Redox potential (reduction potential or ORP) is an abbreviation for reduction and oxidation potential. A redox potential is the potential value when the reduction and oxidation of a certain metal or other chemical substance are in balance with each other. Redox potential is measured in Volt (V).
Redox potential values (under certain standard conditions) are fixed and can be found in tables like this one. For example, the redox potential of silver (Ag) is +0.7994 V and the redox potential of zinc (Zn) is ‑0.763 V.
Redox potential values
The positive and negative value of a redox potential are set off against the redox potential of Hydrogen (H2), which is set by definition at 0 V under standard conditions. Metals with a positive redox potential are called noble metals. Metals with a negative redox potential are called base metals.
What causes a redox potential reached?
If a metal is immersed into a liquid, the metal atoms will oxidize (loose an electron) and the positive metal particles (ions) dissolve into the liquid. The electrons stay in the metal, while the positive metal ions dissolve. The metal is negatively charged, this charge pulls the positive ions back to the metal surface.
The dissolving and returning of metal ions form an equilibrium that is specific for the metal. This process actually defines the redox potential. From the outside it looks like nothing is happening anymore. A steady state has formed and a constant potential can be measured.
Scientific tests using redox potentials
Because the redox potential values of different substances are known, their presence in a liquid can easily be demonstrated. This is done by gradually increasing or decreasing the potential using a potentiostat. Such tests are often carried out in electrochemical research.
If the potential is gradually increased from a point below the redox potential, a reductive electron current (I) is measured, which will become smaller and smaller until the redox potential is reached. After that point an oxidative current will occur and grow larger, as the potential is further increased. This development can be seen in a voltammogram.
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