Titrations Based on Redox Reactions
Redox titrations were introduced shortly after the development of acid–base
titrimetry. The earliest
methods took advantage of the oxidizing power of chlorine. In 1787, Claude Berthollet introduced a method
for the quantitative analysis of chlo- rine water (a mixture
of Cl2, HCl, and HOCl)
based on its ability to oxidize solutions of the dye indigo
(indigo is colorless in its oxidized state). In 1814,
Joseph Louis Gay- Lussac (1778–1850), developed a similar method
for chlorine in bleaching powder. In both methods the end point was signaled
visually. Before the equivalence point, the solution remains clear 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 I as oxidizing titrants, and Fe2+ and S2O32– as
reducing titrants. Even with the availability of these new titrants, however,
the rou- tine application of redox titrimetry to a wide
range of samples
was limited by the
lack of suitable indicators. Titrants whose oxidized and
reduced forms differ
signifi- cantly in color
could be used as their
own indicator. For example, the intensely pur- ple
MnO4– ion serves as its
own indicator since
its reduced form,
Mn2+, is almost
colorless. The utility
of other titrants, however, required a visual indicator that could be added to the solution. The first such indicator was diphenylamine, which was
introduced in the 1920s. Other redox indicators soon followed, increasing the applicability of redox
titrimetry.
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