Redox indicator
The requirement for fast and reversible color change means that the oxidation-reduction equilibrium 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)
- 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 Ox form | Color of Red form |
|---|---|---|---|
| 2,2'-Bipyridine (Ru complex) | +1.33 V | colorless | yellow |
| Nitrophenanthroline (Fe complex) | +1.25 V | cyan | red |
| n-Phenylanthranilic acid | +1.08 V | violet-red | colorless |
| 1,10-Phenanthroline (Fe complex) | +1.06 V | cyan | red |
| n-Ethoxychrysoidine | +1.00 V | red | yellow |
| 2,2`-Bipyridine (Fe complex) | +0.97 V | cyan | red |
| 5,6-Dimethylphenanthroline (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 | +0.76 V | violet | colorless |
pH dependent redox indicators
| Indicator | E0, V at pH=0 | E0, V at pH=7 | Color of Ox form | Color of Red form |
|---|---|---|---|---|
| Sodium 2,6-Dibromophenol-indophenol or Sodium 2,6-Dichlorophenol-indophenol | +0.64 V | +0.22 V | blue | colorless |
| Sodium o-Cresol indophenol | +0.62 V | +0.19 V | blue | colorless |
| Thionine (syn. Lauth's violet) | +0.56 V | +0.06 V | violet | colorless |
| Methylene blue | +0.53 V | +0.01 V | blue | colorless |
| Indigotetrasulfonic acid | +0.37 V | -0.05 V | blue | colorless |
| Indigotrisulfonic acid | +0.33 V | -0.08 V | blue | colorless |
| Indigocarmine (syn. Indigodisulfonic acid | +0.29 V | -0.13 V | blue | colorless |
| Indigomono sulfonic acid | +0.26 V | -0.16 V | blue | colorless |
| Phenosafranin | +0.28 V | -0.25 V | red | colorless |
| Safranin T | +0.24 V | -0.29 V | red-violet | colorless |
| Neutral red | +0.24 V | -0.33 V | red | colorless |
See also
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Last updated on Wednesday September 10, 2008 at 15:15:37 PDT (GMT -0700)
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