Redox indicator

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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:

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 Ox form Color of Red 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

pH dependent redox indicators

Indicator E0, V

at pH=0

E0, V

at pH=7

Color of Ox form Color of Red 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

(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

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