Redox Indicator Definition (Chemistry)

This is the chemical structure of 2,2'-bipyridine.
This is the chemical structure of 2,2'-bipyridine, a redox indicator. Brian Derksen/PD

A redox indicator is an indicator compound that changes color at specific potential differences.

A redox indicator compound must have a reduced and oxidized form with different colors and the redox process must be reversible. Further, the oxidation-reduction equilibrium needs to be reached quickly. Only a few classes of compounds are useful as redox indicators:

  • Phenanthroline and bipyridine metal complexes: The metallorganic systems change color as the metal changes its oxidation state.
  • Organic redox compounds: In these indicators, a proton participates in the redox reaction. An example of this type of indicator is methylene blue.

Redox Indicator Examples

The molecule 2,2'-Bipyridine is a redox indicator. In solution, it changes from light blue to red at an electrode potential of 0.97 V.

Sources

  • Hewitt, L.F. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. 6th Ed. (1950).
  • Ram W. Sabnis, Erwin Ross, Jutta Köthe, Renate Naumann, Wolfgang Fischer, Wilhelm-Dietrich Mayer, Gerhard Wieland, Ernest J. Newman, Charles M. Wilson (2009). Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_127.pub2