Editing Redox indicator

{{short description|Indicator which undergoes a definite color change at a specific electrode potential}}
{{refimprove|date=December 2009}}
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 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 classes of redox indicators:
* [[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]].

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==
{| border="1" 
!Indicator
!E<sup>0</sup>, V
!Color of Oxidized form
!Color of Reduced form
|-
|[[2,2'-bipyridine]] (Ru complex)
|align=center| +1.33 
|align=center bgcolor=white|colorless
|align=center bgcolor=yellow|yellow
|-
|Nitro[[phenanthroline]] (Fe complex)
|align=center| +1.25 
|align=center bgcolor=cyan|cyan
|align=center bgcolor=red|red
|-
|N-[[Phenylanthranilic acid]]
|align=center| +1.08 
|align=center bgcolor=violet|violet-red
|align=center bgcolor=white|colorless
|-
|1,10-Phenanthroline iron(II) sulfate complex ([[Ferroin]])
|align=center| +1.06 
|align=center bgcolor=cyan|cyan
|align=center bgcolor=red|red
|-
|N-[[Ethoxychrysoidine]]
|align=center| +1.00 
|align=center bgcolor=red|red
|align=center bgcolor=yellow|yellow
|-
|2,2`-[[Bipyridine]] (Fe complex)
|align=center| +0.97 
|align=center bgcolor=cyan|cyan
|align=center bgcolor=red|red
|-
|5,6-Dimethyl[[phenanthroline]] (Fe complex)
| +0.97 
|align=center bgcolor=#CCFF00|yellow-green
|align=center bgcolor=cyan|cyan
|-
|[[o-Dianisidine]]
|align=center| +0.85 
|align=center bgcolor=red|red
|align=center bgcolor=white|colorless
|-
|Sodium [[diphenylamine sulfonate]]
|align=center| +0.84 
|align=center bgcolor=#E41B9E|red-violet
|align=center bgcolor=white|colorless
|-
|[[Diphenylbenzidine]]
| +0.76 
|align=center bgcolor=violet|violet
|align=center bgcolor=white|colorless
|-
|[[Diphenylamine]]
|align=center| +0.76 
|align=center bgcolor=violet|violet
|align=center bgcolor=white|colorless
|-
|[[Viologen]]
|align=center| -0.43 
|align=center bgcolor=white|colorless
|align=center bgcolor=blue|blue
|}

==pH dependent==
{| border="1" 
!Indicator
!E<sup>0</sup>, V
at pH=0
!E, V
at pH=7
!Color of 
Oxidized form
!Color of 
Reduced form
|-
|Sodium 2,6-Dibromophenol-indophenol
or Sodium [[DCPIP|2,6-Dichlorophenol-indophenol]]
|align=center bgcolor=white| +0.64 
|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 
|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 
|align=center bgcolor=white| +0.06 
|align=center bgcolor=violet|violet
|align=center bgcolor=white|colorless
|-
|[[Methylene blue]]
|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=blue|blue
|align=center bgcolor=white|colorless
|-
|Indigotetra[[sulfonic acid]]
|align=center bgcolor=white| +0.37 
|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 
|align=center bgcolor=white| -0.08 
|align=center bgcolor=blue|blue
|align=center bgcolor=white|colorless
|-
|[[Indigo carmine]] 
(syn. Indigodi[[sulfonic acid]]
|align=center bgcolor=white| +0.29 
|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 
|align=center bgcolor=white| -0.16 
|align=center bgcolor=blue|blue
|align=center bgcolor=white|colorless
|-
|[[Phenosafranin]] 
|align=center bgcolor=white| +0.28 
|align=center bgcolor=white| -0.25 
|align=center bgcolor=red|red
|align=center bgcolor=white|colorless
|-
|[[Safranin|Safranin '''T''']] 
|align=center bgcolor=white| +0.24 
|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 
|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]]

==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:Physical chemistry]]