Actinide elements solvent extraction behavior

Ion exchange (qv see also Chromatography) is an important procedure for the separation and chemical identification of curium and higher elements. This technique is selective and rapid and has been the key to the discovery of the transcurium elements, in that the elution order and approximate peak position for the undiscovered elements were predicted with considerable confidence (9). Thus the first experimental observation of the chemical behavior of a new actinide element has often been its ion-exchange behavior—an observation coincident with its identification. Further exploration of the chemistry of the element often depended on the production of larger amounts by this method. Solvent extraction is another useful method for separating and purifying actinide elements. 

In over 600 experiments, Maly et al. subjected about 50 000 atoms of No to cation-exchange and co-precipitation studies [51]. These tracer experiments showed that nobelium exhibits a chemical behavior substantially different from the trivalent actinides but similar to the divalent alkaline-earth elements, Sr, Ba, and Ra. Thus, the divalent ion of nobelium is the most stable species in aqueous solution. Additional experiments, which used the technique of solvent extraction chromatography, were performed under a variety of oxidizing conditions and allowed an estimate of +1.45 V for °(No - No ) [53]. 

Investigations may be carried out on the tracer level, where solutions are handled in ordinary-sized laboratory equipment, but where the substance studied is present in extremely low concentrations. Concentrations of the radioactive species of the order of 10 m or much less are not unusual in tracer work with radioactive nuclides. A much larger amount of a suitably chosen non-radioactive host or carrier is subjected to chemical manipulation, and the behavior of the radioactive species (as monitored by its radioactivity) is determined relative to the carrier. Thus the solubility of an actinide compound can be judged by whether the radioactive ion is carried by a precipitate formed by the non-radioactive carrier. Interpretation of such studies is made difficult by the formation of radiocolloids, and by adsorption on glass surfaces or precipitates. Tracer studies provide information on the oxidation states of ions and complex-ion formation, and are used in the development of liquid-liquid solvent extraction and chromatographic separation procedures. Tracer techniques are not applicable to solid-state and spectroscopic studies. Despite the difficulties inherent in tracer experiments, these methods continue to be used with the heaviest actinide and transactinide elements, where only a few to a few score atoms may be available [11]. 

The first study of the solution chemistry of Rf was performed at Berkeley in 1970 and showed that Rf had a stable tetravalent state with properties similar to the gronp-4 elements Zr and Hf and different from Lr and the other trivalent actinides. This established that Rf shonld be placed in the Periodic Table as the heaviest member of group 4 and the first member of a new 6d transition series. It also confirmed the 1945 prediction of Seaborg that the actinide series should end with element 103. The first studies of the solution chemistry of element 105, condncted at the 88-Inch Cyclotron at Berkeley, were reported in 1988 and showed that the element behaved similarly to the group-5 elements Nb and Ta in its sorption properties, bnt differently from the group-4 elements. However, in extractions into certain organic solvents, Db(Ha) and Ta extracted but Nb did not, creating a renaissance of interest in more detailed studies of the behavior of element 105. 

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