For this post, I'm sharing an infographic I made a while ago as a reference for key oxidation-reduction potential (ORP) ranges in water.
Oxidation-Reduction Potential (ORP) is analogous to pH, except that it concerns the aqueous activity of electrons instead of protons (i.e. hydrogen ions). All life requires energy, and oxidation-reduction (redox) reactions have energy that can be conveniently captured in the form of transferred electrons. Different types of microorganisms live off of different redox reactions that take place at different ORPs. In biological wastewater treatment, knowing the ORP can provide insight into which biological processes are prevailing or viable.
For my infographic, below, I took the ORP ranges cited by Michael Gerardi in this excellent article.
In practice, ORP values can depart from theoretical values which assume ideal conditions, so these ranges are a rough guide only.