= Solution
A natural feed can contain <thorium> and <uranium>, as well as other <lanthanides>. Th(IV) hydrolyses readily and can accompany the cerium precipitate; <uranium> speciation depends on its <oxidation state> and the ligands present. Raising <pH> too far also precipitates other rare-earth hydroxides. Even before their bulk <precipitation (chemistry)> thresholds, adsorption and <coprecipitation> on a high-area hydrated cerium oxide can contaminate the solid. Local high-pH zones during base addition and incomplete Ce(III) oxidation make the separation less reproducible.
Thus \b[a cerium-rich precipitate need not be pure cerium]. A sensible test measures the Ce <oxidation state>, analyses both filtrate and washed solid, and includes thorium-containing and thorium-free mixtures; otherwise low dissolved cerium could be mistaken for selective purification.
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