Protactinium V

However, the quantity of Pa produced in this manner is much less than the amount (more than 100 g) that has been isolated from the natural source. The methods for the recovery of protactinium include coprecipitation, solvent extraction, ion exchange, and volatility procedures. AH of these, however, are rendered difficult by the extreme tendency of protactinium(V) to form polymeric coUoidal particles composed of ionic species. These caimot be removed from aqueous media by solvent extraction losses may occur by adsorption to containers and protactinium may be adsorbed by any precipitate present. 

The actinide elements exhibit uniformity in ionic types. In acidic aqueous solution, there are four types of cations, and these and their colors are hsted in Table 5 (12—14,17). The open spaces indicate that the corresponding oxidation states do not exist in aqueous solution. The wide variety of colors exhibited by actinide ions is characteristic of transition series of elements. In general, protactinium(V) polymerizes and precipitates readily in aqueous solution and it seems unlikely that ionic forms ate present in such solutions. 

The extensive hydrolysis of protactinium in its V oxidation state makes the chemical investigation of protactinium extremely difficult. Ions of protactinium(V) must be held in solution as complexes, eg, with fluoride ion, to prevent hydrolysis. 

Table 49 Some Hydrates of Protactinium(V), Uranium(V), Neptunium(V) and Plutonium(V)...

Hydroxides and oxides. Protactinium(V), neptunium(V) and plutonium(V) hydroxides precipitate from alkaline aqueous solutions of the actinides(V) the last two appear to be of the form Mv02(0H)-xH20. 

Phosphates. Some hydrated phosphates and phosphatocomplexes are included in Table 49, p. 1180. Hydrated protactinium(V) phosphate, Pa0(H2P04)3-2H20, dehydrates at 250 °C and decomposes to Pa0(P03)3 at 700 °C, then to (Pa0)4(P207)3 at 1000 °C. The structures of these compounds are not known. 

The only recorded complexes appear to be the protactinium(V) compounds, Pa(Et2NCS2)4X (X = Cl, Br), prepared by treating a suspension of the pentahalide in dichloromethane with an excess of Na(Et2NCS2), followed by vacuum evaporation of the filtrate. 

The protactinium(V) complexes of composition PaXsL (X = Cl, Br R = Ph3PS, (Ph2PS)2CH2) have been recorded. 

A few salts of composition (Et4N)2[MvOX5] (Mv = Pa, X = C1, Br MV = U, X = F) are known the uranium(V) compound is obtained from the dihydrate under vacuum, and the protactinium(V) chlorocompound is prepared by hydrolysis of the hexachloro complex salt in methyl cyanide containing 0.5% water. Salts of composition (Et4N)[Pa(OEt)2X4j (X = Cl, Br) are also known. 

Niobium and tantalum(V) chlorides and tantalum(V) bro-midef yield the complexes MX6-CHSCN, and protactinium(V) bromide yields orange PaBr8 3CH8CN if the same procedure is used. 

As mentioned above, protactinium(V) oxytribromide is invariably obtained as a by-product during the preparation of the pentabromide. It is also formed (32, 45) when stoichiometric amounts of the penfa-bromide and either oxygen [Eq. (6)] or antimony sesquioxide [Eq. (7)] are heated together in a sealed vessel at 350°C. Although it is found with... 

Protactinium(V) oxytribromide possesses monoclinic symmetry (Table III). The structure (53, 54) comprises chains of protactinium atoms linked by bridging bromine atoms and cross-linked by 3-coordinate oxygen atoms. Each protactinium atom is 7-coordinate (Pig. 5) and protactinium-bromine bond lengths lie within the range 2.69-3.02 A. It would be interesting to have structural information on the relatively unstable uranium(V) oxytribromide since the limited X-ray powder... 

Protactinium(V) is stable in aqueous hydrofluoric acid at quite high concentrations, but irreversible hydrolytic condensation occurs in other halogen acid solutions even at protactinium(V) concentrations of the order of 10 to 10 M. Consequently, only fluoroprotactinates(V) have been prepared in aqueous solution. Raman studies (97) have established... 

Although protactinium(V) chloro complexes can also be prepared using methyl cyanide as the solvent (32) (cf. PaBrg" and Pale ), the use of thionyl chloride has several advantages. Thus, it means that one can start with protactinium(V) hydroxide and not the pentachloride, thionyl chloride itself affords protection against atmospheric moisture and dry-atmosphere boxes are not necessary for the preparations, and, in addition, protactinium(V) concentrations up to 0.5 M have been obtained by dissolving the hydroxide in thionyl chloride (75). Such solutions are quite stable in contrast to the hydrolytic condensation reactions which occur in concentrated hydrochloric acid at Pa(V) concentrations as low as 10 M. 

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. 

Although it is known that protactinium(V) precipitates from dilute sulfuric acid on the addition of potassium sulfate, the product, believed to be K3Pa0(S04)3, has not been satisfactorily characterized (19, 28, 105). There is obviously scope for further research on the preparation of sulfato (and selenato) complexes of pentavalent protactinium. In addition, tetravalent protactinium sulfates, selenates, and their complexes have not been studied. 

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. 

Protactinium(V) dioxymonofluoride, PaOgF, is obtained by thermal decomposition of Pa20Fg in air at 270°C (Bagnall et al.). It is a white, air-stable solid which decomposes to another, as yet unidentified, oxyfluoride at 450°-470°C which, in turn, decomposes to PagOg above 650°C. Other papers on the protactinium halides describe a new, im-... 

Attempts to characterize the pentavalent iodate obtained by the addition of iodic acid to a solution of protactinium(V) in sulfuric acid (Muxart et al.) have indicated that the product contains a variable amount of iodate (PailOa between 1 0.75 and 1 2.0) and more than twelve molecules of water. The infrared vibration associated with the iodate was observed as a broad band between 600 and 870 cm . The white solid slowly decomposes with the liberation of iodine. The pentavalent phosphate, which precipitates from sulfuric acid solution on the addition of orthophosphoric acid is reported to have the composition Pa0(0H)P04 H20 (Lecloarec and Muxart). Although infrared studies confirmed the presence of the P04 ion it was not possible to unambiguously identify a band due to the Pa=0 group [cf. H3Pa0(S04)3 (19)]. 

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