2.8: Cerium

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Cerium(IV)-based chemiluminescence systems involve the reduction of cerium(IV), which suggests that the emitter is a cerium(III) species. The chemiluminescence reaction is carried out in an acidic medium (generally sulfuric acid) and has been applied for the determination of substances of biological interest^[1]. A few pharmaceuticals in dosage forms can reduce the cerium(IV) and produce luminescence directly. As a result, many flow-injection-chemiluminescence methods have been established for such species as naproxen, acetaminophen and fluphenazine hydrochloride. The sensitivity of the assays can be improved by increasing cerium (IV) concentration. Almost all cerium(IV) chemiluminescence systems need sensitization procedures to transfer the excited-state energy to a sensitizer, which then emits light of greater intensity. Thus most determinations involving cerium(IV) as the oxidant are indirect, based on the enhancement of chemiluminescence of the cerium(IV)-sulfite system by some analytes. This type of process is used to determine reducing compounds, such as cortisone, ofloxacin, nomoxacin, ciprofloxacin, lomefloxacin, flufenamic acid, mefenamic acid and salicylic acid.

Cerium(IV) chemiluminescence systems are very popular to determine sulfur-containing substances such as sodium-2-mercaptoethane, tiopronin, captopril, menadione sodium bisulfite and some sulfur-substituted benzamides but also other substances such as paraben, phenolic compounds (by LC), phentolamine, barbituric acid and erythromycin. In addition, light emission resulting from the chemical reaction of cerium(IV) with some mercapto-containing compounds in pharmaceutical preparations can be enhanced by certain fluorometric reagents such as quinine, rhodamine B and rhodamine 6G or by lanthanide ions such as terbium(III) and europium(III). Thus, a range of flow-injection chemiluminescence methods have been developed for determination of compounds of this kind.