Oxidation Reduction Potential (ORP) is a method to measure oxidation in water. ORP is a common measurement of water quality in a wide variety of water treatment applications. So, what is ORP and how does it work?
Fundamentals of ORP Measurement in water
ORP stands for oxidation-reduction potential, this is a measure, in millivolts of the tendency of water to oxidize or reduce substances within that water or that the water comes in contact with.
Oxidation is the loss of electrons, or an increase in oxidation state by a molecule, atom or ion. When a substance has been oxidized it’s oxidation state increases. Greater ORP values in water equate to create potentials for oxidation by that water. Any compounds within the water that increase the ORP of the water will be indicated by the ORP meter. In an ozone system, an elevated ORP value can be primarily attributed to the increase of ozone in water along with the resulting ozone by-products, oxygen, hydrogen peroxide, hydroxyl radical, etc.
An ORP sensor in water will measure very small voltages generated by the construction of that probe placed in water. The ORP sensor consists of an ORP electrode and a reference electrode. The principle behind the ORP measurement is the use of an inert metal electrode (commonly platinum) which, due to its low resistance, will give up electrons to an oxidant or accept electrons from a reductant. The ORP electrode will continue to accept or give up electrons until it develops a potential, due to the build-up of charge, which is equal to the ORP of the solution.
ORP sensors will, over time, become less accurate, or cease to provide any voltage changes or usable ORP values. Contamination in the water can coat the ORP sensor probes and cause less accurate ORP readings and slower reactions to changes in ORP values in water. The electrode used in the ORP sensor will also age, due to normal wear of the platinum electrode due to water flowing past the electrode. Due to normal aging and coating of the sensor an ORP sensor should be replaced annually, or as needed to provide accurate ORP readings in water.
Measure ozone in water with ORP
To use the ORP measurement to measure ozone in water a correlation must be made. ORP is a mV signal while ozone is a PPM signal. If the only oxidant added to the water is ozone, a safe assumption can be made that any changes to ORP (mV) are changes in ozone in water. The ozone level in the water can be measured with a manual ozone test kit or meter. This ppm level can be correlated to the mV reading to provide an accurate ozone reading with ORP.
As ORP levels go up the ORP sensor is less and less responsive. To measure ozone in water at levels above 1.0 ppm the ORP sensor will not be suitable.
Why use ORP to measure ozone?
Any time the water is discolored or dirty a dissolved ozone meter will not accurately measure ozone in water. In these cases, ORP is your only option. Also, in most of these applications, dissolved ozone levels are very low, therefore ORP is a valuable method of measuring ozone in water.