What is Oxidation Reduction Potential (ORP)?

The Oxidation Reduction Potential (ORP) is a measure used to assess the macroscopic redox properties of substances in an aqueous solution. A higher ORP indicates a stronger oxidizing property, while a lower ORP indicates a stronger reducing property. A positive potential signifies an oxidizing property in the solution, while a negative potential signifies a reducing property.

In a water body, multiple redox pairs often form a complex redox system, and the ORP reflects the overall result of the redox reaction between oxidizing and reducing substances. Regardless of the reaction form, oxidation involves losing electrons, while reduction involves gaining electrons, accompanied by the process of electron donation.

When a platinum electrode is inserted into a reversible redox system, it will give electrons to the electrode and become a half-cell with a potential corresponding to the reduction capacity of the system. The potential measured by combining it with a standard hydrogen electrode is the Oxidation Reduction Potential of the system.

How to Measure ORP?

ORP is directly measured by using a high quality electrochemical sensor, which is typically referred to as an ORP sensor. The working principle of an ORP (Oxidation Reduction Potential) sensor is based on the electrochemical potential difference between two electrodes in contact with a solution. The sensor typically consists of a measuring electrode and a reference electrode. The measuring electrode, often made of inert materials like platinum or gold, is sensitive to the redox potential of the solution. When it comes into contact with the solution, it exchanges electrons with the oxidizing or reducing agents present, depending on their respective tendencies to gain or lose electrons.

The reference electrode, on the other hand, provides a stable potential against which the potential of the measuring electrode can be compared. It is usually a silver/silver chloride (Ag/AgCl) or calomel electrode, which maintains a constant potential regardless of the solution's composition. When the two electrodes are connected, they form a complete circuit, and the potential difference between them is measured by the ORP sensor. This potential difference is a result of the chemical reactions occurring at the surface of the measuring electrode and is proportional to the ORP of the solution. The ORP sensor converts this potential difference into an electrical signal, which is then displayed on a meter in millivolts (mV).

Y533-A Digital ORP Sensor-Water Quality Sensor Manufacturer-Yosemitech (yosemitesensors.com)

Factors influencing ORP measurement

pH: The acidity or alkalinity of the solution affects the ORP reading because the concentration of hydrogen ions changes the potential of the electrode.

Temperature: Temperature changes affect the rate of chemical reactions in the solution, which in turn affects the ORP value.

Dissolved oxygen: The concentration of dissolved oxygen in the solution affects the redox reaction, which in turn affects the ORP reading.

Ionic strength: The high or low ion concentration in the solution affects the double layer structure on the electrode surface, which affects the ORP measurement.

Electrode material: Electrodes of different materials respond differently to the redox potential, so the choice of electrode material will affect the measurement results.

Contamination and wear: Contamination or wear on the electrode surface will affect its response to the redox reaction in the solution, resulting in inaccurate ORP readings.

Mixed uniformity of the solution: Uneven distribution of oxidants and reductants in the solution will affect the accuracy of ORP measurements.

Polarization and stabilization time of the electrode: The electrode needs to be polarized to reach a stable state before use. If the polarization is insufficient or the stabilization time is insufficient, the measurement results will be inaccurate.

Interfering substances: Interfering substances that may be present in the solution (such as surfactants, suspended solids, etc.) will affect the reaction of the electrode and thus affect the measurement of ORP.

Reference electrode stability: The potential of the reference electrode must remain stable in order to accurately measure ORP. If the reference electrode is unstable, it will cause measurement errors.

Why We Need to Measure ORP in Water?

Water Quality Monitoring: Oxidation-Reduction Potential (ORP) is utilized to monitor the disinfection process in water treatment, ensuring that the water is free from harmful pathogens by measuring the effectiveness of disinfectants such as chlorine or ozone.

Process Control: In industrial processes, ORP can be employed to control reactions involving oxidation or reduction. By monitoring ORP, operators can adjust process conditions to maintain optimal reaction rates and product quality.

Environmental Monitoring: ORP measurements can offer insights into the health of aquatic environments. For instance, a sudden change in ORP could indicate pollution or other environmental disturbances.

Food and Beverage Industry: ORP is utilized in the food and beverage industry to monitor the sanitation of food contact surfaces and to ensure that the products are safe for consumption.

Aquaculture: ORP is used in aquaculture to monitor water quality and ensure it is suitable for fish and other aquatic organisms.