Oxidation Reduction Potential is a key indicator of the effectiveness of sanitizers and the overall disinfecting power of the water. Simply put, in the term “Oxidation- Reduction,” Oxidation is the gain of oxygen and reduction is the loss of oxygen. Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons between two substances. In these reactions, the substance that accepts electrons is called the oxidizer, and the substance that donates electrons is called the reductant.
ORP – HOW IT WORKS
In swimming pools, oxidation-reduction potential (ORP) refers to an electrochemical probe which is used to measure output signals for chemical controllers. The ORP itself is also referred to as “redox” or reduction oxidation activity within the pool water. As an example, chlorine is an oxidizing agent. When chlorine oxidizes a contaminant, it gains electrons and is reduced to chloride. The contaminant loses electrons and is oxidized. This reaction of chlorine with contaminants is considered as a redox reaction. ORP devices that are part of an automated system consist of two electrode probes in order to measure the potential of the redox reaction along with a stable comparison potential to calculate the concentration of certain chemical species e.g., free chlorine or combined chlorine. The ORP probe is made of platinum due to its stability. The second probe is known as the reference electrode which provides a continual output for comparison. The reference electrode is usually made of silver or silver chloride. Some handheld units are single probe.
ORP – HOW IT IS USED
ORP is a measurement of the oxidizing capacity of a solution expressed in millivolts (mV). ORP manufacturers recommend positive ORP readings between 650 to 750 mV to ensure sufficient sanitation. Elevated levels of oxidizers will account for a higher ORP reading. Chlorine, bromine, and ozone are some examples of oxidizers that increase the oxidation capacity of water, which serves to increase the ORP reading. These readings do not reflect the quantity of an oxidizer but rather, are purely qualitative. In other words, ORP tells you what the chemical is doing as opposed to the amount of the chemical in the water. Chemical reducing agents, such as sodium thiosulfate and sodium sulfite, and amines which bind free chlorine, including cyanuric acid, ammonia and organic amines, reduce the oxidation capacity of the water and lower ORP readings by decreasing the hypochlorous acid (HOCl) concentration. Free chlorine has a higher ORP reading than chloramine. The presence of ammonia or organic nitrogen contributes to the formation of chloramines which cause the ORP to decrease. While ORP is an efficient means for measuring the oxidative potential of a sanitizer in the water, it is not intended as a direct measure of the sanitizer. ORP acts as a surrogate indicator of oxidizers such as chlorine, potassium monopersulfate, and ozone.
ORP – HIGHS AND LOWS
ORP is affected by any substance that is capable of donating or receiving electrons. In pool water, this action is mostly a result of chlorine. However, non-chlorine oxidizers — such as ozone, potassium monopersulfate, and even dissolved oxygen — serve to increase ORP readings. A higher ORP reading generally is an indicator of more sanitized pool water, but this is not the case with non-chlorine oxidizers. ORP readings should increase as more chlorine is added to the water. However, it is important to note that a decrease in pH will also lead to an increase in ORP readings. And contrarily, a high pH will cause a decrease in the ORP reading. Other substances that lead to decreased ORP readings are bacteria, algae, swimmer waste, high CYA, and debris such as dirt or leaves. Bacteria and algae will consume dissolved oxygen which will cause lower ORP readings. The pH plays a role upon ORP levels. High pH causes greater dissociation of chlorine where less hypochlorous acid (HOCl) is present compared to the hypochlorite ion (-OCl). Since -OCl is a weaker sanitizer and oxidizer than HOCl, the ORP will be reduced. Lower pH levels increase the percentage of the stronger sanitizer and oxidizer HOCl. Therefore, at lower pH, the ORP readings will increase. ORP is also sensitive to free chlorine even at low concentrations. A slight change in chlorine causes a substantial change in ORP, due to its oxidative reactions in the water.
When using a Salt Chlorine Generator, you may observe a drop in the ORP reading while the unit is generating, due to hydrogen gas being produced by the Chlorine Generator. In this case, turning the salt generator off and allowing for recovery time, the ORP should rise.
ORP AND AUTO CONTROLLERS
ORP systems are a part of automatic chemical control systems that monitor the chemical feeders of the pool. Most auto controllers used in commercial pools test and display ORP. The controllers are adjusted to a specific set point based on desired chemical levels. Readings from the ORP sensors are fed back to the controllers. If the value from the ORP sensors does not match the set point, the appropriate chemical feeder will be signaled to either increase or reduce chemical input into the pool. Some controllers have microprocessors that can calculate the parts per million ppm of free chlorine based on the ORP reading and the pH. The ORP mV, along with the calculated free chlorine ppm, are displayed on the controller. However, due to variables as a result of other substances that may be in the water, the controller ppm value may not always be the same as a manual DPD test. These controllers must be corrected based on manual DPD test readings to better ensure a more accurate auto controller display reading. ORP systems in commercial pools also incorporate a pH probe. When the pH probe senses a rise in pH, the controller will respond by turning on the acid feed system. Once the pH is lowered to the set point, the reading will notify the controller to shut off the feed system.
ORP – PROBE MAINTENANCE
Swimmer waste, organics and elevated concentrations of CYA can lead to fouling of the ORP probes, which can lead to reduced accuracy and slower responses to the chemical controllers. This could lead to the system incorrectly responding to changes in the water chemistry.
Fouling can occur from a non-visible film of dirt and oil. It is recommended that probes be cleaned according to the manufacturer’s directions. Most manufacturers advise monthly cleaning. Use a simple cleaning routine: With a mild detergent and a soft brush, such as a toothbrush, gently scrub and thoroughly rinse the probe. Do not let the probe get dry and replace it into the controller immediately.
ORP systems managed and maintained properly provide an approximation of the levels of sanitizers and oxidizers like chlorine, bromine, or ozone. It is important to note that the use of ORP controllers is NOT intended to be a replacement for required manual testing. The operator should understand that ORP is only reliable to report the oxidative activity in the water. ORP should not be used as a replacement for the proper method of measuring sanitizer concentration in ppm. DPD free chlorine tests are still required to ensure the correct minimum level of sanitizer is present at all times.
(ANSI/APSP/ICC-11 2019 American National Standard for Water Quality in Public Pools and Spas, Sections 8.5.1,8.5.2).
This article first appeared in the October 2025 issue of AQUA Magazine — the top resource for retailers, builders and service pros in the pool and spa industry. Subscriptions to the print magazine are free to all industry professionals. Click here to subscribe.
