Redox indicator

{{short description|Indicator which 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 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.{{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}}

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

border="1"

!Indicator

!E⁰, 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

Nitrophenanthroline (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-Dimethylphenanthroline (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

!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

|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| +0.01HEWITT, LF. "Oxidation-Reduction Potentials in Bacteriology and Biochemistry." Oxidation-Reduction Potentials in Bacteriology and Biochemistry. Edn 6 (1950).

|align=center bgcolor=blue|blue

|align=center bgcolor=white|colorless

Indigotetrasulfonic acid

|align=center bgcolor=white| +0.37

|align=center bgcolor=white| -0.05

|align=center bgcolor=blue|blue

|align=center bgcolor=white|colorless

Indigotrisulfonic acid

|align=center bgcolor=white| +0.33

|align=center bgcolor=white| -0.08

|align=center bgcolor=blue|blue

|align=center bgcolor=white|colorless