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.
There are two common types of redox indicators:
- metal-organic complexes (Ex. phenanthroline)
- true organic redox systems (Ex. Methylene blue)
Sometimes colored inorganic oxidants or reductants (Ex. Potassium manganate, 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 independent redox indicators
| 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 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 | +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 | 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 |