Oxidation/Reduction Potential, also known as redox (REDuction-OXidation)
potential, or simply redox, is a common, single-value measurement for water
quality, often used with aquaculture, pools, hot tubs and
drinking water. ORP also plays an important role in sewage
wastewater treatment and food processing facilities.
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Follow the Electrons
ORP is a complicated measurement.
In its simplest form, we typically think
of redox reactions as the transfer of
electrons, although a more accurate
description would be a change in
In more practical terms, think
of ORP as the ability, or potential,
of water to break down the
contaminants it contains. The higher
the ORP reading, the stronger the
water’s ability to destroy contaminants
such as pathogenic microorganisms,
chemicals, disinfectants, etc.
Digital meters measure ORP in
positive and, in some special
applications, negative readings,
from -2,000mV (millivolts) to +2.000
mV. Unlike pH and EC-TDS, ORP
measurements are not affected by
water temperature. It is also important
to note that changes in ORP levels are
not instantaneous; however, the higher
the ORP reading, the faster the
contaminant breakdown occurs.
Why Does ORP Matter?
There is a strong correlation between
bacterial activity and ORP levels.
Therefore, ORP readings are good
indicators of disinfection levels, and
when targeted levels are achieved,
can assure reliable disinfection. For
instance, chlorine, bromine, and
ozone are oxidizers and make good
water sanitizers because they “steal”
electrons (which increases the ORP
material and causing them to die.
How Does an ORP
An ORP meter is actually a millivolt
meter, measuring tiny voltages that are
generated when its probe is placed
in water containing oxidizing and
reducing agents. The meter’s probe
houses two electrodes; one for
measuring (usually made of platinum)
made of silver
wire. The voltage
measured across the
circuit formed by the
two metal electrodes has a direct
relationship with the potential of
the oxidizer or reducer to perform a
chemical task (disinfection).
Negative ORP Readings?
Readings below zero are utilized for
some health/medical quality waters,
and you’re probably asking, “What
the higher, the better?
Fair question. Negatively charged
water is brought to this “unnatural”
condition by special equipment, and
the process begins with water that has
already been puriﬁed.
To take negative ORP readings, the
probe requires special conditioning -
an oxidized coating must be formed
on the diode. For more details
about conditioning and storage, see
“Negative Probe Preparation” in the
Tech Tips section of our website.
LOSS of negatively charged
electrons resulting in an increase
in oxidation state.
GAIN of negatively charged
electrons resulting in a decrease
in oxidation state.