Cerimetry

Cerimetry or cerimetric titration, also known as cerate oximetry, is a method of volumetric chemical analysis developed by Ion Atanasiu. It is a redox titration in which a Fe²⁺-1,10-phenanthroline complex (ferroin) color change indicates the end point. Ferroin can be reversibly discolored in its oxidized form upon titration with a Ce⁴⁺ solution. The use of cerium(IV) salts as reagents for volumetric analysis was first proposed in the middle of 19th century, but systematic studies did not start until about 70 years later. Standard solutions can be prepared from different Ce⁴⁺ salts, but often cerium sulfate is chosen.

Since cerimetry is linked to the Fe³⁺/Fe²⁺redox pair, it can be used for analyses of nonstoichiometric levels that either oxidize Fe²⁺ or reduce Fe³⁺. For the case of oxidation, a precise excess of high-purity crystalline Mohr’s salt is added upon the oxide digestion in aqueous hydrogen chloride (HCl), while for the case of reduction, an excess of 1 M iron trichloride (FeCl₃) is added. In the both cases, this is Fe²⁺ ions which will be titrated subsequently. Because the Ce⁴⁺ solution is prone to hydrolysis, the titration is done in a strongly HCl-acidic solution into which some phosphoric acid (H₃PO₄) is added to obtain a less colored phosphato complex of Fe³⁺.

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