Protactinium, element 91, is one of the most rare of the naturally occurring elements and may well be the most difficult of all to extract from natural sources. Protactinium is, formally, the third element of the actinide series and the first having a 5f electron. The superconducting properties of protactinium metal provide clear evidence that Pa is a true actinide element (Smith et al., 1979). Its chemical behavior in aqueous solution, however, would seem to place it in group VB of the Mendeleev’s table, below Ta and Nb.
The predominant oxidation state is 5+. Pa(V) forms no simple cations in aqueous solution and, like Ta, it exhibits an extraordinarily high tendency to undergo hydrolysis, to form polymers, and to be adsorbed on almost any available surface. These tendencies undoubtedly account for the many reports of erratic and irreproducible behavior of protactinium as well as for its frustrating habit of disappearing in the hands of inexperienced or unwary investigators. A useful review of the chemical properties of Pa important in an analytical context has been made by Pal’shin et al. (1970) and Myasoedov et al. (1978).
The most important natural isotope is 231Pa, but the industrial importance of Pa stems chiefly from the role of its artificial isotope, 233Pa, as an intermediate in the production of fissile 233U in thorium breeder reactors. It was, in fact, the need for a relatively stable isotope that could be used for macroscopic chemical studies, which was responsible for the revival of interest in the recovery of 23IPa from natural sources (Katzin, 1952). The result has been a rapid growth in our understanding of Pa chemistry, as summarized in numerous critical review articles.
KeywordsThermal Ionization Mass Spectroscopy Actinide Element Pure 231Pa Phenylarsonic Acid Weighable Amount
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