What is "redox indicator"

n. (context chemistry English) A substance that exists in an oxidized and reduced forms, these forms having different colours, changing colour at a specific redox potential

A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific electrode potential.

The requirement for fast and reversible color change means that the oxidation-reduction equilibrium for an indicator redox system needs to be established very quickly. Therefore only a few classes of organic redox systems can be used for indicator purposes.

There are two common types of redox indicators:

• metal-organic complexes (Ex. phenanthroline)
• true organic redox systems (Ex. Methylene blue)

Sometimes colored inorganic oxidants or reductants (Ex. Potassium manganate, Potassium dichromate) are also incorrectly called redox indicators. They can’t be classified as true redox indicators because of their irreversibility.

Almost all redox indicators with true organic redox systems involve a proton as a participant in their electrochemical reaction. Therefore sometimes redox indicators are also divided into two general groups: independent or dependent on pH.