Redox indicator

Short description: Indicator which undergoes a definite color change at a specific electrode 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.[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 participates 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. Redox Indicator example: The molecule 2,2'- Bipyridine is a redox Indicator. In solution, it changes from light blue to red at an electrode potential of 0.97 V.

## Contents

* 1 pH independent
* 2 pH dependent
* 3 See also
* 4 References
* 5 External links

## 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  | cyan
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

| E, 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

* Chemical analysis
* pH indicator
* Complexometric indicator

## References

1. ↑ Ram W. Sabnis; Erwin Ross; Jutta Köthe; Renate Naumann; Wolfgang Fischer; Wilhelm-Dietrich Mayer; Gerhard Wieland; Ernest J. Newman et al. (2009). "Indicator Reagents". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_127.pub2. ISBN 978-3-527-30673-2.
2. ↑ HEWITT, LF. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. Edn 6 (1950).

## External links

* Redox Indicators. Characteristics And Applications

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