WHAT IS ORP?
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). For instance, chlorinated water will show a positive ORP
value whereas sodium sulfite (a reducing agent) loses electrons and will show a
negative ORP value.
ORP is measured in millivolts (mV), 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 measurable 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 -
the same type used by a pH sensor.
HISTORY OF ORP
ORP electrodes were first studied at Harvard University in
1936. These studies showed a strong correlation of ORP and bacterial activity.
These tests were confirmed by studies on drinking water and swimming pools in
other areas of the world. In 1971 ORP (700 mV) was adopted by the World Health
Organization (WHO) as a standard for drinking water. 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 previous paragraphs, ORP is used
for drinking water, swimming pools and spas. However, ORP is also used for
cooling tower disinfection, groundwater remediation, bleaching, cyanide
destruction, chrome reductions, metal etching, fruit and vegetable disinfection
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 of 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.*
* Courtesy of Siemens Stranco Products.
WHY USE ORP?
ORP is a convenient measure of the oxidizer’s 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 accomplished
using in-line and handheld instrumentation. It is by far a more consistent and
reliable measurement than say chlorine alone.
LIMITATIONS FOR ORP
As with all testing, ORP has certain limitations. The
speed of response is directly related to the exchange current density which is
derived from concentration, the oxidation reduction system, and the electrode.
If the ORP of a sample is similar to the ORP of the electrode, the speed will be
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. ORP can be correlated to a system
by checking the oxidizer or reducer in a steady state system with a wet test,
and measuring pH. If the system stays within the confines of this steady state
parameter (usually maintained by inline or continuous control), a good
correlation can be made. The best recommendation for ORP is to use wet tests,
and over three test periods correlate the ORP values to those test parameters.
FREE CHLORINE CONVERSION USING ORP
The most ubiquitous and cost-effective sanitizing agent used in disinfection systems is chlorine. When chlorine is used as the sanitizer, free chlorine measurements are required to ensure residual levels high enough for ongoing bactericidal activity. Myron L® digital handheld instrumentation accurately converts ORP measurements to free chlorine based on the understanding of the concentrations of the forms of free chlorine at a given pH and temperature. The conversion is accurate when chlorine is the only oxidizing/reducing agent in solution and pH is stable between 5 and 9. This pH range fits most applications because pH is usually maintained such that the most effective form of free chlorine, hypochlorous acid, exists in the greatest concentration with respect to other variables such as human tolerance.
MYRON L® INSTRUMENTS
Myron L® Company offers a variety of handheld instruments and in-line Monitor/controllers that may be used to measure, monitor and/or control ORP. The following table lists some of the model numbers for measuring, monitoring, and/or controlling ORP. The Ultrameter III™ 9PTK, Ultrameter II™6PFCE, POOLPRO™ PS9TK and PS6FCE and D-6 DIGITAL DIALYSATE METER™ are multi-parameter handheld instruments with ORP and free chlorine measuring capabilities. These instruments also have the capability to measure conductivity, TDS, resistivity, pH, mineral/salt concentration and temperature, making them the preferred instruments for all water treatment professionals. The 720 Series II Monitor/controllers are an excellent choice for continuous in-line measurements.
For additional information, contact your local distributor
or the Myron L® Company directly. You may also email us at