A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific electrode potential.
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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.
| 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 |
| 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 |
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