A redox indicator, also known as an oxidation-reduction indicator, are indicator compounds that exhibit color variation at varied values of potential difference and undergoes a definite change in hue at specific electrode potential. The equilibrium for simultaneous oxidation and reduction reactions should be established rather quickly, which is why fast and reversible color change is required in the first place, and also explains as to why only a selected few groups of redox systems can be used for the indication purpose. Redox indicators can be divided into two major categories : Metal Organic complexes (Phenanthroline) and True Organic Redox Systems (Methylene Blue).
1) A condition for being a true redoc indicator is that a compound must have a reduced and oxidized form with varying colors and the process has to be reversibility
2) Colored inorganic oxidants and reductants can not be called redox indicators because of the factor of irreversibility associated with their reaction mechanism.
3) The participation of proton in their electromechanical reaction is a defining characteristic of true organic redox systems
4) The change of color between the oxidized and reduced forms of the redox indicators is fully reversible, even though the shades of color may vary depending upon the electrostatic conditions and values of potential.
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