Protactinium , complex fluorides

Complex fluorides containing tetravalent protactinium and the alkali fluorides (Li, Na, K, Rb, and NH4) were prepared from MF -j- PaF4 starting mixtures and by reducing MF-PaF compounds at 400°—500°C. using pure hydrogen, obtained from UH3. The x-ray results on compounds obtained by these methods are presented in Table I. 

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. 

The element is difficult to maintain in aqueous solution in the form of simple salts. Solubility data seem to indicate that such amounts as can be dissolved probably do so entirely by formation of complex ions. Fluoride ion strongly complexes protactinium, and it is due to this that protactinium compounds are in general soluble in hydrofluoric acid. 

The protactinium(IV) fluoro complexes have been prepared either by hydrogen reduction of a pentavalent complex at 400°C or by heating together appropriate amounts of MF and PaF4 in sealed vessels. The reaction between ammonium fluoride and protactinium tetrafluoride to yield (NH4)4PaFg, the only octafluoro complex known, takes place when the component halides are ground together at room temperature (4,114). 

A recent study of the alkali metal/uranium fluoride complex systems has shown that KriUFo, K3UF7, and K UFs are isomorphous (99), the crystal symmetry being unaffected by the fluoride ion absences. The structure of K PaFy has been determined only recently (39), and it has been shown that each protactinium atom is surrounded by nine fluorine atoms in what is effectively a trigonal prism with three added equatorial fluorine atoms the PaFo groups are linked in infinite chains by two fluorine bridges. 

Table I. Alkali Fluoride Complexes of Tetravalent Protactinium...

Table IV. Some Alkali Fluoride Protactinium ( V ), Uranium ( V ), Neptunium(V), Plutonium(V) Fluoride Complexes"...

Binary halides. A number of homoleptic halides of pentavalent protactinium, uranium, and neptunium have been reported. In particular, the fluoride complexes AnFs are prepared by high... 

Protactinium(V) octafluoro complexes, MlPaF, have been prepared (5, 35, 37, 59) by precipitation from aqueous hydrofluoric acid (M = Li, Na, and Rb), by heating the alkali metal fluoride with the corresponding heptafluoro complex in argon (M = K and Cs) or by fluorinating the product obtained by evaporation of a hydrofluoric acid solution containing 3 1 mixtures of MF and Pa(V). 

Figure 10.26 compares the low-concentration distribution coefficients of uranium, thorium, plutonium, protactinium, and the principal fission products. The spread between thorium and fission-product zirconium is greatest between 1 and 2 M HNO3, the range used in the decontamination step of the acid Thorex process. Because the distribution coefficient of protactinium is close to that of thorium, it is necessary to remove protactinium or complex it with fluoride or phosphate ion to prevent its extraction with thorium. 

Protactinium can be recovered from solutions 2-8M in nitric or hydrochloric acids by extraction with tributyl phosphate, isobutyl methyl ketone or other organic solvents. The protactinium can be stripped from the solvent by aqueous acid fluoride solutions the addition to these solutions of Al3 + ion or boric acid, which form stronger complexes with fluoride ion than... 

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