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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 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:

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

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
Sodium diphenylamine sulfonate	+0.84	red-violet	colorless
Diphenylbenzidine	+0.76	violet	colorless
Diphenylamine	+0.76	violet	colorless
Viologen	-0.43	colorless	blue

pH dependent

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 Sodium 2,6-Dichlorophenol-indophenol	+0.64	+0.22	blue	colorless
Sodium o-Cresol indophenol	+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
Indigomono sulfonic acid	+0.26	-0.16	blue	colorless
Phenosafranin	+0.28	-0.25	red	colorless
Safranin T	+0.24	-0.29	red-violet	colorless
Neutral red	+0.24	-0.33	red	colorless

References

^ Ram W. Sabnis; Erwin Ross; Jutta Köthe; Renate Naumann; Wolfgang Fischer; Wilhelm-Dietrich Mayer; Gerhard Wieland; Ernest J. Newman; Charles M. Wilson (2009). "Indicator Reagents". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_127.pub2. ISBN 978-3-527-30673-2.
^ HEWITT, LF. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. Edn 6 (1950).

External links

Redox Indicators. Characteristics And Applications

[1] Ram W. Sabnis; Erwin Ross; Jutta Köthe; Renate Naumann; Wolfgang Fischer; Wilhelm-Dietrich Mayer; Gerhard Wieland; Ernest J. Newman; Charles M. Wilson (2009). "Indicator Reagents". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_127.pub2. ISBN 978-3-527-30673-2.

[2] HEWITT, LF. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. Edn 6 (1950).

[1]

[2]

@@ Line 1: / Line 1: @@
+{{short description|Indicator which undergoes a definite color change at a specific electrode potential}}
-{{Unreferenced|date=December 2009}}
+{{refimprove|date=December 2009}}
-A '''redox indicator''' (also called an '''oxidation-reduction indicator''') is an indicator that undergoes a definite color change at a specific [[electrode potential]].
+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 [[Chemical equilibrium|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.
+The requirement for fast and reversible color change means that the oxidation-reduction [[Chemical equilibrium|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.<ref>{{cite encyclopedia| encyclopedia = Ullmann's Encyclopedia of Industrial Chemistry|author=Ram W. Sabnis |author2=Erwin Ross |author3=Jutta Köthe |author4=Renate Naumann |author5=Wolfgang Fischer |author6=Wilhelm-Dietrich Mayer |author7=Gerhard Wieland |author8=Ernest J. Newman |author9=Charles M. Wilson |title=Indicator Reagents|year=2009|publisher=Wiley-VCH|place=Weinheim|doi=10.1002/14356007.a14_127.pub2|isbn=978-3-527-30673-2}}</ref>
-There are two common type of redox indicators:
+There are two common classes of redox indicators:
-* metal-organic complexes (Ex. [[phenanthroline]])
+* [[metal complex]]es of [[phenanthroline]] and [[bipyridine]].  In these systems, the metal changes oxidation state.
+* [[organic redox system]]s 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]].
-* true [[organic redox system]]s (Ex. [[Methylene blue]])
-Sometimes colored inorganic [[oxidant]]s or [[reductant]]s (Ex. [[Potassium manganate]], [[Potassium dichromate]]) are also ''incorrectly'' called redox indicators.  They can’t be classified as '''true''' redox indicators because of their '''irreversibility'''.
+The most common redox indicator are [[organic compounds]].
-Almost all redox indicators with true [[organic redox system]]s  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]].
+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 redox indicators==
+==pH independent==
 {| border="1"
 !Indicator
@@ Line 20: / Line 21: @@
 |-
 |[[2,2'-bipyridine]] (Ru complex)
-|align=center| +1.33 V
+|align=center| +1.33
 |align=center bgcolor=white|colorless
 |align=center bgcolor=yellow|yellow
 |-
 |Nitro[[phenanthroline]] (Fe complex)
-|align=center| +1.25 V
+|align=center| +1.25
 |align=center bgcolor=cyan|cyan
 |align=center bgcolor=red|red
 |-
 |N-[[Phenylanthranilic acid]]
-|align=center| +1.08 V
+|align=center| +1.08
 |align=center bgcolor=violet|violet-red
 |align=center bgcolor=white|colorless
 |-
-|1,10-[[Phenanthroline]] (Fe complex)
+|1,10-Phenanthroline iron(II) sulfate complex ([[Ferroin]])
-|align=center| +1.06 V
+|align=center| +1.06
 |align=center bgcolor=cyan|cyan
 |align=center bgcolor=red|red
 |-
 |N-[[Ethoxychrysoidine]]
-|align=center| +1.00 V
+|align=center| +1.00
 |align=center bgcolor=red|red
 |align=center bgcolor=yellow|yellow
 |-
 |2,2`-[[Bipyridine]] (Fe complex)
-|align=center| +0.97 V
+|align=center| +0.97
 |align=center bgcolor=cyan|cyan
 |align=center bgcolor=red|red
 |-
 |5,6-Dimethyl[[phenanthroline]] (Fe complex)
-| +0.97 V
+| +0.97
 |align=center bgcolor=#CCFF00|yellow-green
-|align=center bgcolor=red|red
+|align=center bgcolor=cyan|cyan
 |-
-|o-[[Dianisidine]]
+|[[o-Dianisidine]]
-|align=center| +0.85 V
+|align=center| +0.85
 |align=center bgcolor=red|red
 |align=center bgcolor=white|colorless
 |-
 |Sodium [[diphenylamine sulfonate]]
-|align=center| +0.84 V
+|align=center| +0.84
 |align=center bgcolor=#E41B9E|red-violet
 |align=center bgcolor=white|colorless
 |-
 |[[Diphenylbenzidine]]
-| +0.76 V
+| +0.76
 |align=center bgcolor=violet|violet
 |align=center bgcolor=white|colorless
 |-
 |[[Diphenylamine]]
-|align=center| +0.76 V
+|align=center| +0.76
 |align=center bgcolor=violet|violet
 |align=center bgcolor=white|colorless
 |-
 |[[Viologen]]
-|align=center| -0.43 V
+|align=center| -0.43
 |align=center bgcolor=white|colorless
 |align=center bgcolor=blue|blue
 |}
-==pH dependent redox indicators==
+==pH dependent==
 {| border="1"
 !Indicator
 !E<sup>0</sup>, V
 at pH=0
-!E<sup>0</sup>, V
+!E, V
 at pH=7
 !Color of
@@ Line 94: / Line 95: @@
 |Sodium 2,6-Dibromophenol-indophenol
 or Sodium [[DCPIP|2,6-Dichlorophenol-indophenol]]
-|align=center bgcolor=white| +0.64 V
+|align=center bgcolor=white| +0.64
-|align=center bgcolor=white| +0.22 V
+|align=center bgcolor=white| +0.22
 |align=center bgcolor=blue|blue
 |align=center bgcolor=white|colorless
 |-
 |Sodium o-Cresol [[indophenol]]
-|align=center bgcolor=white| +0.62 V
+|align=center bgcolor=white| +0.62
-|align=center bgcolor=white| +0.19 V
+|align=center bgcolor=white| +0.19
 |align=center bgcolor=blue|blue
 |align=center bgcolor=white|colorless
 |-
 |[[Thionine]] (syn. [[Lauth's violet]])
-|align=center bgcolor=white| +0.56 V
+|align=center bgcolor=white| +0.56
-|align=center bgcolor=white| +0.06 V
+|align=center bgcolor=white| +0.06
 |align=center bgcolor=violet|violet
 |align=center bgcolor=white|colorless
 |-
 |[[Methylene blue]]
-|align=center bgcolor=white| +0.53 V
+|align=center bgcolor=white| +0.53
+|align=center bgcolor=white| +0.01<ref>HEWITT, LF. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. Edn 6 (1950).</ref>
-|align=center bgcolor=white| +0.01 V
 |align=center bgcolor=blue|blue
 |align=center bgcolor=white|colorless
 |-
 |Indigotetra[[sulfonic acid]]
-|align=center bgcolor=white| +0.37 V
+|align=center bgcolor=white| +0.37
-|align=center bgcolor=white| -0.05 V
+|align=center bgcolor=white| -0.05
 |align=center bgcolor=blue|blue
 |align=center bgcolor=white|colorless
 |-
 |Indigotri[[sulfonic acid]]
-|align=center bgcolor=white| +0.33 V
+|align=center bgcolor=white| +0.33
-|align=center bgcolor=white| -0.08 V
+|align=center bgcolor=white| -0.08
 |align=center bgcolor=blue|blue
 |align=center bgcolor=white|colorless
@@ Line 131: / Line 132: @@
 |[[Indigo carmine]]
 (syn. Indigodi[[sulfonic acid]]
-|align=center bgcolor=white| +0.29 V
+|align=center bgcolor=white| +0.29
-|align=center bgcolor=white| -0.13 V
+|align=center bgcolor=white| -0.13
 |align=center bgcolor=blue|blue
 |align=center bgcolor=white|colorless
 |-
 |[[Indigo]]mono [[sulfonic acid]]
-|align=center bgcolor=white| +0.26 V
+|align=center bgcolor=white| +0.26
-|align=center bgcolor=white| -0.16 V
+|align=center bgcolor=white| -0.16
 |align=center bgcolor=blue|blue
 |align=center bgcolor=white|colorless
 |-
 |[[Phenosafranin]]
-|align=center bgcolor=white| +0.28 V
+|align=center bgcolor=white| +0.28
-|align=center bgcolor=white| -0.25 V
+|align=center bgcolor=white| -0.25
 |align=center bgcolor=red|red
 |align=center bgcolor=white|colorless
 |-
-|[[Safranin]] '''T'''
+|[[Safranin|Safranin '''T''']]
-|align=center bgcolor=white| +0.24 V
+|align=center bgcolor=white| +0.24
-|align=center bgcolor=white| -0.29 V
+|align=center bgcolor=white| -0.29
 |align=center bgcolor=#E41B9E|red-violet
 |align=center bgcolor=white|colorless
 |-
 |[[Neutral red]]
-|align=center bgcolor=white| +0.24 V
+|align=center bgcolor=white| +0.24
-|align=center bgcolor=white| -0.33 V
+|align=center bgcolor=white| -0.33
 |align=center bgcolor=red|red
 |align=center bgcolor=white|colorless
 |-
 |}
 ==See also==
+*[[Analytical chemistry|Chemical analysis]]
 *[[pH indicator]]
+*[[Complexometric indicator]]
-*[http://pac.iupac.org/publications/pac/pdf/1978/pdf/5005x0463.pdf Redox Indicators. Characteristics And Applications]
+==References==
+{{Reflist}}
+==External links==
+*[http://old.iupac.org/publications/pac/1978/pdf/5005x0463.pdf Redox Indicators. Characteristics And Applications]
 {{DEFAULTSORT:Redox Indicator}}
-[[Category:Redox indicators| ]]
+[[Category:Redox indicators]]
 [[Category:Physical chemistry]]
-[[it:Indicatore (chimica)#Indicatori di ossido-riduzione]]