Complexes of protactinium

Extraction by means of oxine is illustrated in Fig. 21.7. The percentage extracted is given as a function of pH. The value of pH is commensurate with log[AA] as long as [HAA] is practically constant (pH — log[AA] = log — log[HAA]). Protactinium(v) and neptunium(iv) are strongly preferred to the trivalent actinides (and even more to radium (ii)). At high values of pH (i.e. [AA]), anionic complexes of protactinium(v) are formed in the aqueous phase, resulting in a steep decrease in extraction. 

In studying the complexation of protactinium(IV) with acetylacetone, Lundqvist (1974) indicated that in solution the metal ion was present as PaO " (or equally PafOH) )- However, Lundqvist also noted that this behaviour was not present in other tetravalent metal ions such as Zr(IV), Hf(IV) or other actinide(IV) ions. As shown in Section 9.2.3, protactinium(V) exists as PaO ", a behaviour that is also not followed by other actinide(V) ions. The observations of Lundqvist (1974) differ from those of Guillaumont (1965, 1968) who indicated the formation of three protactinium(IV) hydrolysis species, PaOH, Pa(OH)2 and PafOHlg, with respective stability constants of log = -0.14, log 2 = -0.52 and log = -1.77 in measurements carried out in 3.0 mol 1 LiClO. Given that there are no supporting data for either of these interpretations, no data are retained for protactinium(IV). 

---