Understanding ORP Measurement in Accuracy, Applications and Maintenance

Contents

• What does ORP mean? What is an ORP meter?
• What is an ORP Combination electrode?
• How should we correctly understand the accuracy of ORP values?
• What are the main applications of ORP testing?
• Is it necessary to calibrate the ORP electrode during ORP testing? How to verify the ORP electrode performance?
• How to maintain ORP combination electrode?

The Basics of Oxidation-Reduction Potential (ORP)

What does ORP mean? What is an ORP meter?

ORP stands for Oxidation-Reduction Potential, which represents the oxidation-reduction potential of a solution. The ORP value is a measurement index of the oxidation-reduction capacity of aqueous solutions, and its unit is millivolts (mV). An ORP meter is a specialized instrument for measuring the redox potential of a solution. It consists of an ORP combination electrode and an mV meter.

What is an ORP Combination electrode?

ORP electrodes absorbs or releases electrons on their surface without chemical reactions with the sample solution. The sensitive elements are made of inert metals, usually platinum or gold, and are typically in the form of needles, discs or rings. The electrode shaft material is usually plastic, or glass and the reference electrode is an Ag/AgCl electrode, similar to a pH electrode.

How should we correctly understand the accuracy of ORP values?

Natural water contains a variety of ions and dissolved oxygen. When industrial wastewater containing a large number of ions and organic matter is discharged into water. Due to the different properties of the ions, oxidation-reduction reactions occur in the water and tend to equilibrium. Therefore, natural water is not a single redox system, but rather a mixed redox system. ORP electrodes is also designed for reflecting a mixed potential, which is subject to large experimental errors.

Furthermore, since changes in the solution’s pH and temperature also affect the ORP value and cannot be corrected. Therefore, the REDOX potential is a qualitative measurement of the REDOX capacity of a solution. It reflects a relative state, or more precisely, a description of the trend and state of the redox equilibrium and the degree of completion of the reaction process, rather than a theoretical quantitative estimate. Therefore, it is meaningless to emphasize the absolute potential of the solution in actual measurement. We can say that the ORP value of a solution represents the reduction or oxidation state of the solution around a certain numerical value or represents a certain property of the solution (such as cleanliness). However, this value can vary greatly, and it is usually impossible to quantitatively determine it completely accurately, which is different from the accuracy of pH testing.

What are the main applications of ORP testing?

The ORP test is common in wastewater treatment, primarily in redox systems, such as the reduction of chromic acid and the oxidation of oxides. Adding sodium disulfide or sulfur dioxide to wastewater can reduce hexavalent chromium to trivalent chromium. Adding chlorine or sodium hypochlorite can oxidize cyanide, among other substances. Generally, platinum ORP electrodes are used in oxidation systems, while gold ORP electrodes are used in reduction systems.

ORP meters are also commonly used to measure the sterilization and disinfection effectiveness of swimming pool water, mineral water, hydrogen water, and drinking water. Because the sterilization efficiency of E. coli in water is related to the redox potential (REDOX), a certain ORP value can indicate the bacterial content in the water. Conventional freshwater swimming pools typically use platinum ORP electrodes, while saltwater pools often use gold ORP electrodes, especially in salt chlorine machines. This is because:

• Water electrolysis produces hydrogen. This negatively charged molecule adsorbs into the micropores on the surface of the platinum sensitive element, causing the measurement result to be lower than the actual value. 
• When calibrating with an ORP standard solution, the calibration solution can reduce the platinum oxide on the surface of the sensitive element. After calibration, when directly measuring the electrolyzed water in a salt chlorinator system, hydrogen is more likely to adsorb onto the surface of the platinum sensitive element, ultimately resulting in a negative ORP measurement.

The calibration and the maintenance of ORP electrode

Is it necessary to calibrate the ORP electrode during ORP testing? How to verify the ORP electrode performance?

ORP meter does not require calibration and can be used directly. If there is a doubt about the performance of the ORP electrode or test results, you can prepare ORP standard solution and test its mV value to check the the ORP electrode or instrument.

How to maintain ORP combination electrode?

The surface of the platinum or gold sensitive element of ORP probe should be kept bright and shiny. Rough surfaces lead to respond much more slowly to the redox potential than smooth surfaces. Furthermore, contamination of the platinum or gold ORP sensor over extended use can also cause inaccurate measurements and slow response. In such cases, the following cleaning and maintenance procedures are recommended.

• Inorganic contamination: Immerse the electrode in 0.1 mol/L dilute hydrochloric acid for 30 minutes, rinse with pure water, and then soak in electrode soaking solution for 6 hours before use.
• Organic oil and oil film contamination: Clean the platinum or gold sensor surface with a mild detergent, rinse with pure water, and then soak in electrode soaking solution for 6 hours before use.
• If the platinum surface is greatly contaminated, such as with an oxide film, polish the platinum or gold sensor surface with granular solid toothpaste, rinse with pure water, and then soak in electrode soaking solution for 6 hours before use.