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 classes of redox indicators: metal complexes of phenanthroline and bipyridine. In these systems, the metal changes oxidation state. organic redox systems such as methylene blue. In these systems, a proton participates in the redox reaction. Therefore, sometimes redox indicators are also divided into two general groups: independent or dependent on pH.
The most common redox indicator are organic 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.
pH independent IndicatorE0, VColor of Oxidized formColor of Reduced form[RuIII/II(2,2'-bipyridine)3] +1.33 greenorangeNitrophenanthroline (Fe complex) +1.25 cyanredN-Phenylanthranilic acid +1.08 violet-redcolorless1,10-Phenanthroline iron(II) sulfate complex (Ferroin) +1.06 cyanredN-Ethoxychrysoidine +1.00 redyellow2,2`-Bipyridine (Fe complex) +0.97 cyanred5,6-Dimethylphenanthroline (Fe complex) +0.97 yellow-greencyano-Dianisidine +0.85 redcolorlessSodium diphenylamine sulfonate +0.84 red-violetcolorlessDiphenylbenzidine +0.76 violetcolorlessDiphenylamine +0.76 violetcolorlessViologen -0.43 colorlessblue
pH dependent IndicatorE0, V at pH=0E, V at pH=7Color of Oxidized formColor of Reduced formSodium 2,6-Dibromophenol-indophenol or Sodium 2,6-Dichlorophenol-indophenol +0.64 +0.22 bluecolorlessSodium o-Cresol indophenol +0.62 +0.19 bluecolorlessThionine (syn. Lauth's violet) +0.56 +0.06 violetcolorlessMethylene blue +0.53 +0.01HEWITT, LF. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. Edn 6 (1950). bluecolorlessIndigotetrasulfonic acid +0.37 -0.05 bluecolorlessIndigotrisulfonic acid +0.33 -0.08 bluecolorlessIndigo carmine (syn. Indigodisulfonic acid +0.29 -0.13 bluecolorlessIndigomono sulfonic acid +0.26 -0.16 bluecolorlessPhenosafranin +0.28 -0.25 redcolorlessSafranin T +0.24 -0.29 red-violetcolorlessNeutral red +0.24 -0.33 redcolorless
See also Chemical analysis pH indicator Complexometric indicator
References
External links Redox Indicators. Characteristics And Applications