Protactinium pentoxide

The preparation of the metal was first reported by von Grosse (80) who obtained it by bombarding protactinium pentoxide with 35 keV electrons in a high vacuum and by decomposing the pentachloride on a hot wire. No properties were reported for these products and more recently the pure metal has been obtained by reduction of protactinium tetrafluoride with lithium (73) or barium (65,125) vapor at 1300°-1400°C using the double crucible technique and on a larger scale by reduction with barium (106) or 10% magnesium in zinc alloy (107). 

Protactinium is separated by solvent extraction and anion exchange processes by using sulfate solutions. After chemical separation, the protactinium salts are ignited to a pentoxide, Pa205, which may be converted into an arsenazo(III) complex. The absorbance of the solution is measured at 630 nm with a spectrophotometer. Protactinium-231 is an alpha emitter and also forms photons at 300 KeV, which can be measured by various radioactive counters and spectrophotometric techniques. Protactinium also can be measured by neutron activation analysis. 

The probable existence of protactinium was predicted as early as 1871 by Mendeleev to fill up the space on his peiiodic table between thorium (at, no. 90) and uranium (at. no, 92). He termed the unconfirmed element ekatantalum. In 1926, O. Hahn predicted the properties of the element in considerable detail, including descriptions of its compounds. In 1930, Aristid v. Grosse isolated 2 milligrams of what then was termed ekatantalum pentoxide and showed that element 91 differed m all reactions with comparable amounts of tantalum compounds with exception of precipitation by NH3. However, credit for the discovery of protactinium generally is attributed to Lise Meitner and Otto Hahn in 1917,... 

Protactinium oxides can be stabilized in the tetravalent and pentavalent state. The most stable oxide phase obtained by the burning of metal or protactinium compoimds is the white pentoxide, Pa20s. The structme of the pentoxide is related to fluorite and has cubic symmetry. Pa02 is a black solid that crystallizes in the cubic fluorite structure. 

The results of solid state reactions of protactinium dioxide and pentoxide with other metal oxides (89, 93-96) support the view that the oxide systems of protactinium resemble those of other actinide elements rather than those of niobium and tantalum. However, when assessing results of this type one must always bear in mind the relative ionic radii of the respective M " and M + ions since they obviously play a large part in determining the structures of the complex phases. This comment applies equally well, of course, to the structural properties of other types of compound and in particular to the high coordination numbers exhibited by protactinium(V) in its chloro and nitrato complexes. 

Attempts to prepare protactinium pentanitrate by reacting penta-halides with liquid dinotrogen pentoxide have resulted in the formation of HPalNOglfl, possibly as a result of traces of anhydrous nitric acid present in the N Os 49). The presence of the jiroton has not been confirmed by electron spin resonance studies, but infrared results have shown that all the nitrate is covalently bound and vibrations associated with the nitronium and nitrosonium cations were not observed. Niobium and tantalum pentahalides react under similar conditions to form the anhydrous oxytrinitrates, M 0(N0g)3 20, 87). 

No binary sulfide has been reported, but the bright yellow oxysulfide, PaOS, isostructural with other actinide oxysulfides, has been obtained (125) by heating protactinium pentachloride or pentoxide in a mixture of carbon disulfide and hydrogen sulfide at 900° and 1200°C, respectively... 

It is well known that protactinium( V) can be precipitated from oxalic acid solution by the addition of hydrochloric acid. The resulting white solid has recently been reported (111, 112) to have the composition Pa0(0H)C204. TH.20 (1.5 <. r < 4.0). Protactinium oxygen stretching vibrations observed at 778 om indicate the presence of Pa-O-Pa groups (112), but there is no evidence for the presence of discrete Pa=0 groups. It is insoluble in dilute hydrochloric and oxalic acid solutions it dissolves in 8 ilf HCl and is unstable in air, decomposing to the pentoxide at 340°C. 

A few derivatives of oxyanions have also been prepared protactinium (V) forms hexanitrato complexes, Pa(N03)e , by reaction of the hexachloro complex with dinitrogen pentoxide, in contrast to niobium and tantalum which, under similar conditions, yield only tetranitrato complexes, M0(N03)4 (35). Neptunium (V) nitrates, NPO2NO3 and Np0(N03)3 have also been reported 71). Protactinium (V) sulfato-and selenato complex acids, H3PaO( 804)3 and H3Pa0(Se04)3, have been obtained from aqueous solution (13), but no fully sulfated or sele-nated species have been recorded. 

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