Redox indicator
A redox indicator (also called an oxidation-reduction indicator) is an indicator that undergoes a definite colour change at a specific electrode potential.
Fast and reversible colour change means that oxidation-reduction equilibrium for indicator redox system to be established very fast. Therefore only few classes of organic redox systems could be used for indicator aims.
There are two common type of redox indicators:
- metal-organic complexes (Ex. phenanthroline)
- true organic redox systems (Ex. Methylene blue)
Sometimes colour inorganic oxidants or reductants (Ex. Potassium manganate, Potassium dichromate) also are incorrectly mentioned as redox indicator. It couldn’t be classifed as true redox indicator because of its irreversibility.
Almost all redox indicators which are concerned to the true organic redox systems, involve into its electrochemical reaction proton as a participant. Therefore sometimes redox indicators also are 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 |
| Indigotetrasulphonic acid | +0.37 V | -0.05 V | blue | colorless |
| Indigotrisulphonic acid | +0.33 V | -0.08 V | blue | colorless |
| Indigocarmine
(syn. Indigodisulphonic acid |
+0.29 V | -0.13 V | blue | colorless |
| Indigomono sulphonic 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 |