What Exactly is Oxidation Reduction Potential (ORP)?

Author: Troy McGinty, Product Manager Hayward Commercial Pool


I would like to start this article by reminding everyone that ORP (Oxidation Reduction Potential) is not exactly the easiest concept to understand. However, I will attempt to portray the information in this article in the most Laymen terms as possible. When we are measuring ORP, we are measuring the quality and the capability of the oxidizing component in the water. The oxidizing component can come from chlorine, bromine, Ozone, non-chlorine shocks, hydrogen peroxide, etc. As we all know, sanitizers will only work predictably on a properly balanced body of water. This is where ORP comes into play. ORP specifically measures the ability and/or quality of the oxidizer present in the water. Its ability/quality are decreased or increased in relation to the other water chemistry variables; pH, total alkalinity, total dissolved solids, temperature, the presence of cyanuric acid, the presence of chlorine neutralizers, etc. Simply put, it does not matter how much sanitizer is in your body of water, if the other chemistry variables are not balanced, the sanitizer WILL NOT WORK to its full “potential”.

Oxidation, in the past, was merely defined as the interaction between oxygen molecules and all dissimilar matter that they may come in contact with. Upon the discovery of electrons, oxidation came to be more accurately defined as the loss of an electron when two or more matters interact. So when a sanitizer interacts with unwanted substances it steals an electron from the substance changing its chemical make-up, in turn helping to burn the substance up. However, now that the sanitizer has gained an electron it has “reduced” the sanitizer’s electrical energy making the sanitizer less effective.

Reduction occurs when a substance gains an electron after the oxidation process has occurred. The atom or oxidizer that gains the electron may lose their ability to further oxidize substances or they will combine with other substances in the water resulting in by-products and/or combined chlorine.

Potential is the concept of stored energy within. The potential as it relates to ORP relates to the potential of the sanitizer present in the water to oxidize contaminants. The potential is measured in millivolts, a very small electrical reading. When an oxidizer is present in water it is attempting to steal negatively charged electrons from any substance. If the substance is a platinum or gold electrode (ORP Sensor) once the oxidizer steals the negatively charged electron from the electrode the electrode will then generate more positively charged energy, therefore increasing ORP. The more oxidizer present in the water the more positively charged the electrode becomes.

When discussing ORP, often times people like to leave out the reduction part of it. Why? Because we are measuring the potential of the oxidizer to steal electrons – not the potential of substance being reduced or gaining electrons. However, if there are exactly the same amounts of reducers, both oxidizer atoms that have gained electrons and oxidizer atoms that have not yet gained electrons, will cause there to be no “Potential” for future oxidation. That is why we must continually sanitize water or eventually it will result in the sanitizer being used up.

pH is the most important factor in keeping the strength of your sanitizer. The Potential of Hydrogen or Power of Hydrogen relates to HOCL and OCL. If you have a lower pH the HOCL – the stronger chlorine, will be more active and the sanitizer will be more effective. If the stronger part of your chlorine is working harder and is more active, the more oxidizing potential it will have. Lower pH means more available H+ in the water, which means that the water will not take the H+ as frequently from the HOCL, resulting in more HOCL and less OCL (your weaker free chlorine).

In conclusion ORP (Oxidation Reduction Potential) is the measurement of the potential of the oxidizer present in the water to remove electrons from unwanted substances. Once the unwanted substance loses it’s electron its chemical structure is changed and it is “burned up”.

ORP is not tricked by the effects of other variables present in the water. ORP tells you the true effectiveness of your sanitizer based solely on its ability to oxidize in that specific body of water. The millivolt reading of 650mV has been designated as the potential needed for safe drinking water. The relationship of pH, FAC, and ORP are very important. We must keep a more acidic (7.5 or lower) pH in our water to make sure our HOCL is more active which gives it more oxidizing ability, in turn raising our ORP reading. Automated pH and ORP controllers constantly monitor both the pH and ORP in the water and make the necessary adjustments to keep a proper pH and a proper amount of oxidizer present in the water to keep your water safe 24/7.

Troy McGinty HeadShotAbout Author: M. Troy McGinty is the global product manager for commercial products for Hayward Commercial Pools, a division of Hayward Industries, a pool equipment manufacturer. He first became involved in the pool and spa industry in 2003 and has developed and commercialized a number of swimming pool and spa products. He is a Certified Pool Operator/Instructor (CPOI), Aquatic Facility Operator (AFO) and has led numerous seminars and training sessions across North America on the topic of pH and ORP automation, communication, and business development. He can be reached via e-mail at mtmcginty@haywardnet.com.