Modern inorganic chemistry

106 ACIDS AND BASES: OXIDATION AND REDUCTION

be more satisfactorily found by potential measurements of a cell

incorporating the redox reaction.

Consider the estimation of iron(II) ions by cerium(IV) ions in

aqueous solution:

Fe^2 + (aq) + Ce^4 + (aq) -> Ce^3 + (aq) + Fe^3 + (aq)

The electrode potential for the iron(II)-iron(III) system is given by

[Fe^3 + (aq)j

and for the cerium(IV)-cerium(III) system by

^ RT [Ce^4 + (aq)]

E 2 = E 2 + _loge^JT^

Experimentally, the aqueous iron(II) is titrated with cerium(IV)
in aqueous solution in a burette. The arrangement is shown in
Figure 4.6; the platinum indicator electrode changes its potential
(with reference to a calomel half-cell as standard) as the solution is
titrated. Figure 4.7 shows the graph of the cell e.m.f. against added
cerium(IV). At the equivalence point the amount of the added
Ce^4 + (aq) is equal to the original amount of Fe^2 + (aq); hence the
amounts of Ce^3 + (aq) and Fea + (aq) are also equal. Under these
conditions the potential of the electrode in the mixture is (£~f-f Ef)/2 ;
this, the equivalence point, occurs at the point indicated.
Potentiometric methods can be used for the study of a large

Ce 02) solution,

in burette

Fe(n)solution"

being titrated

a

Calomel standard

r electrode

Platinum indicator

electrode

Stirrer

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