Oxidation-Reduction Potential (ORP), also known as Redox Potential, is a measure of water’s ability to facilitate chemical processes that either oxidize or reduce substances within it. It indicates the presence and activity of agents that can accept or donate electrons.
Oxidizing agents accept electrons and are reduced during a chemical reaction. Reducing agents donate electrons and are oxidized.
ORP is measured in terms of electric potential with standard units of millivolts (mV). Readings can be either positive or negative. Typically, a negative value signifies a reducing environment with substances that can accept electrons, while a positive value indicates an oxidizing environment where electrons are more readily available. Greater distance in either direction from zero generally indicates the relative strength of the reducing or oxidizing nature of the water.
Negative redox values indicate a reducing environment, while positive redox values indicate oxidizing conditions.
Why Measure ORP?
Oxidation-Reduction Potential provides insight into the chemical balance of natural waters and an indication of what types of reactions are likely to occur. Taken on its own, it can be difficult to draw conclusions from ORP because any chemical substances present in the water can influence ORP values, and dissolved oxygen is a dominating factor in many well-aerated natural waters.
However, if specific substances are known to be present in a body of water, ORP may be able to indicate their activity. For example, a lake or wetland may be known to periodically contain hydrogen sulfide released from bacterial decomposition of organic material under anaerobic conditions. In such a case, changes in measured redox potential not linked to corresponding changes in DO may offer a method to detect presence. The same is true in some cases of pollution, such as wastewater discharges.
ORP can also impact the biogeochemical cycles of elements like nitrogen and phosphorus, thereby affecting nutrient availability in aquatic ecosystems. This, in turn, may influence the productivity and types of species present in an ecosystem.
Typical redox potential values of various waters.
In summary, ORP’s usefulness is enhanced when the chemical composition of a water body is known, allowing for more targeted insights into chemical activities and pollution sources. However, the complexity of natural water systems means that ORP should ideally be used alongside other water quality parameters and analysis for a more comprehensive understanding of the chemical processes occurring.
How Is ORP Measured?
The principle for measuring Oxidation-Reduction Potential is very similar to that of pH, where the measurement is based on the voltage produced between a sensing electrode and a reference electrode when the probe is submerged in water. In fact, ORP is sometimes combined with pH in a combination sensor with a shared reference electrode.
A typical ORP sensor has a platinum electrode and a reference electrode and measures the voltage between them when placed into a solution.
In the case of ORP, the sensing electrode includes a metal tip or band, often constructed of platinum or another non-reactive substance, that comes in contact with the water when submerged. The electrode facilitates electron exchange occurring during oxidation or reduction reactions in the water without participating in the reactions. The resulting potential difference (voltage) defines the redox potential of the water.
The reference electrode of ORP sensors typically produces a constant base potential of around 200-220 mV regardless of water composition. Most sensors automatically account for this, such that 0 mV is a roughly neutral value. A typical measurement range from -1000 mV to +1000 mV is normally sufficient to cover the range of ORP values that may be encountered in natural waters.
How to Select an ORP Sensor?
Sensor maintenance is one of the most important considerations when evaluating ORP sensors. As the electrolyte in the reference electrode is consumed over time, it must be periodically replenished or replaced.
Sensor fouling can also be impactful in long-term applications. If a sensor will be placed in a high-fouling environment, consider whether anti-fouling measures such as a wiper can be incorporated into the system.
What to Consider When Preparing an ORP Sensor?
ORP is normally calibrated using a solution specially formulated to produce a specific mV value consistently and quickly. Examples include Zobell or Light’s solution, but many are available at varying standard values. A single-point calibration is sufficient for most sensors, though multiple standards can be used for verification.
Prior to calibration, the sensor should be thoroughly cleaned and inspected. The platinum electrode may need periodic polishing to maintain good performance and response time. Rinse the sensor with deionized (DI) or distilled water, followed by the calibration solution, before beginning a calibration.
Note that although ORP measurements are generally not temperature-corrected, like pH, temperature does have a small effect on ORP readings. A table of values by temperature will normally be provided with the calibration solution.
How to Deploy an ORP Measurement System?
Due to the similarities of ORP sensor construction to pH sensors, and since these are often combined into a single probe, the methods for deployment of ORP sensors are nearly identical to those of pH sensors. Refer to the section on the deployment of pH measurement systems for more information.
ORP sensors can be placed in wells for measurement of groundwater.
An additional common application for ORP measurement is in groundwater, where low DO conditions may increase the significance of ORP measurement. In such cases, the probe is placed into a groundwater well with data logging and telemetry mounted at the surface.
Conclusion
ORP or redox potential is a measure that indicates whether substances in water are more likely to undergo oxidation or reduction by chemical agents found in the water. Such oxidation-reduction reactions are characterized by electron transfer that can significantly alter the water chemistry. In some cases, this can affect the solubility, toxicity, and bioavailability of various substances, amongst other effects.
While further analysis is needed to determine the precise accumulation of chemical substances contributing to a water body’s ORP, monitoring ORP in conjunction with other water quality parameters offers a more comprehensive understanding of the aquatic environment. By understanding and monitoring redox potential, water resource managers can make informed decisions to maintain the chemical balance and health of water resources.