Berkelium stability

Its stability then decreases progressively until we reach curium where aqueous solutions containing the tetra-positive state must be complexed by ligands such as fluoride or phosphotungstate. Even then, they oxidize water and revert to cur-ium(lll). The expected drop in I4 between curium and berkelium provides Bk" (aq) with a stability similar to that of Ce (aq), but the decrease in stability is then renewed, and beyond californium, the +4 oxidation state has not yet been prepared [2, 10, 15]. 

Berkelium exhibits both the III and IV oxidation states, as would be expected from the oxidation states displayed by its lanthanide counterpart, terbium. Bk(III) is the most stable oxidation state in noncomplex-ing aqueous solution. Bk(IV) is reasonably stable in solution, undoubtedly because of the stabilizing influence of the half-filled Sf7 electronic configuration. Bk(III) and Bk(IV) exist in aqueous solution as the simple hydrated ions Bk3+(aq) and Bk4+(aq), respectively, unless com-plexed by ligands. Bk(III) is green in most mineral acid solutions. Bk(IV) is yellow in HC1 solution and is orange-yellow in H2S04 solution. A discussion of the absorption spectra of berkelium ions in solution can be found in Section IV,C. 

The preparation and characterization of intermetallic compounds and alloys of berkelium should be pursued, as well as the determination of the stability constants of Bk(IV) complexes. The range of oxidation states accessible to berkelium might be expanded by stabilizing Bk(II) and/or Bk(V) in highly complexing aqueous, nonaqueous, or even molten salt media and/or in appropriate solid-state matrices. 

Although the outermost electrons in the actinides are apparently more readily available for bonding than those in the lanthanides, even occurring, the preference for increases with atomic number. Redox potentials indicate that this trend is still more rapid than in the lanthanides. The fact that Cm is dominant (cf, gadolinium in the lanthanides), and that berkelium gives also Bk, lends support to the suggestion that these elements have a half-filled 5f shell with its characteristic stability. 

Many of the actinoids are also separated by exploiting their redox behavior. Thorium is exclusively tetravalent and berkelium is chemically similar to cerium, so iodate precipitation of Th and extraction of Bk(IV) with bis(2-ethylhexyl)orthophos-phoric acid (HDEHP) are used to isolated these elements. The differing stabilities of the (III), (IV), (V), and (VI) states of U, Np, and Pu have be exploited in precipitation and solvent extraction separations of these elements from each other and from fission product and other impurities with which they are found. Because of its technical importance, the process chemistry to separate U and Pu in nuclear materials has been highly developed. Extraction of Bk(IV) with HDEHP is used to separate Bk from neighbouring elements. 

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... 

This is the only important oxidation state for Th, and is one of the two for which U is stable in aqueous solution it is moderately stable for Pa and Np also. In water Pu, like Pu, dispropor-tionates into a mixture of oxidation states III, IV, V and VI, while Am not only disproportionates into Am -I- Am 02" " but also (like the strongly oxidizing Cm ) undergoes rapid self-reduction due to its a-radioactivity. As a result, aqueous Am and Cm require stabilization with high concentrations of F ion. Berkelium(IV), though easily reduced, clearly has an enhanced stability, presumably due to its f configuration, and the only other -1-4 ion is Cf, found in the solids CfFq and Cf02. 

The electronic configurations 5f or 4f representing the half-filled f shells of curium and gadolinium, have special stability. Thus, tripositive curium and gadolinium, are especially stable. A consequence of this is that the next element in each case readily loses an extra electron through oxidation, so as to obtain the f structure, with the result that terbium and especially berkelium can be readily oxidized from the III to the IV oxidation state. Another manifestation of this is that europium (and to a lesser extent samarium) -just before gadolinium - tends to favor the 4f structure with a more stable than usual II oxidation state. Similarly, the stable f electronic configuration leads to a more stable than usual II oxidation state in ytterbium (and to a lesser extent in thuUum) just before lutetium (whose tripositive ion has the 4f structure). This leads to the prediction that element 102, the next to the last actinide element, will have an observable II oxidation state. 

The quantity of information available for the berkelium oxygen system is far less than that for the previous actinides, due mainly to the scarcity and the shorter half-life (325 days) of the Bk-249 isotope used in solid-state studies. The sesquioxide and the dioxide are the two well-established oxides of Bk, and the dioxide is much more stable than the dioxides of Cm and Am. This stability can be attributed in part to the fact that the Bk(IV) state results in a half-filled 5f state, as found for TbOj. However, BkOj is much more stable and is formed more easily than is TbOj. 

In comparing berkelium and terbium, the situation with their configurations seems clearer both form a half-filled f orbital in their dioxides, although there is a large difference in ease of formation and stability of these two dioxides. The dioxide of Bk is very stable and readily forms in air, whereas the formation of TbOj requires highly oxidizing conditions and Tb02 is less stable thermally. 

As was the case for the previously discovered transuranium elements, element 97 was first produced via a nuclear bombardment reaction. In December 1949 ion-exchange separation of the products formed by the bombardment of Am with accelerated alpha particles provided a new electron-capture activity eluting just ahead of curium [1,2]. This activity was assigned to an isotope (mass number 243) of element 97. The new element was named berkelium after Berkeley, California, the city of its discovery, in a parallel manner to the naming of its lanthanide analog, terbium, after Ytterby, Sweden. The initial investigations of the chemical properties of berkelium were limited to tracer experiments (ion exchange and co-precipitation), but these were sufficient to establish the stability of Bk(iii) and the accessibility of Bk(iv) in aqueous solution and to estimate the electrochemical potential of the Bk(iv)/Bk(iii) couple [2,3]. 

In solution, the range of oxidation states accessible to berkelium should be further examined by using strong complexing agents in an effort to stabilize Bk(ii), Bk(iv), and possibly Bk(v), produced chemically or electrochemically in non-aqueous or molten-salt media. New organometallic complexes of Bk(iii)... 

Berkelium is the first member of the second half of the actinide series of elements. Extended knowledge of the stability and accessibility of the various oxidation states of berkelium is important to the understanding and predictability of its physicochemical behavior. In addition, such information would enable more accurate extrapolations to the physicochemical behavior of the transberkelium elements for which experimental studies are severely limited by lack of material and/or by intense radioactivity. 

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]. 

Desire, Hussonnois and GuUlaumont (1969) determined stability constants for the species AnOH + for the actinides, plutonium(III), americium(III), curium (III), berkelium(III) and californium (III) using a solvent extraction technique. The stability constants obtained for americium(III) and curium(III) are two orders of magnitude larger than other similar data available in the literature. The stability constants of the lanthanide(III) and actinide(III) ions are very difficult to obtain using solvent extraction due to problems associated with attainment of maximum extraction into the solvent phase before the narrow band of pH between the onset of hydrolysis reactions and the precipitation of solid hydroxide phases. Consequently, the data of Desire, Hussonnois and GuUlaumont (1969) are not retained in this review. 

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