A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific
electrode potential
In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode ...
.
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:
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metal complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es of
phenanthroline
1,10-Phenanthroline (phen) is a heterocyclic organic compound. It is a white solid that is soluble in organic solvents. The 1,10 refer to the location of the nitrogen atoms that replace CH's in the hydrocarbon called phenanthrene.
Abbreviate ...
and
bipyridine Bipyridines also known as bipyridyls, dipyridyls, and dipyridines, are a family of chemical compounds with the formula (C5H4N)2, consisting of two pyridyl (C5H4N) rings. Pyridine is an aromatic nitrogen-containing heterocycle. Bipyridines are of si ...
. In these systems, the metal changes oxidation state.
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organic redox systems such as
methylene blue
Methylthioninium chloride, commonly called methylene blue, is a salt used as a dye and as a medication. Methylene blue is a thiazine dye. As a medication, it is mainly used to treat methemoglobinemia by converting the ferric iron in hemoglobin ...
. In these systems, a proton participant 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
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
.
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
pH dependent
See also
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Chemical analysis
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
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pH indicator
A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...
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Complexometric indicator
A complexometric indicator is an ionochromic dye that undergoes a definite color change in presence of specific metal ions. It forms a weak complex with the ions present in the solution, which has a significantly different color from the form ex ...
References
External links
Redox Indicators. Characteristics And Applications
{{DEFAULTSORT:Redox Indicator
Redox indicators
Physical chemistry