Redox indicator

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

{| border="1"

!Indicator

!E⁰, V

!Color of Oxidized form

!Color of Reduced form

|-

|[Ru^III/II(2,2'-bipyridine)₃]

|align=center| +1.33

|align=center bgcolor=green|green

|align=center bgcolor=orange|orange

|-

|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| +0.01

|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

|-

|Indigo carmine

(syn. Indigodisulfonic acid

|align=center bgcolor=white| +0.29

|align=center bgcolor=white| -0.13

|align=center bgcolor=blue|blue

|align=center bgcolor=white|colorless

|-

|Indigomono 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 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

• Chemical analysis
• pH indicator
• Complexometric indicator

References

External links

• Redox Indicators. Characteristics And Applications