Oxidation-Reduction Potential (ORP) is measured by inserting an ORP sensor into water. This can be either a handheld sensor or it can be built into a system. The meter then reads the electrical potential (voltage) from the sensor, and it may apply a correction or offset before reporting the value. The reported values, which can be positive or negative, are usually reported in millivolts (mV).
Anatomy of an ORP sensor
ORP sensors work by measuring the electrical potential (voltage) between two electrodes in contact with water. Both electrodes are often contained in a single unit called a combination electrode. One electrode is called the indicator electrode, and it is usually made of platinum (other materials such as gold or graphite are also sometimes used).
The other electrode is called the reference electrode. The reference electrode is usually made of silver and silver chloride (Ag/AgCl electrodes), although electrodes made of mercury and mercury chloride (called “calomel electrodes”) are sometimes used. This electrode contains a filling solution containing potassium chloride.
Electrons from the water interact with both electrodes, creating a voltage between them that is read by the meter.
The Standard Hydrogen Electrode (SHE)
The voltage created between the two electrodes actually depends on the type of reference electrode. For example, for the same water, a higher voltage will be read when using a Ag/AgCl electrode than when using a calomel electrode. Because of this, it is sometimes useful to report an ORP reading that has been adjusted to correspond to a standard reference electrode called the 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 bubbling hydrogen gas through a strong acid solution, so it is not practical for routine laboratory or field use. This reference electrode produces a voltage that is about 200 mV higher than what is produced using the Ag/AgCl electrode. When converting a Ag/AgCl ORP value to SHE, +200 mV is added to the Ag/AgCl ORP number.
Technical note: The actual mV value added when reporting an ORP value in reference to the SHE may depend on the concentration of the filling solution.
ORP Standard Solutions and Calibration
Because the meter reads the ORP voltage directly from the sensor, a calibration is not necessarily required. However, because the response of the sensor can degrade over time, it is sometimes desirable to test the sensor using a standard solution to verify that it is giving the correct response, such as within +/-10 mV. Two common ORP standard solutions are Zobell’s solution and Light’s solution. At 25°C, Zobell’s solution has an ORP of approximately +228 mV when measured using a Ag/AgCl reference electrode, and Light’s solution has an ORP of approximately +475 mV under the same conditions.
Caption: Typical ORP values (in mV) of Zobell’s and Light’s solutions for the three types of reference electrodes
Technical note: The actual ORP values of Zobell’s and Light’s solutions depend on temperature and the concentration of the filling solution[1,2].
The testing with the standard solution can also be used to adjust the actual measured reading so that it matches the value of the standard. This is accomplished by adding an offset to the actual measured mV reading (the offset is the difference between the ideal mV reading and actual mV reading for the standard). This can be thought of as a type of calibration, and most meters can be set up to add the offset automatically.
An offset can also be applied in other situations. For example, a user may want the ORP sensor to read a certain 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 may be used to make ORP readings consistent between different ORP sensors, or a new offset may be calculated when an old electrode is replaced with a new one.
Reporting ORP Measurements
ORP measurements are most often reported in mV values relative to the reference electrode used. Most often, the Ag/AgCl reference electrode is used, so values are usually reported based on measurements with this electrode. Calomel electrodes are less common because they contain mercury, so calomel ORP values are relatively uncommon.
Sometimes, however, values are reported relative to the SHE. As explained above, these values are approximately 200 mV higher than values for the Ag/AgCl electrode.
When ORP values are reported, the reference electrode that the values are based on should also be reported in order to avoid confusion. If values are seen where the reference electrode is not specified, they are usually assumed to be reported relative to the Ag/AgCl electrode.
ORP is a common, simple and straightforward measurement that provides a quick indication of how oxidizing or reducing a water is. The reading provided by the meter depends on the condition of the sensor, any offset value provided during calibration, and the type of reference electrode used, so care must be taken in interpreting the results. Nonetheless, ORP is especially useful for providing either a quick snapshot of the oxidation/reduction state of a solution or for monitoring its changes over time.
 American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.
 U.S. Environmental Protection Agency (2017) Field measurement of oxidation reduction potential (ORP). SESDPROC-113-R2.