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.


  • 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
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Helmenstine, Anne Marie, Ph.D. "Redox Indicator Definition (Chemistry)." ThoughtCo, Aug. 25, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 25). Redox Indicator Definition (Chemistry). Retrieved from Helmenstine, Anne Marie, Ph.D. "Redox Indicator Definition (Chemistry)." ThoughtCo. (accessed April 17, 2021).