Protactinium aqueous chemistry

Lundqvist, R. (1974) Aqueous chemistry of protactinium(IV). Stability constants for Pa(IV)-acetylacetone complexes. Acta Chem. Scand., 28A, 243-247. 

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. 

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. 

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