A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific
electrode potential
An electrode is an electrical conductor used to make contact with a nonmetallic part of a Electronic circuit, circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can c ...
.
The requirement for fast and reversible color change means that the oxidation-reduction
equilibrium
Equilibrium may refer to:
Film and television
* ''Equilibrium'' (film), a 2002 science fiction film
* '' The Story of Three Loves'', also known as ''Equilibrium'', a 1953 romantic anthology film
* "Equilibrium" (''seaQuest 2032'')
* ''Equilibr ...
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 complex
A coordination complex is a chemical compound consisting 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 ...
es of
phenanthroline and
bipyridine. In these systems, the metal changes oxidation state.
*
organic redox systems such as
methylene blue
Methylthioninium chloride, commonly called methylene blue, is a salt used as a dye and as a medication. As a medication, it is mainly used to treat methemoglobinemia. It has previously been used for treating cyanide poisoning and urinary trac ...
. 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
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
.
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
*
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. Separa ...
*
pH indicator
A pH indicator is a halochromism, halochromic chemical compound added in small amounts to a Solution (chemistry), solution so the pH (acidity or Base (chemistry), basicity) of the solution can be determined visually or spectroscopically by chang ...
*
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 exis ...
References
External links
Redox Indicators. Characteristics And Applications
{{DEFAULTSORT:Redox Indicator
Redox indicators
Physical chemistry