Redox indicator

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.[1]

There are two common classes of redox indicators:

  • metal complexes of phenanthroline and bipyridine. In these systems, the metal changes oxidation state.
  • organic redox systems such as Methylene blue. In these systems, a proton participant in the redox reaction. Therefore, sometimes redox indicators are also divided into two general groups: independent or dependent on pH.

The most common redox indicator are organic compounds]].

pH independent

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

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[2] 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

References

  1. ↑ 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.
  2. ↑ HEWITT, LF. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. Edn 6 (1950).
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