Instant Notes: Analytical Chemistry

Redox equilibria In order to establish a scale of oxidative power, it is necessary to have a stan-
dard, and since these reactions involve electrons, measurement of the reduction
electrode potentialis a convenient way to do this. The details are given more
fully in Topic C3.
Some standard reduction electrode potentials, where the reagents are at unit
activity, at 25∞C are given in Table 3. These potentials allow the prediction of
which ions will oxidize other ions, under standard conditions, that is when the
concentrations are molar. A more poisitve electrode potential will oxidize a
more negative potential.
It was shown in Topic C2 that the electrochemical cell e.m.f. is related to the
free energy change, and hence to the equilibrium constant:

En=(RT/nF) ln K

Therefore, the larger the cell e.m.f, the larger the equilibrium constant, and the

more complete the reaction.

Example

For the reaction of cerium(IV) ions with iron(II) ions, what is the likely reaction,

and what is the equilibrium constant? Which reagent is the oxidizing agent, and

which the reducing agent?

Cell: Pt |Fe^2 +, Fe^3 +||Ce^4 +, Ce^3 +|Pt

En(cell) =En(rhs) – En(lhs) =1.44 -0.77 =0.67 V

88 Section C – Analytical reactions in solution

CH 3 C CCH 3

NOHNOH

OH

N

(a)

(b)

Fig. 2. Reagents for the precipitation of metal ions. (a) Dimethylglyoxime. (b) Oxine.

Table 3. Standard reduction electrode potentials of some common redox systems at

25 ∞C

Reaction En/V

H 2 O 2 +2H++2e-=2H 2 O 1.77

MnO 4 - +8H++5e-=Mn^2 ++5H 2 O 1.51

Ce^4 ++e-=Ce^3 + 1.44

Cr 2 O 72 - +14H++6e-=2Cr^3 ++7H 2 O 1.33

I 2 + 2e-=2I- 0.54

Fe^3 ++e-=Fe^2 + 0.77

S 4 O 62 - +2e-=2S 2 O 32 - 0.08

2CO 2 +2e-=C 2 O 42 - -0.49