A redox indicator is an indicator that undergoes a definite color change at a specific electrode potential

Electrode potential

Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...

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The requirement for fast and reversible color change means that the oxidation-reduction equilibrium

Chemical equilibrium

In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...

 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:

• metal-organic complexes (Ex. phenanthroline
  Phenanthroline
  Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, it forms strong complexes with most metal ions...
  
  )
• true organic redox systems (Ex. Methylene blue
  Methylene blue
  Methylene blue is a heterocyclic aromatic chemical compound with the molecular formula C16H18N3SCl. It has many uses in a range of different fields, such as biology and chemistry. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in...
  
  )

Sometimes colored inorganic oxidants or reductants (Ex. Potassium manganate

Potassium manganate

Potassium manganate is the inorganic compound with the formula K2MnO4. This green-colored salt is an intermediate in the industrial synthesis of potassium permanganate , a common chemical...

, 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

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

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pH independent redox indicators

Indicator	E0, V	Color of Oxidized form	Color of Reduced form
2,2'-bipyridine 2,2'-Bipyridine 2,2'-Bipyridine is a organic compound with the formula . This colorless solid, commonly abbreviated bipy or bpy , is an important isomer of the bipyridine family. It is a bidentate chelating ligand, forming complexes with many transition metals...  (Ru complex)	+1.33 V	colorless	yellow
Nitrophenanthroline Phenanthroline Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, it forms strong complexes with most metal ions...  (Fe complex)	+1.25 V	cyan	red
N-Phenylanthranilic acid	+1.08 V	violet-red	colorless
1,10-Phenanthroline Phenanthroline Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, it forms strong complexes with most metal ions...  (Fe complex)	+1.06 V	cyan	red
N-Ethoxychrysoidine	+1.00 V	red	yellow
2,2`-Bipyridine Bipyridine Bipyridines are a family of chemical compounds with the formula 2, which are formed by the coupling of two pyridine rings. Six isomers of bipyridine exist, but two isomers are prominent: 2,2'-bipyridine is a popular ligand in coordination chemistry and 4,4'-bipyridine is a precursor to the...  (Fe complex)	+0.97 V	cyan	red
5,6-Dimethylphenanthroline Phenanthroline Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, it forms strong complexes with most metal ions...  (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 Diphenylamine Diphenylamine is the organic compound with the formula 2NH. It is a colourless solid, but samples are often yellow due to oxidized impurities.-Preparation and reactivity:...	+0.76 V	violet	colorless
Viologen Viologen Viologens are toxic bipyridinium derivatives of 4,4'-bipyridyl. The name is because this class of compounds is easily reduced to the radical mono cation, which is intensely blue coloured....	-0.43 V	colorless	blue

pH dependent redox indicators

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 V	+0.22 V	blue	colorless
Sodium o-Cresol indophenol Indophenol Indophenol is an artificial blue metachromatic dye, obtained by the action of phenol on certain nitrogenous derivatives of quinone. Indophenol resembles the color indigo in appearance...	+0.62 V	+0.19 V	blue	colorless
Thionine (syn. Lauth's violet)	+0.56 V	+0.06 V	violet	colorless
Methylene blue Methylene blue Methylene blue is a heterocyclic aromatic chemical compound with the molecular formula C16H18N3SCl. It has many uses in a range of different fields, such as biology and chemistry. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in...	+0.53 V	+0.01 V	blue	colorless
Indigotetrasulfonic acid Sulfonic acid Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...	+0.37 V	-0.05 V	blue	colorless
Indigotrisulfonic acid Sulfonic acid Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...	+0.33 V	-0.08 V	blue	colorless
Indigo carmine Indigo carmine Indigo carmine, or 5,5'-indigodisulfonic acid sodium salt, also known as indigotine or FD&C Blue #2 is a pH indicator with the chemical formula C16H8N2Na2O8S2. It is approved for use as a food colorant in the USA and the EU and has the E number E132.-Uses:The primary use of Indigo carmine is as a... (syn. Indigodisulfonic acid Sulfonic acid Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...	+0.29 V	-0.13 V	blue	colorless
Indigo Indigo Indigo is a color named after the purple dye derived from the plant Indigofera tinctoria and related species. The color is placed on the electromagnetic spectrum between about 420 and 450 nm in wavelength, placing it between blue and violet... mono sulfonic acid Sulfonic acid Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...	+0.26 V	-0.16 V	blue	colorless
Phenosafranin	+0.28 V	-0.25 V	red	colorless
Safranin Safranin Safranin is a biological stain used in histology and cytology. Safranin is used as a counterstain in some staining protocols, colouring all cell nuclei red. This is the classic counterstain in a Gram stain...  T	+0.24 V	-0.29 V	red-violet	colorless
Neutral red Neutral red Neutral Red is a eurhodin dye used for staining in histology. It stains lysosomes red. It is used as a general stain in histology, as a counterstain in combination with other dyes, and for many staining methods. Together with Janus Green B, it is used to stain embryonal tissues and supravital...	+0.24 V	-0.33 V	red	colorless