ORP (Oxidation Reduction Potential)


What is it?
Oxidation Reduction Potential or Redox is the activity or strength of oxidizers and reducers in relation to their concentration. Oxidizers accept electrons. Reducers lose 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. A single voltage is called the Oxidation-Reduction Potential, where a positive voltage shows a solution attracting electrons (oxidizing agent). Chlorinated water will show a positive ORP value, for instance.

ORP is measured in millivolts, with no correction for solution temperature. Like pH, it is not a measurement of concentration directly, but of activity level. In a solution of only one active component, ORP indicates concentration. As with pH, a very dilute solution will take time to accumulate a readable charge.

An ORP sensor uses a small platinum surface to accumulate charge without reacting chemically. That charge is measured relative to the solution, so the solution "ground" voltage comes from the reference junction – same as the pH sensor.

History of ORP
ORP electrodes were first studied at Harvard University in 1936. In 1982 the German Standards Agency adopted the ORP (750 mV) for public pools and in 1988 the National Swimming Pool Institute adopted ORP (650 mV) for public spas.

Where is ORP used?
As you can tell by the above paragraphs, ORP is used for drinking water, swimming pools and spas. ORP is also used for cooling tower disinfection, groundwater remediation, bleaching, cyanide destruction, chrome reductions, metal etching, fruit and vegetable disinfection, and dechlorination.

In test after test on poliovirus, E. coli, and other organisms, a direct correlation between ORP and the rate of inactivation was determined. It is, therefore, possible to select an individual ORP value, expressed in millivolts, at which a predictable level of disinfection will be achieved and sustained regardless of variations in either oxidant demand or oxidant concentration. Thus, individual ORP targets, expressed in millivolts, can be determined for each application which will result in completely reliable disinfection of pathogens, oxidation or organics, etc. Any level of oxidation for any purpose can be related to a single ORP number which, if maintained, will provide utterly consistent results at the lowest possible dosage.

Why use ORP?
ORP is a convenient measure of the oxidizer or reducer’s ability to perform a chemical task.  ORP is not only valid over a wide pH range, but it is also a rugged electrochemical test which can easily be adapted for n-line and portable measurement. 

Limitations for ORP
As with all testing, ORP has certain limitations. This may occur with swimming pools at 650 mV or other systems with low concentration of Redox pairs.

Carryover is also a possible problem when checking strong oxidizers or reducers, and rinsing well will help greatly.

Although a better indicator of bactericidal activity, ORP cannot be used as a direct indicator of the residual of an oxidizer, due to the effect of pH and temperature on the reading. The best recommendation for ORP is to use wet tests, and over three test periods correlate the ORP
values to those test parameters.

Myron L Instruments
Myron L 3P and 6P Ultrameters are multi parameter instruments, with ORP measuring capabilities. Along with ORP, our instruments can measure conductivity, TDS, resistivity, pH and temperature making them the preferred instrument for all water treatment professionals.

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