The Foundations of Chemistry

STOICHIOMETRY OF REDOX REACTIONS

One method of analyzing samples quantitatively for the presence of oxidizableor reducible
substances is by redox titration.In such analyses, the concentration of a solution is deter-
mined by allowing it to react with a carefully measured amount of a standardsolution of
an oxidizing or reducing agent.
As in other kinds of chemical reactions, we must pay particular attention to the mole
ratio in which oxidizing agents and reducing agents react.
Potassium permanganate, KMnO 4 , is a strong oxidizing agent. Through the years it
has been the “workhorse” of redox titrations. For example, in acidic solution, KMnO 4
reacts with iron(II) sulfate, FeSO 4 , according to the balanced equation in the following
example. A strong acid, such as H 2 SO 4 , is used in such titrations (Example 11-15).
A word about terminology. The reaction involves MnO 4 ions and Fe^2
ions in acidic
solution. The source of MnO 4 ions usually is the soluble ionic compound KMnO 4. We
often refer to “permanganate solutions.” Such solutions also contain cations — in this case,
K
. Likewise, we often refer to “iron(II) solutions” without specifying what the anion is.

EXAMPLE 11-22 Redox Titration

What volume of 0.0200 MKMnO 4 solution is required to oxidize 40.0 mL of 0.100 MFeSO 4
in sulfuric acid solution (Figure 11-2)?

Plan
The balanced equation in Example 11-15 gives the reaction ratio, 1 mol MnO 4 /5 mol Fe^2
.
Then we calculate the number of moles of Fe^2
to be titrated, which lets us find the number
of moles of MnO 4 required andthe volume in which this number of moles of KMnO 4 is
contained.

11-8

Because it has an intense purple color,

KMnO 4 acts as its own indicator. One

drop of 0.020 MKMnO 4 solution

imparts a pink color to a liter of pure

water. When KMnO 4 solution is added

to a solution of a reducing agent, the

end point in the titration is taken as

the point at which a pale pink color

appears in the solution being titrated

and persists for at least 30 seconds.

11-8 Stoichiometry of Redox Reactions 425

One mole of KMnO 4 contains one

mole of MnO 4 ions. The number of

moles of KMnO 4 is therefore always

equal to the number of moles of

MnO 4 ions required in a reaction.

Similarly, one mole of FeSO 4 contains

1 mole of Fe^2 
ions.

Figure 11-2 (a) Nearly colorless FeSO 4 solution is titrated with deep-purple KMnO 4.
(b) The end point is the point at which the solution becomes pink, owing to a very small
excess of KMnO 4. Here a considerable excess of KMnO 4 was added so that the pink color
could be reproduced photographically.

(a) (b)