Redox indicator explained

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.

pH independent

Indicator	E0, V	Color of Oxidized form	Color of Reduced form
2,2'-bipyridine (Ru complex)	+1.33	align=center bgcolor=white	colorless	align=center bgcolor=yellow	yellow
Nitrophenanthroline (Fe complex)	+1.25	align=center bgcolor=cyan	cyan	align=center bgcolor=red	red
N-Phenylanthranilic acid	+1.08	align=center bgcolor=violet	violet-red	align=center bgcolor=white	colorless
1,10-Phenanthroline iron(II) sulfate complex (Ferroin)	+1.06	align=center bgcolor=cyan	cyan	align=center bgcolor=red	red
N-Ethoxychrysoidine	+1.00	align=center bgcolor=red	red	align=center bgcolor=yellow	yellow
2,2`-Bipyridine (Fe complex)	+0.97	align=center bgcolor=cyan	cyan	align=center bgcolor=red	red
5,6-Dimethylphenanthroline (Fe complex)	+0.97	align=center bgcolor=#CCFF00	yellow-green	align=center bgcolor=cyan	cyan
o-Dianisidine	+0.85	align=center bgcolor=red	red	align=center bgcolor=white	colorless
Sodium diphenylamine sulfonate	+0.84	align=center bgcolor=#E41B9E	red-violet	align=center bgcolor=white	colorless
Diphenylbenzidine	+0.76	align=center bgcolor=violet	violet	align=center bgcolor=white	colorless
Diphenylamine	+0.76	align=center bgcolor=violet	violet	align=center bgcolor=white	colorless
Viologen	-0.43	align=center bgcolor=white	colorless	align=center bgcolor=blue	blue

pH dependent

Indicator	E0, Vat pH=0	E, Vat pH=7	Color of Oxidized form	Color of Reduced form
Sodium 2,6-Dibromophenol-indophenolor Sodium 2,6-Dichlorophenol-indophenol	align=center bgcolor=white	+0.64	align=center bgcolor=white	+0.22	align=center bgcolor=blue	blue	align=center bgcolor=white	colorless
Sodium o-Cresol indophenol	align=center bgcolor=white	+0.62	align=center bgcolor=white	+0.19	align=center bgcolor=blue	blue	align=center bgcolor=white	colorless
Thionine (syn. Lauth's violet)	align=center bgcolor=white	+0.56	align=center bgcolor=white	+0.06	align=center bgcolor=violet	violet	align=center bgcolor=white	colorless
Methylene blue	align=center bgcolor=white	+0.53	align=center bgcolor=white	+0.01[2]	align=center bgcolor=blue	blue	align=center bgcolor=white	colorless
Indigotetrasulfonic acid	align=center bgcolor=white	+0.37	align=center bgcolor=white	-0.05	align=center bgcolor=blue	blue	align=center bgcolor=white	colorless
Indigotrisulfonic acid	align=center bgcolor=white	+0.33	align=center bgcolor=white	-0.08	align=center bgcolor=blue	blue	align=center bgcolor=white	colorless
Indigo carmine (syn. Indigodisulfonic acid	align=center bgcolor=white	+0.29	align=center bgcolor=white	-0.13	align=center bgcolor=blue	blue	align=center bgcolor=white	colorless
Indigomono sulfonic acid	align=center bgcolor=white	+0.26	align=center bgcolor=white	-0.16	align=center bgcolor=blue	blue	align=center bgcolor=white	colorless
	align=center bgcolor=white	+0.28	align=center bgcolor=white	-0.25	align=center bgcolor=red	red	align=center bgcolor=white	colorless
	align=center bgcolor=white	+0.24	align=center bgcolor=white	-0.29	align=center bgcolor=#E41B9E	red-violet	align=center bgcolor=white	colorless
	align=center bgcolor=white	+0.24	align=center bgcolor=white	-0.33	align=center bgcolor=red	red	align=center bgcolor=white	colorless

See also

• Chemical analysis
• pH indicator
• Complexometric indicator

External links

• Redox Indicators. Characteristics And Applications

Notes and References

1. Encyclopedia: Ullmann's Encyclopedia of Industrial Chemistry. Ram W. Sabnis . Erwin Ross . Jutta Köthe . Renate Naumann . Wolfgang Fischer . Wilhelm-Dietrich Mayer . Gerhard Wieland . Ernest J. Newman . Charles M. Wilson . Indicator Reagents. 2009. Wiley-VCH. Weinheim. 10.1002/14356007.a14_127.pub2. 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).