The oxidation-reduction potential (ORP) is measured by inserting an ORP sensor into water. This can be either a pocket meter or a meter with a separate electrode. The meter then reads the electrical potential (voltage) from the sensor and may make a correction or offset before reporting the value. The specified values, which can be positive or negative, are usually given in millivolts (mV).
Structure of a redox sensor
ORP sensors measure the electrical potential (voltage) between two electrodes in contact with water. Both electrodes are often contained in a single unit called a combination electrode. An electrode is called an indicator electrode and is usually made of platinum (sometimes other materials such as gold or graphite are used).
The other electrode is called the reference electrode. The reference electrode is usually made of silver and silver chloride (Ag / AgCl electrodes), although sometimes electrodes of mercury and mercuric chloride (so-called “calomel electrodes”) are used. This electrode contains a filling solution containing potassium chloride.
Electrons from the water interact with both electrodes and generate a voltage between them, which is read by the meter.
The standard hydrogen electrode (SHE)
The voltage generated between the two electrodes actually depends on the type of reference electrode. For example, a higher voltage is read for the same water when using an Ag / AgCl electrode than when using a calomel electrode [1]. For this reason, it is sometimes useful to report a redox reading that has been adjusted to correspond to a standard reference electrode called a standard hydrogen electrode (SHE). SHE ORP values are comparable to a quantity called Eh, which describes the theoretical ORP of the solution and has the same scale as SHE ORP measurements.
The SHE requires the injection of hydrogen gas through a strongly acidic solution, so it is not practical for routine laboratory or field use. This reference electrode generates a voltage that is about 200 mV higher than that generated with the Ag / AgCl electrode. When converting an Ag / AgCl-ORP value to SHE, +200 mV is added to the Ag / AgCl-ORP number.
| Technical note: The actual mV value added when expressing a redox valuein terms of SHE may depend on the concentration of the filling solution [1]. |
ORP standard solutions and calibration
Since the meter reads the ORP voltage directly from the sensor, calibration is not absolutely necessary. Since the response of the sensor may degrade over time, it is sometimes desirable to test the sensor with a standard solution to ensure that it provides the correct response, e.g., within +/- 10 mV [2]. Two common standard ORP solutions are the Zobell solution and the Light solution. At 25 ° C, the Zobell solution has an ORP of approximately +228 mV when measured with an Ag / AgCl reference electrode, and the Light solution has an ORP of approximately +475 mV under the same conditions [1].
Heading: Typical redox values (in mV) of the solutions of Zobell and Light for the three types of reference electrodes [1].
| Calomel | Ag/AgCl | SHE | |
| Zobell’s | +183 | +228 | +428 |
| Lights | +430 | +475 | +675 |
| Technical note: The actual ORP values of the Zobell and Light solutions depend on the temperature and concentration of the filling solution [1,2]. |
Testing with the standard solution can also be used to adjust the actual measured value to match the value of the standard. This is achieved by adding an offset to the actual measured mV value (the offset is the difference between the ideal mV value and the actual mV value for the standard). This can be considered a form of calibration, and most gauges can be set up to add the offset automatically.
An offset can also be applied in other situations. For example, you may want the ORP sensor to indicate a specific mV value when certain chemical conditions are reached, such as a specific combination of pH and chlorine concentration in a water disinfection application. This technique can be used to make ORP readings consistent between different ORP sensors, or a new offset can be calculated when an old electrode is replaced with a new one.
Information about ORP measurements
ORP measurements are most often expressed in mV values relative to the reference electrode used. The Ag / AgCl reference electrode is most commonly used, so values are usually given based on measurements with this electrode. Calomel electrodes are less common because they contain mercury. Therefore, calomel redox values are relatively rare.
Sometimes, however, values are given relative to the SHE. As explained above, these values are about 200 mV higher than the values for the Ag / AgCl electrode.
When redox values are given, the reference electrode on which the values are based should also be given to avoid confusion. If values are displayed where the reference electrode is not specified, it is normally assumed that they are specified relative to the Ag / AgCl electrode.
Conclusion
ORP is a common, simple and straightforward measurement that quickly indicates how oxidizing or reducing a water is. The reading provided by the meter depends on the condition of the sensor, the offset value provided during calibration, and the type of reference electrode used. Therefore, care must be taken in interpreting the results. Nevertheless, ORP is particularly useful for either providing a quick snapshot of the oxidation / reduction state of a solution or for monitoring its changes over 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.