The ORP value, also known as the redox potential, is an important parameter in water analysis. It indicates how strongly oxidizing or reducing a liquid is, thereby providing insight into its chemical state.
In this article, you will learn exactly what the ORP value means, how it is measured, which standard solutions are involved, and what you should keep in mind when interpreting the results.
Would you like to start by understanding what ORP actually is? What is the oxidation-reduction potential (ORP)?
What is the ORP value?
ORP stands for Oxidation-Reduction Potential, or redox potential for short. This value describes how strongly a liquid tends to oxidize or reduce other substances.
A positive ORP value indicates that the liquid has an oxidizing effect, for example due to dissolved chlorine in swimming pool water. A negative value indicates a reducing environment, such as that found in oxygen-poor groundwater.
The unit of the ORP value is millivolts (mV). The values can be positive or negative and typically range from -2000 mV to +2000 mV, depending on the application.
ORP is often measured in conjunction with pH, as these two parameters together provide a complete picture of a liquid’s chemical state.
More on this: What is pH?
Structure of an ORP sensor
An ORP sensor measures the electrical potential between two electrodes, both of which are in contact with the sample. In modern devices, both electrodes are often combined into a single unit, known as a combination electrode.
Indicator electrode: This electrode is usually made of platinum, and less commonly of gold or graphite. It reacts to the redox activity of the sample and generates an electrical signal.
Reference electrode: The reference electrode provides a stable reference potential. Silver/silver chloride (Ag/AgCl) electrodes are the most commonly used. Calomel electrodes, which contain mercury and mercury chloride, have become less common because they contain mercury. The reference electrode contains a potassium chloride-based filling solution.
The voltage generated between the two electrodes is measured by the meter and displayed as an ORP value in millivolts.
Measuring ORP Correctly
Measuring the ORP value is straightforward. Unlike pH measurement, calibration is not strictly necessary, as the meter reads the redox potential directly from the sensor.
Nevertheless, it is recommended that you test the sensor regularly using a standard solution to ensure that it is still functioning properly. More on this in the next section.
The basic measurement procedure:
Step 1: Prepare the sensor
Rinse the ORP sensor with distilled water and gently pat it dry. Do not rub it, as this can damage the sensitive electrode surface.
Step 2: Dip the sensor
Immerse the sensor completely in the sample. The measuring tip must be completely wetted. A stable value can be achieved more quickly by swivelling slightly.
Step 3: Read the value
Do not read the result until the device displays a stable value. ORP sensors may take slightly longer than pH electrodes to stabilize.
Step 4: Clean and store the sensor
After taking the measurement, rinse the sensor again with distilled water and place it back in the protective cap containing the storage solution.
ORP standard solutions and calibration
Even though calibration is not strictly necessary for an ORP meter, the sensor’s response may deteriorate over time. A test using a standard solution will show whether the sensor is still functioning reliably—for example, whether it remains within ±10 mV of the setpoint.
Two common standard ORP solutions are the Zobell solution and the Light solution:
| Solution | Ag/AgCl | SHE |
|---|---|---|
| Zobell solution | +228 mV | +428 mV |
| Light version | +475 mV | +675 mV |
| Technical note: The actual redox values of the Zobell and Light solutions depend on the temperature and concentration of the buffer solution [1,2]. |
If the measured value differs from the setpoint, most measuring devices allow you to set an offset value. The device then automatically adds the difference between the actual measured value and the ideal setpoint to provide more reliable results.
About Apera calibration solutions and accessories
Interpreting ORP Results Correctly
When interpreting ORP readings, there is one important point to keep in mind: The displayed value depends on the reference electrode used.
The most commonly used reference electrode is the Ag/AgCl electrode. ORP values are therefore typically reported relative to this electrode.
Sometimes values are given relative to the standard hydrogen electrode (SHE). These values are about 200 mV higher than the corresponding Ag/AgCl values. The SHE itself is not suitable for practical use, as it requires the introduction of hydrogen gas through a strongly acidic solution.
When comparing ORP values from different sources, you should always check which reference electrode they are based on. If no information is provided, Ag/AgCl is usually assumed.
Typical ORP values in practice
The ORP value alone doesn’t tell you much without knowing the context. Here are some guidelines for common applications:
Drinking water: typically between +200 mV and +600 mV, depending on the treatment process and chlorine content.
Swimming pool water: For effective disinfection, ORP values between +650 mV and +750 mV are recommended.
Aquaculture: Healthy water for fish typically has a pH between +200 mV and +400 mV.
Reducing groundwater: may have negative ORP values, sometimes below -200 mV.
ORP and pH are closely related. Many Apera devices measure both parameters simultaneously, which is particularly useful in water treatment and aquaculture.
Conclusion
The ORP value is a simple and quick measurement that provides valuable information about the chemical state of a liquid. The measurement itself is straightforward, but it requires proper handling of the sensor and a basic understanding of the reference electrode to which the values refer.
Monitoring ORP and pH levels together provides a complete picture of water quality.
Frequently Asked Questions About the ORP Value
What is a good ORP value?
That depends on the application. For swimming pool water, values between +650 mV and +750 mV are considered ideal for reliable disinfection. In aquaculture, values between +200 mV and +400 mV are typical for healthy water.
What is the difference between ORP and redox potential?
ORP and redox potential refer to the same thing: a liquid’s ability to oxidize or reduce other substances. ORP is the abbreviation for oxidation-reduction potential.
Does an ORP meter need to be calibrated?
Calibration is not strictly necessary for the ORP meter, as the device reads the voltage directly from the sensor. Nevertheless, it is recommended to perform regular tests using a standard solution, such as the Zobell or Light solution, to ensure that the sensor is still responding correctly.
What does a negative ORP value mean?
A negative ORP value indicates that the liquid has a reducing effect. This is the case, for example, in oxygen-depleted groundwater or in certain industrial processes. In drinking water or swimming pool water, a negative ORP value is a sign that disinfection is not taking place adequately.
Why do ORP values vary depending on the reference electrode?
The voltage displayed depends on the reference electrode used for the measurement. Ag/AgCl electrodes and calomel electrodes yield different values for the same sample. When comparing ORP values from different sources, the reference electrode used should always be specified.
Can I measure ORP and pH at the same time?
Yes. Many Apera devices measure both parameters simultaneously, which is particularly useful in water treatment, aquaculture, and swimming pool applications.
How long does an ORP sensor last?
The service life depends on the frequency of use and how well it is maintained. If stored properly in a storage solution and cleaned regularly, a lifespan of one to two years is realistic. Signs that replacement is necessary include consistently unstable readings or a very long response time.
References
[1] American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.
[2] U.S. Environmental Protection Agency (2017) Field measurement of oxidation reduction potential (ORP). SESDPROC-113-R2.