actinides. V. I. Spitsyn and G. V. Ionova, Russ. Chem. Rev. (Engl. Transl.), 1984, 53, 725 (138). 

The known oxidation states of plutonium present a 5f -series, starting from f1 [Pu(VII)] up to f5 [Pu(III)]. But contrary to the 4f - and 5f series across the period table, where the properties can be described by some smooth varying parameters, changing of the oxidation states influences the electronic properties drastically. Due to the large range of available oxidation states plutonium represents a favorable element among the actinides to study these effects. 

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. 

The above information was used to develop conceptual flowsheets for the extraction of all of the actinides (U, Np, Pu, Am, and Cm) from high-level liquid waste from PUREX processing using 0.4 M 0fuel using 0.8 M DHDECMP in DEB. In both flowsheets, no oxidation state of Pu is necessary since the III, IV, and VI state extract into the organic phase. 

Most of the U-series nuclides are metals. Five of them belong to the actinide family corresponding to the filling of the internal orbitals while the orbitals 7s are filled. A sixth, Ra is an alkali earth and shares some chemical properties with other alkali earths, particularly the heavier ones (Sr and Ba), while a seventh, Rn, is a noble gas. The filling of the orbitals prescribes the possible oxidation states of these elements. Their preferred oxidation state is obtained when the electronic configuration is that of the closest rare gas (Rn). 

Asprey, L. B. and Cunningham, B. B., Unusual Oxidation States of Some Actinide and Lanthanide Elements. 2 267... 

A primary goal of chemical separation processes in the nuclear industry is to recover actinide isotopes contained in mixtures of fission products. To separate the actinide cations, advantage can be taken of their general chemical properties [18]. The different oxidation states of the actinide ions lead to ions of charges from +1 (e.g., NpOj) to +4 (e.g., Pu" " ) (see Fig. 12.1), which allows the design of processes based on oxidation reduction reactions. In the Purex process, for example, uranium is separated from plutonium by reducing extractable Pu(IV) to nonextractable Pu(III). Under these conditions, U(VI) (as U02 ) and also U(IV) (as if present, remain in the... 

Fig. 12.1 Oxidation states of the actinide elements most stable ions in aqueous solutions ++ oxidation states observed in aqueous solutions +, unstable ions observed only as transient species. In solids precipitated from alkaline solutions.

As with most other transuranic elements of the actinide series, fermium has an oxidation state of +3, as well as possibly a +2 oxidation state. Thus, this ion can combine with nonmetals, such as oxygen and the halogens, as do many of the other elements in this series. Two examples follow ... 

Many oxidation states of the actinides are poorly stable or stable only under certain conditions. Great care must thus be taken in preparing samples for relaxometry studies. Working under the same chemical conditions with different actinides in the same oxidation state is sometimes impossible. Plutonium is particularly noteworthy because it is the only element in the Mendeleev table that can exist simultaneously in solution in four different oxidation states. This unusual situation stems from the fact that the ions and PuO have a tendency to undergo dismuta-... 

The preparation and properties of numerous actinide haUdes have been described by D. Brown Although the oxidation numbers of actinides in halides can vary from II to VI, most solid state studies are limited to di-, tri- and tetrahalides. 

When more than one oxidation state of the actinide is present in the oxides, superimposition of the spectral response due to the different ions occurs. 

The oxidation states of the actinide elements are given in Table 8.7. 

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... 

Table 1 Known Oxidation States of the Actinide Elements...

Diselenides, MSe2, and ditellurides, MTe2 (M = Th, U, Pu), are known but the oxidation state of the metal in these compounds is uncertain. Other selenides and tellurides [e.g. Th2Se5, USe3, MTe3 (M = Th, U) and UTe5] are presumably analogous to the actinide(IV) polysulfides (p. 1135). A more detailed description of these systems is available.139... 

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