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.