assay of ferrous sulphate by cerimetry

Assay of Ferrous sulphate by Cerimetry

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Cerimetry or cerimetric titration, also known as cerateoximetry.It is a redox titration in which a Fe2+-1,10-phenanthroline complex (ferroin) color change indicates the endpoint. (Ferroin) can be reversibly discolored in its oxidized form upon titration with a Ce4+ solution.
Standard solutions can be prepared from different Ce4+ salts, but often cerium sulfate is chosen.

Procedure:

Since cerimetry is linked to the Fe3+/Fe2+ redox pair, it can be used for analyses of non-stoichiometric 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, 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-acidic condition (Ceric solution in sulphuric acid does not oxidize chloride and can be used to titrate HCl solutions of analytes).

2FeSO4 + 2Ce(SO)4.2(NH4) 2 SO4  →  Fe2 (SO4)3+ Ce2(SO4)3+ 2(NH4)2SO4

The redox potentials were measured using the combined redox electrode (Pt–Ag/AgCl)(Refer potentiometric titrations & electrodes) in equimolar Ce4+/Ce3+ solutions in sulfuric acid electrolyte. The Ce4+/Ce3+ redox potentials significantly decrease (i.e.shift to more negative values) with rising sulfuric acid concentration.

Ceric ammonium sulphate is a powerful oxidizing agent in the acid medium. It is bright yellow in colour both in solid and in the solution state. In the titration ferrous sulphate reacts with ceric ammonium sulphate and the corresponding ferrous salt formed is colorless and acts as a self indicator oxidizes ferrous sulphate to ferric sulphate.

(Each ml of 0.1 N ceric ammonium sulphate ~ 0.6326 gm of ceric ammonium sulphate ~ 4.946 grams of arsenic trioxide)

Important Facts about standard solution and stability:

1)Salt is dissolved in sulphuric acid –to prevent precipitation of basic salts.

2)Solution in sulfuric acid is indefinitely stable.

3)Solution in HNO3 undergoes photochemical decomposition but slowly.

4)Ceric salts hydrolyses to ceric hydroxide

Applications:

Cerimetry titrations are used for the determination of some drugs,
like vitamins (Vit. K3),
Hydrogen peroxide &
Iron (II) solution/ Ferrous sulphate

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