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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 fast. Therefore only a few classes of organic redox systems can be used for indicator purposes.
There are two common type of redox indicators:
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|
|1,10-Phenanthroline iron(II) sulfate complex (Ferroin)||+1.06||cyan||red|
|2,2`-Bipyridine (Fe complex)||+0.97||cyan||red|
|5,6-Dimethylphenanthroline (Fe complex)||+0.97||yellow-green||red|
|Sodium diphenylamine sulfonate||+0.84||red-violet||colorless|
pH dependent redox indicators
or Sodium 2,6-Dichlorophenol-indophenol
|Sodium o-Cresol indophenol||+0.62||+0.19||blue||colorless|
|Thionine (syn. Lauth's violet)||+0.56||+0.06||violet||colorless|
(syn. Indigodisulfonic acid
|Indigomono sulfonic acid||+0.26||-0.16||blue||colorless|