ORP Measurements and ORP Meters

Posted by 28 Dec, 2010

What is the ORP of water?

ORP stands for Oxidation Reduction Potential, also known as Redox. It is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. The ORP measurement is displayed in millivolts (mV) and the usual range for ORP meters is (+1000 mV) to (-1000 mV) with no temperature compensation. It is not a direct measurement of concentration, but rather an indicator of the activity level or strength of an oxidizer or reducer.

An oxidizer gains electrons, while a reducer loses electrons. Examples of oxidizers are: chlorine, hydrogen peroxide, bromine, ozone, and chlorine dioxide. Examples of reducers are sodium sulfite, sodium bisulfate and hydrogen sulfide. Like acidity and alkalinity, the increase of one is at the expense of the other.

When measuring with an ORP meter, a positive voltage shows a solution gaining electrons (oxidizing) and a negative voltage shows a solution losing electrons (reducing). For instance, chlorinated water will show a positive ORP value and a solution with sodium sulfite will have a negative ORP value.

ORP Applications

ORP meters are used for swimming pools and spas, drinking water, cooling tower disinfection, and groundwater remediation. ORP meters are also crucial for bleaching applications, cyanide destruction, chrome reductions, metal etching, fruit and vegetable disinfection and dechlorination.

Many organizations around the world have accepted ORP as a standard for disinfection. An ORP of 750 mV was adopted by the German Standards Agency in 1982 for public pools. The National Swimming Pool Institute also adopted an ORP of 650 mV for public spas. The World Health Organization (WHO) adopted an ORP of 700 mV as a standard for drinking water.

How to use ORP?

Oxidation Reduction Potential is a convenient measure of the oxidizer’s or reducer’s ability to perform a chemical task. It is a more consistent and reliable measurement than chlorine alone. You can establish an ORP measurement at which acceptable sanitation is achieved and maintained for your application by testing the rate of inactivation of various microorganisms. There are published standards for many applications or a lab water analysis should be able to provide these results if necessary. For example, the above paragraph mentions the 750 mV standard for public swimming pools. Once that has been determined you can use an ORP controller to maintain that level of sanitation.

ORP Limitations

Similar to pH, high purity solutions will take extra time to stabilize a reading due to the low ionic strength. Also, if the ORP measurement of a sample solution is similar to the mV of the ORP probe, it will take slightly longer for a reading to respond and stabilize. Rinsing the probe with a strong oxidizer will help to increase the reaction time and give accurate readings quickly. Ensure that there is not residual solution in the ORP probe by rinsing thoroughly with the solution to be tested.

ORP measuements

ORP meters like the 728II are used to monitor the sanitizer level. A pH controller, such as the 723II, is also used to monitor pH. The controllers automatically turn the appropriate chemical feeders on and off, as required to maintain the proper sanitizer and pH levels. This results in good water quality and elimination of chloramines and other unpleasant contaminants, as well as in savings in chemical and labor.

Generally, ORP meters without pH control should not be used with alkaline sanitizers such as sodium hypochlorite or calcium hypochlorite. If the pH remains fairly constant, these inexpensive controllers provide a very cost effective solution to the problem of chemical control in small commercial applications.

Other handheld instruments like the POOLPRO PS6 have been designed to test ORP reliably for swimming pools and other applications. If you are in the medical dialysis industry, you will definitely be interested in the Digital Dialysate Meter D-6 that test ORP, among other parameters.

Categories : Technical Tips
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