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
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
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
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
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
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
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.