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Aredox indicator (also called anoxidation-reduction indicator) is an indicator which undergoes a definite color change at a specificelectrode potential.
The requirement for fast and reversible color change means that the oxidation-reductionequilibrium for an indicatorredox 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:
The most common redox indicator areorganic 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 |
| Sodiumdiphenylamine 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 Sodium2,6-Dichlorophenol-indophenol | +0.64 | +0.22 | blue | colorless |
| Sodium o-Cresolindophenol | +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 |
| Indigomonosulfonic acid | +0.26 | -0.16 | blue | colorless |
| Phenosafranin | +0.28 | -0.25 | red | colorless |
| SafraninT | +0.24 | -0.29 | red-violet | colorless |
| Neutral red | +0.24 | -0.33 | red | colorless |