Redox indicator

A redox indicator (also called an oxidation-reduction indicator) is an indicator that 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 fast. Therefore only a few classes of organic redox systems can be used for indicator purposes.

There are two common type 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.

pH independent redox indicators

Indicator	E0, V	Color of Oxidized form	Color of Reduced form
2,2'-bipyridine (Ru complex)	+1.33 V	colorless	yellow
Nitrophenanthroline (Fe complex)	+1.25 V	cyan	red
N-Phenylanthranilic acid	+1.08 V	violet-red	colorless
1,10-Phenanthroline (Fe complex)	+1.06 V	cyan	red
N-Ethoxychrysoidine	+1.00 V	red	yellow
2,2`-Bipyridine (Fe complex)	+0.97 V	cyan	red
5,6-Dimethylphenanthroline (Fe complex)	+0.97 V	yellow-green	red
o-Dianisidine	+0.85 V	red	colorless
Sodium diphenylamine sulfonate	+0.84 V	red-violet	colorless
Diphenylbenzidine	+0.76 V	violet	colorless
Diphenylamine	+0.76 V	violet	colorless
Viologen	-0.43 V	colorless	blue

pH dependent redox indicators

Indicator	E0, V at pH=0	E0, V at pH=7	Color of Oxidized form	Color of Reduced form
Sodium 2,6-Dibromophenol-indophenol or Sodium 2,6-Dichlorophenol-indophenol	+0.64 V	+0.22 V	blue	colorless
Sodium o-Cresol indophenol	+0.62 V	+0.19 V	blue	colorless
Thionine (syn. Lauth's violet)	+0.56 V	+0.06 V	violet	colorless
Methylene blue	+0.53 V	+0.01 V	blue	colorless
Indigotetrasulfonic acid	+0.37 V	-0.05 V	blue	colorless
Indigotrisulfonic acid	+0.33 V	-0.08 V	blue	colorless
Indigo carmine (syn. Indigodisulfonic acid	+0.29 V	-0.13 V	blue	colorless
Indigomono sulfonic acid	+0.26 V	-0.16 V	blue	colorless
Phenosafranin	+0.28 V	-0.25 V	red	colorless
Safranin T	+0.24 V	-0.29 V	red-violet	colorless
Neutral red	+0.24 V	-0.33 V	red	colorless

See also

• pH indicator