Reduction Potential testers are used in water treatment applications to check
the sanitizing power of the chlorine in the water. We offer three great
new testers to make measurement easy and accurate:
The HI 98201 is an economical hand held-tester designed for quick and accurate measurements.
The HI 98120 is a combination thermometer and ORP tester.
The HI 98121 is a combination thermometer, pH and ORP tester.
(Click the part numbers above for pricing and specifications)
Oxidation reduction potential (ORP), also known as redox, is the measurement of a solution's oxidizing and reducing activity. Whenever one material is oxidized, another material is reduced.
Rust is an example of an oxidation/reduction reaction. Oxygen combines with iron to form iron oxides. In this process, the iron is oxidized and, once again, the oxygen has been reduced. More pertinent to water treatment is the oxidation/reduction potential of chlorine reacting with bacteria or algae.
Bacteria and algae essentially are hydrocarbons, and chlorine is a powerful oxidizing reagent. Chlorine destroys bacteria and algae by literally burning their carbon and hydrocarbon into CO2 and H2O. When all of the oxidizing and reducing materials have reacted, an equilibrium is reached and there is usually a surplus. It is this surplus material that creates the oxidation or reduction potential of a solution.
ORP can be measured by colorimetric or potentiometric means. Colorimetric techniques take advantage of the fact that certain chemicals can change their color as for example, the amount of chlorine in water changes. Colorimetric kits are inexpensive but subject to errors from the color of the water. They are not well-suited for monitoring or control applications.
The ORP potential generated at the electrode varies as the chemicals in the solution change. This signal is compared to that of a reference electrode.
Another factor to consider when making ORP measurements is that they can be pH- dependent (remember that pH is a measure of hydrogen ions). For example, chlorine exists in solution as hypochlorous acid (OCl). Depending on the pH, this hypochlorous acid will shift its equilibrium to provide more or less free chlorine (this accounts for chlorine reacting more strongly at low pH values-as pH is lowered, more free chlorine is generated).
Even though the concentration of chlorine remains constant, its oxidizing power is pH-dependent. To obtain accurate residual chlorine information, the pH must either be constant or adjusted.
ORP measurement is slow when compared to a pH measurement. Whereas a pH electrode will respond in seconds, a new or cleaned ORP electrode can take several hours to initially equilibrate or re-equilibrate to a sample. Once equilibrated, an electrode's response time is measured in minutes, not seconds.
Calibration of the ORP testers is not normally required. However, measurement error can occur due to contamination or coatings on the electrode. Even though the meter cannot be adjusted, calibration verification can be helpful.
The higher the ORP value, the greater the sanitizing power of your water. The ORP tester provides a valuable indication of water quality.