 Solution

A better controlled route is selective solvent extraction in acid, followed by selective stripping, rather than precipitating everything near overlapping hydrolysis thresholds. Oxidize Ce(III) to Ce(IV) in a controlled acidic nitrate medium, extract its nitrate complexes into an organic donor phase, and separate the phases. Then reduce the extracted Ce(IV) to Ce(III) and back-extract it into an aqueous phase. Trivalent rare earths are less strongly extracted under suitable conditions.

This still needs a thorium separation: tributyl phosphate can coextract Th(IV). One demonstrated strategy deliberately coextracts Ce(IV) and Th(IV), then uses reducing peroxide solution to strip Ce as Ce(III), followed by a separate thorium strip. The appropriate controls are distribution measurements for each ion at the chosen acid concentration, a cerium oxidation-state check, and a complete mass balance over extraction and stripping. Selectivity comes from combining oxidation state and complexation, not from assuming that one extraction removes every contaminant.^[ref]