The compact (1-inch) ORP sensors from PREI Instrumentation use three measuring electrodes compared with conventional ORP sensors, which use only two electrodes. Compared to the conventional combination technique, the differential electrode technique provides measurements of greater stability over longer periods of time with less maintenance.
All oxidation-reduction potential measurement systems are based on the principle of an electrochemical cell in which two electrodes (either platinum or gold) are placed in a solution capable of carrying a charge. An electrical potential develops between the electrodes. In a combination sensor the reference electrode is easily contaminated by the solution being monitored. For this reason, the Hach GLI differential system uses a reference electrode in a concentrated buffer, which is mechanically isolated from the solution being measured. A double junction salt bridge allows the electrical circuit to be completed, and makes it extremely difficult for a contaminating amount of solution being measured, to migrate into the inner reference chamber.
A measure of the power of a substance to gain electrons is called the Oxidation Reduction Potential (sometimes referred to as the Redox Potential). Oxidation and reduction always occur together, with one substance accepting the electrons that the other loses. For example, in the reaction of sodium with chlorine, the sodium atom loses an electron and is therefore oxidised while the chlorine atom gains an electron and is said to be reduced.
The response of an ORP sensor is non-specific, and therefore it is difficult to correlate the output with the concentration of a species. For this reason it is usual to use the ORP millivolt measurement (as it stands) to control a process, rather then converting the millivolts into concentration or other units.
Typical applications are in the textile industry, wastewater and metal finishing and include chlorine proportioning, nitrite detoxication, cyanide detoxication and chromate detoxication (converting Chrome 6 to Chrome 3 etc).
For more information contact Buks Brits, PREI Instrumentation, 011 448 2172.
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