Protactinium solution chemistry

Protactinium-231, an a-emitter, is the only isotope suitable for macrochemical studies [fj/2 = 32,340 years (36)] and in view of the radiochemical hazards associated with weighable amounts of this isotope, it is necessary to perform all manipulations in glove boxes or, in the case of solution chemistry, in well-ventilated fume hoods. An indication of the toxicity of protactinium-231 is given by the fact that the maximum permissible concentration in air is 10 mg/m whereas that of hydrogen cyanide is 10 mg/m. Details of suitable handling procedures are adequately dealt with in other publications (11, 136). 

The chemical properties span a range similar to the representative elements in the first few rows of the periodic table. Francium and radium are certainly characteristic of alkah and alkaline earth elements. Both Fr and Ra have only one oxidation state in chemical comhina-tions and have little tendency to form complexes. Thallium in the 1+ oxidation state has alkah-like properties, but it does form complexes and has extensive chemistry in its 3+ state. Similarly, lead can have alkaline earth characteristics, hut differs from Ra in forming complexes and having a second, 4+, oxidation state. Bismuth and actinium form 3+ ions in solution and are similar to the lanthanides and heavy (Z > 94) actinides. Thorium also has a relatively simple chemistry, with similarities to zirconium and hafiuum. Protactinium is famous for difficult solution chemistry it tends to hydrolyze and deposit on surfaces unless stabilized (e.g., by > 6 M sulfuric acid). The chemistry of uranium as the uranyl ion is fairly simple, hut... 

With the exception of thorium and protactinium, all of the early actinides possess a stable +3 ion in aqueous solution, although higher oxidation states are more stable under aerobic conditions. Trivalent compounds of the early actinides are structurally similar to those of their trivalent lanthanide counterparts, but their reaction chemistry can differ significantly, due to the enhanced ability of the actinides to act as reductants. Examples of trivalent coordination compounds of thorium and protactinium are rare. The early actinides possess large ionic radii (effective ionic radii = 1.00-1.06 A in six-coordinate metal complexes),and can therefore support large coordination numbers in chemical compounds 12-coordinate metal centers are common, and coordination numbers as high as 14 have been observed. 

The solution photochemistry of the actinides begins with uranium none has been reported for actinium, thorium, and protactinium. Spectra have been obtained for most of the actinide ions through curium in solution (5). Most studies in actinide photochemistry have been done on uranyl compounds, largely to elucidate the nature of the excited electronic states of the uranyl ion and the details of the mechanisms of its photochemical reactions (5a). Some studies have also been done on the photochemistry of neptunium (6) and plutonium (7). Although not all of these studies are directed specifically toward separations, the chemistry they describe may be applicable. 

Nobelium is a member of the actinide series of elements. The ground state electron configuration is assumed to be (Rn)5fl47s2, by analogy with the equivalent lanthanide element ytterbium ([Kr]4fl46s2) there has never been enough nobelium made to experimentally verify the electronic configuration. Unlike the other actinide elements and the lanthanide elements, nobelium is most stable in solution as the dipositive cation No ". Consequently its chemistry resembles that of the much less chemically stable dipositive lanthanide cations or the common chemistry of the alkaline earth elements. When oxidized to No, nobelium follows the well-estabhshed chemistry of the stable, tripositive rare earth elements and of the other tripositive actinide elements (e.g., americium and curium), see also Actinium Berkelium Einsteinium Fermium Lawrencium Mendele-vium Neptunium Plutonium Protactinium Ruthereordium Thorium Uranium. 

Protactinium as 231Pa occurs in pitchblende, but even the-richest-ores contain only about 1 part of Pa in 107. The isolation of protactinium from residues in the extraction of uranium from its minerals is difficult, as indeed is the study of protactinium chemistry generally, owing to the" extreme tendency of the compounds to hydrolyze. In aqueous solution, polymeric ionic species and colloidal particles are formed, and these are carried on precipitates and adsorbed on vessels in solutions other than those containing appreciable amounts of mineral acids or complexing agents or ions such as F , the difficulties are almost insuperable. 

Protactinium. No efforts have been made to achieve high solubilities of protactinium in order to use it as a component of reactor fuel solutions. Rather, the chemistry of protactinium has been examined in order to devise processes for removing Pa continuously from thorium breeder blanket systems. A project was undertaken by the Mound Laboratories [28] to separate gram quantities of the longer-lived Pa which could be used in studies of the chemistry of protactinium. 

---