Properly measuring ORP (Oxidation-Reduction Potential) and understanding its limitations when used to monitor and control oxidizing biocide feed is important to minimize the potential for increased corrosion rates, excessive chemical feeds, and biofouling.
What is ORP and How is it Used?
ORP is a measure of a system’s ability to oxidize or reduce a substance in the water. In simple terms, a water with an ORP greater than 0 mV is in an oxidizing environment, while water with an ORP less than 0 mV is in a reducing environment.
ORP can be used as an estimation of oxidizing biocide residuals and activity. However, using ORP to control oxidizer additions is not as simple as targeting a particular ORP value or increase above the normal background operating levels. When the free and/or total chlorine readings are in the desired range, the corresponding ORP level should be the target range for oxidizing biocide feeds. It does not matter what the ORP reading is or how much the ORP increases as long as it maintains the desired chlorine residuals.
An ORP probe connected to a controller or a handheld meter is used to measure water’s oxidizing potential in millivolts (mV). When using a handheld meter, it is important to note that ORP readings can drift and take as long as 30 minutes to stabilize, and they may not exactly match the controller readings. The ORP reading itself is not as important as the oxidizer residuals in the system. ORP readings should only be used to establish the desired halogen residuals within the system itself.
Routine probe maintenance is required for consistent ORP readings. Like pH probes, which are similar in design, ORP sensors must remain wet at all times. When not in use, they should be immersed in a pH/ORP probe storage solution or a neutral pH buffer. Storing ORP/pH probes in DI water will cause the electrolyte to leach from the probe and degrade its life and performance. Monthly cleaning with a soft cloth or brush is very important to help prevent deposits on the probe surfaces. If deposits do form, they may be removed with a cloth, brush, or by soaking in dilute acid. A properly maintained probe can last 12-18 months before a replacement is needed and should never require field calibration.
Factors Affecting ORP Readings
Monitoring and controlling oxidizing biocide additions based on ORP readings can be challenging. Oxidizing agents such as chlorine increase ORP while reducing agents such as sulfite reduce ORP. There are several other factors that will impact readings. For example, ORP has an inverse relationship with temperature, which means the higher the temperature, the lower the ORP. A higher pH will see a smaller ORP response to oxidizer feeds. Due to these factors, the same oxidizing biocide dosage added to two different water systems can give very different oxidizer residuals and ORP readings.
A basic understanding of the relationship between pH and ORP is particularly important in cooling water treatment. The higher the pH, the lower the ORP response due to the higher concentration of the less active hypochlorite/hypobromite to hypochlorous/hypobromous acid. Keep in mind that the free and total chlorine residuals do not vary with pH, only the ORP response changes. When the pH in a system varies due to changes in makeup water quality, inconsistent cycles, extended prebleed, bleed lockout times, or high pH chemical additions, the ORP setpoint required to maintain a particular oxidizer residual also varies, and the oxidizer residual associated with a particular ORP setpoint can vary even within the same system.
The stabilizer in some oxidizing biocides, the excess ammonia in chloramine treated makeup water, and the presence of organics in a dirty tower creates additional oxidizer demand that can also significantly reduce the ORP response associated with a particular free/total chlorine reading. It is recommended that you routinely monitor the split between free and total chlorine readings during oxidizer feeds as an indicator of concerns related to high demand. If you find a large split between the readings (e.g. low free and high total), the system may need to be cleaned, flushed, and/or disinfected for effective microbiological control. Where chloramine treated makeup water is used, you may also find that high oxidizer dosages are required to achieve a free chlorine residual, and that it is more practical to control the feeds based on total readings.
Correlating ORP values with the target free and total halogen residuals on each individual system is required to control an oxidizer program with ORP controls. Using ORP to control halogen feed can be useful in maintaining target residuals as long as you realize that it's the residuals that matter and not the ORP reading itself.