= Solution
A better controlled route is \b[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. \b[Selectivity comes from combining <oxidation state> and <complexation>, not from assuming that one extraction removes every contaminant.]<Coextraction and successive selective stripping of cerium(IV) and thorium from monazite leach solutions with tributyl phosphate (TBP) by A. H. Ali>[2017, abstract: coextraction from nitric acid and successive <peroxide>/water stripping]{ref}
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