University Of Anbar / College Of Engineering Lecturer: Assist. Prof. Dr. Hamad K. Abdulkadir
Department of Chem. & Petrochemical Engineering 2nd . Stage / Analytical Chemistry
Lectures- 12
Titrations Based on Redox reactions
Analytical titrations using redox reactions were introduced shortly after the
development of acid–base titrimetry. The earliest Redox titration took advantage
of the oxidizing power of chlorine. In 1787, Claude Berthollet introduced a method
for the quantitative analysis of chlorine water (a mixture of Cl2, HCl, and HOCl)
based on its ability to oxidize indigo, a dye that is colorless in its oxidized state. In
1814, Joseph Gay-Lussac developed a similar method for determining chlorine in
bleaching powder. In both methods the end point is a change in color. Before the
equivalence point the solution is colorless due to the oxidation of indigo. After the
equivalence point, however, unreacted indigo imparts a permanent color to the
solution.
The number of redox titrimetric methods increased in the mid-1800s with the
introduction of MnO4–, Cr2O72–, and I2 as oxidizing titrants, and of Fe2+ and S2O32–
as reducing titrants. Even with the availability of these new titrants, redox titrimetry
was slow to develop due to the lack of suitable indicators. A titrant can serve as its
own indicator if its oxidized and reduced forms differ significantly in color. For
example, the intensely purple MnO4– ion serves as its own indicator since its
reduced form, Mn2+, is almost colorless. Other titrants require a separate indicator.
The first such indicator, diphenylamine, was introduced in the 1920s. Other redox
indicators soon followed, increasing the applicability of redox titrimetry.
Redox Titration Curves
To evaluate a redox titration we need to know the shape of its titration curve. In
an acid–base titration or a complexation titration, the titration curve shows how the
concentration of H3O+ (as pH) or Mn+ (as pM) changes as we add titrant. For a
redox titration it is convenient to monitor the titration reaction’s potential instead of
the concentration of one species.
You may recall from Chapter 6 that the Nernst equation relates a solution’s
potential to the concentrations of reactants and products participating in the redox
reaction. Consider, for example, a titration in which a titrand in a reduced
state, Ared, reacts with a titrant in an oxidized state, Box.
A red + Box ⇌ Bred + A ox
where Aox is the titrand’s oxidized form, and Bred is the titrant’s reduced form. The
reaction’s potential, Erxn, is the difference between the reduction potentials for each
half-reaction.
Erxn = E Box / Bred – E Aox /Ared
