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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: a) metal-organic complexes (Ex. phenanthroline) b) 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-Dibromphenol indophenol or Sodium 2,6-Dichlorphenol 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
Indigocarmin (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

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-A [[redox indicator]] (also called an '''oxidation-reduction indicator''' or a '''potentiometric indicator''') is an [[indicator]] that undergoes a definite colour change at a specific oxidation potential.
+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:
+a) metal-organic complexes (Ex. [[phenanthroline]])
+b) true [[organic redox system]]s (Ex. [[Methylene blue]])
+Sometimes colour inorganic [[oxidant]]s or [[reductant]]s (Ex. [[Potassium manganate]], [[Potassium dichromate]]) also are <i>incorrectly</i> mentioned as redox indicator. It couldn’t be classifed as <b>true</b> redox indicator because of its <b>irreversibility</b>.
+Almost all redox indicators which are concerned to the true [[organic redox system]]s, 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]].
@@ Line 151: / Line 161: @@
 [[category:Physical chemistry]]
 [[category:Analytical chemistry]]
-{{chem-stub}}

Revision as of 09:10, 22 October 2005

pH independent redox indicators

pH dependent redox indicators

See Also