Redox indicator

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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 fast. Therefore only a few classes of organic redox systems can be used for indicator purposes.

There are two common type of redox indicators:

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[edit]

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

pH dependent redox indicators[edit]

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 +0.22 blue colorless
Sodium o-Cresol indophenol +0.62 +0.19 blue colorless
Thionine (syn. Lauth's violet) +0.56 +0.06 violet colorless
Methylene blue +0.53 +0.01 blue colorless
Indigotetrasulfonic acid +0.37 -0.05 blue colorless
Indigotrisulfonic acid +0.33 -0.08 blue colorless
Indigo carmine

(syn. Indigodisulfonic acid

+0.29 -0.13 blue colorless
Indigomono sulfonic acid +0.26 -0.16 blue colorless
Phenosafranin +0.28 -0.25 red colorless
Safranin T +0.24 -0.29 red-violet colorless
Neutral red +0.24 -0.33 red colorless

See also[edit]