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pH/ORP Measurement Tips For All Users


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pH and ORP and widely used measurements of water's character and suitability. This is a reprint of an article that appeared in on 6/29/2017.

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pH/ORP Measurement Tips For All Users

  1. 1. 1Water Online • ARTICLE Think You Know ORP? Get 5 pH/ORP Measurement Tips Water and wastewater treatment professionals are constantly looking for as much information as possible about the quality of their water. If knowledge is power, then understanding the properties of their water is key to running an effective and efficient facility. Of course, there are many measurements of water quality and several ways to determine them. Two of the most fundamental quality parameters are pH and oxidation-reduction potential (ORP). Below, you’ll find the five keys for measuring pH/ORP. 1. Know What You’re Measuring It might sound obvious, but to control pH and ORP in water, the first step is to understand why these measurements need to be monitored and what acceptable levels look like. To begin with, pH is possibly the most fundamental parameter for maintaining good water quality. It is basically a measure of the acidity of a sample, and that acidity can have a significant impact on treatment results. • “High-pH water often tastes bitter and can indicate the scaling potential of the water,”per • “Low-pH water may lead to the dissolution of pipes, particularly copper pipes.” • The EPA classifies pH under unregulated “Secondary Drinking Water Standards” and recommends a range between 6.5 and 8.5 pH units. Meanwhile, ORP is lesser known, yet still a fundamental parameter for determining water quality. ORP measures a substance’s ability to oxidize or reduce molecules around it or, in other words, its potential to steal or donate electrons. Most types of water, including tap water and bottled water, contain dissolved oxygen so their ORP value is positive.1 When used in wastewater treatment systems, ORP indicates the ability or potential of wastewater to permit the occurrence of specific biological (oxidation-reduction) reactions. ORP is measured in volts (V) or millivolts (mV). Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species’ affinity for electrons and tendency to be reduced. “The best definition I can give is that ORP is a measure of the cleanliness of the water and its ability to break down contaminants,”reported Ozone Solutions. “It has a range of -2,000 to +2,000 and units are in mV. On the ORP scale, the presence of an oxidizing agent such as oxygen increases the ORP value, while the presence of a reducing agent decreases the ORP value. Since ozone is an oxidizer, we are only concerned with positive ORP levels (above 0 mV).” Oxidation reduction potential can be used for water system monitoring with the benefit of a single-value measure of the disinfection potential, showing the activity of the disinfectant rather than the applied dose. For example, E. coli, Salmonella, Listeria, and other pathogens have survival times of under 30 s when the ORP is above 665 mV, compared against >300 s when it is below 485 mV. 2 2. Know Your ORP Electrodes ORP tests are quick and easy to perform. To measure ORP solutions, utilize a metal electrode made of gold or platinum, which can pass electrons to oxidants or receive them from reductants. Once a charge is adequately built, the electrode will have a potential that is equal to the solution’s ORP. 1 2 Trevor V. Suslow, 2004, Oxidation-Reduction Potential for Water Disinfection Monitoring, Control, and Documentation, University of California Davis, Image credit: “Image from page 318 of “Chemistry of dye-stuffs (1903)” Internet Archive Book Image © 1903 used under an Attribution 2.0 Generic license:
  2. 2. 2Water Online • ARTICLE However, it is possible that a low rate of electron exchange between the electrode and the solution can result in inaccurate readings, as the ORP can respond more strongly to dissolved oxygen or another redox couple. Because of this, operators should be sure to check ORP measurement devices with the process sample in a laboratory before putting them in use. 3. Know Your Points A key to monitoring pH and ORP is calibration. This process essentially measures the response of an instrument to changing an input that it is measuring in a predetermined way. ORP calibration generally means using a single point. The ORP electrode directly measures the mV in the process solution and only requires a single standard solution for calibration. Unlike ORP, pH calibration relies on multiple points. The pH electrode directly measures mV in the process solution but expresses the result as a pH reading using the Nernst equation. Most pH electrodes deviate from ideal efficiency and benefit from calibration with at least two buffers. Thus, it is critical to keep in mind that ORP calibration will be done with a single solution, while pH calibration will require multiple solutions. By monitoring the ORP of wastewater, an operator can determine what biological reaction is happening and if conditions need to be changed to promote or prevent that reaction. 4. Know the Temperature Unbeknownst to many, temperature can have a significant impact on accurate pH and ORP measurement. An ORP measurement value is an indicator of how well the components of the sample can oxidize its contaminants. While oxidation can be dramatically affected by temperature changes, ORP does not measure any single chemical, so it is not generally possible to correct measurements for temperature changes. That being said, it’s always recommended that an operator record temperature levels at which ORP readings were taken. Measurements of pH, on the other hand, can be corrected for temperature for improved accuracy. Temperature increases result in chemical activity increases, which will affect a pH electrode’s reflection of hydrogen ions in the solution. To ensure accurate measurements, a pH sensor will measure the temperature and automatically correct the pH for it using the Nernst equation. 5. Know Your Tools Perhaps most critical to ORP and pH measurement is the use and understanding of proper equipment. There are many solution providers out there with their own proprietary products for water quality measurement. ORP probes are extremely versatile for monitoring biological reactions in wastewater treatment plants. They can indicate if acceptable or unacceptable biological activity is occurring. Knowing the ins and outs of the treatment processes, operators should utilize quick, reliable, and accurate sensors and analyzers by a vendor they can trust and that will be responsive in cases of malfunction or other issues.