Oxidation Reduction Potential (ORP)*
Definition: The ORP measures the level of oxidizing agents (accepts electrons) or reducing agents (donates electrons) in the water.
A positive ORP reading indicates that a substance is an oxidizing agent. The higher the reading, the more oxidizing it is. As such, a substance with an ORP reading of +400 mv is 4 times more oxidizing than a substance with an ORP reading of +100 mv.
A negative ORP reading indicates that a substance is a reducing agent. The lower the reading, the more anti-oxidizing it is. As such, a substance with an ORP reading of -400 mv is 4 times more anti-oxidizing than a substance with an ORP reading of -100 mv.
Ideal range: The reduction potentials in natural systems often lie comparatively near one of the boundaries of the stability region of water. Aerated surface water, rivers, lakes, oceans, rainwater and acid mine water, usually have oxidizing conditions (positive potentials). In places with limitations in air supply, such as submerged soils, swamps and marine sediments, reducing conditions (negative potentials) are the norm. ORP directly impacts the quality of the water.
Oxidation-reduction potential (ORP) monitoring water systems via a single-value measurement of the disinfection potential, showing the activity of the disinfectant rather than the applied dose. For example, the pathogens E. coli, Salmonella and Listeria have survival times of under 30 s when the ORP is above 665 mV, compared against >300 s when the ORP is below 485 mV.
Method of measurement: Reduction potentials of aqueous solutions are determined by measuring the potential difference between an inert sensing electrode in contact with the solution and a stable reference electrode connected to the solution by a salt bridge.
Method of Control: The most common method for adjusting the ORP is ozonation, which generates free radicals (O3) that break down organic compounds.
*Note: pH and ORP are inversely related:
pH determines the water’s relative state in receiving or gaining protons (i.e. hydrogen ions, H+) while ORP indicates the water chemistry’s relative state to receive or gain electrons.
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