Ammonium cerium sulfate, REDOX

Other REDOX reagents include iodine (I2), either by itself in a forward titration or in a back titration with sodium thiosulfate (Na2S2Os), and complex salts of the metal cerium (such as ammonium cerium sulfate,... 

Oxidation states higher than + 3 are exhibited by Ce, Pr, and Tb, but only Ce4+ is stable (kinetically) in water. It is a very strong oxidizing agent in aqueous solution ( ° = 1.74 V) and is used as a volumetric standard in redox titrations. Some of its salts [e.g., cerium(IV) ammonium nitrate, cerium(IV) sulfate] find application in... 

Fluoride ions forms very stable complexes with cerium(IV) and lowers the redox potential considerably. A cerium(IV) sulfate solution is not able to oxidize iodide to iodine when appreciable amounts of fluoride ions are present. The most important fluoro complex is hexafluo-rocerate(IV), [CeFe] ", which can be formed by addition of ammonium fluoride to a solution of cerium(IV). The behavior of cerium(IV) in presence of fluoride ions is markedly different from that of cerium(III). Addition of an ammonium fluoride solution to a solution containing cerium(III) results in the precipitation of gelatinous cerium(III) fluoride, CeF3, which becomes powdery upon standing (Svehla, 1979). The use of cerium(IV) fluoride as a fluori-nating agent is discussed in sections 5.16 and 6.7. 

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