pH and ORP and widely used measurements of water's character and suitability. This is a reprint of an article that appeared in www.wateronline.com on 6/29/2017. https://www.wateronline.com/doc/think-you-know-orp-get-ph-orp-measurement-tips-0001.

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Think You Know ORP?
Get 5 pH/ORP Measurement Tips
Water and wastewater treatment
professionals are constantly looking for
as much information as possible about
the quality of their water. If knowledge is
power, then understanding the properties
of their water is key to running an effective
and efficient facility.
Of course, there are many measurements
of water quality and several ways to
determine them. Two of the most
fundamental quality parameters are
pH and oxidation-reduction potential
(ORP). Below, you’ll find the five keys for
measuring pH/ORP.
1. Know What You’re Measuring
It might sound obvious, but to control
pH and ORP in water, the first step is to
understand why these measurements
need to be monitored and what
acceptable levels look like.
To begin with, pH is possibly the most
fundamental parameter for maintaining
good water quality. It is basically a measure
of the acidity of a sample, and that acidity
can have a significant impact on treatment
results.
• “High-pH water often tastes bitter and
can indicate the scaling potential of
the water,”per HarvestH20.com.
• “Low-pH water may lead to the
dissolution of pipes, particularly
copper pipes.”
• The EPA classifies pH under
unregulated “Secondary Drinking
Water Standards” and recommends a
range between 6.5 and 8.5 pH units.
Meanwhile, ORP is lesser known, yet still a
fundamental parameter for determining
water quality. ORP measures a substance’s
ability to oxidize or reduce molecules
around it or, in other words, its potential
to steal or donate electrons. Most types
of water, including tap water and bottled
water, contain dissolved oxygen so their
ORP value is positive.1
When used in wastewater treatment
systems, ORP indicates the ability or
potential of wastewater to permit
the occurrence of specific biological
(oxidation-reduction) reactions.
ORP is measured in volts (V) or millivolts
(mV). Each species has its own intrinsic
reduction potential; the more positive the
potential, the greater the species’ affinity
for electrons and tendency to be reduced.
“The best definition I can give is that
ORP is a measure of the cleanliness of
the water and its ability to break down
contaminants,”reported Ozone Solutions.
“It has a range of -2,000 to +2,000 and
units are in mV. On the ORP scale, the
presence of an oxidizing agent such as
oxygen increases the ORP value, while the
presence of a reducing agent decreases
the ORP value. Since ozone is an oxidizer,
we are only concerned with positive ORP
levels (above 0 mV).”
Oxidation reduction potential can be
used for water system monitoring with
the benefit of a single-value measure of
the disinfection potential, showing the
activity of the disinfectant rather than
the applied dose. For example, E. coli,
Salmonella, Listeria, and other pathogens
have survival times of under 30 s when the
ORP is above 665 mV, compared against
>300 s when it is below 485 mV. 2
2. Know Your ORP Electrodes
ORP tests are quick and easy to perform.
To measure ORP solutions, utilize a metal
electrode made of gold or platinum, which
can pass electrons to oxidants or receive
them from reductants. Once a charge is
adequately built, the electrode will have
a potential that is equal to the solution’s
ORP.
1
http://www.aquahealthproducts.com/science
2
Trevor V. Suslow, 2004, Oxidation-Reduction Potential for Water Disinfection Monitoring, Control, and Documentation, University of California Davis,
http://anrcatalog.ucdavis.edu/pdf/8149.pdf
Image credit: “Image from page 318 of “Chemistry
of dye-stuffs (1903)” Internet Archive Book Image ©
1903 used under an Attribution 2.0 Generic license:
https://creativecommons.org/licenses/by-nc-nd/2.0/

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However, it is possible that a low rate of
electron exchange between the electrode
and the solution can result in inaccurate
readings, as the ORP can respond
more strongly to dissolved oxygen or
another redox couple. Because of this,
operators should be sure to check ORP
measurement devices with the process
sample in a laboratory before putting
them in use.
3. Know Your Points
A key to monitoring pH and ORP is
calibration. This process essentially
measures the response of an instrument
to changing an input that it is measuring
in a predetermined way.
ORP calibration generally means using a
single point. The ORP electrode directly
measures the mV in the process solution
and only requires a single standard
solution for calibration.
Unlike ORP, pH calibration relies on
multiple points. The pH electrode directly
measures mV in the process solution but
expresses the result as a pH reading using
the Nernst equation. Most pH electrodes
deviate from ideal efficiency and benefit
from calibration with at least two buffers.
Thus, it is critical to keep in mind that
ORP calibration will be done with a single
solution, while pH calibration will require
multiple solutions.
By monitoring the ORP of wastewater, an
operator can determine what biological
reaction is happening and if conditions
need to be changed to promote or
prevent that reaction.
4. Know the Temperature
Unbeknownst to many, temperature can
have a significant impact on accurate pH
and ORP measurement.
An ORP measurement value is an
indicator of how well the components of
the sample can oxidize its contaminants.
While oxidation can be dramatically
affected by temperature changes, ORP
does not measure any single chemical,
so it is not generally possible to
correct measurements for temperature
changes. That being said, it’s always
recommended that an operator record
temperature levels at which ORP readings
were taken.
Measurements of pH, on the other
hand, can be corrected for temperature
for improved accuracy. Temperature
increases result in chemical activity
increases, which will affect a pH
electrode’s reflection of hydrogen ions
in the solution. To ensure accurate
measurements, a pH sensor will measure
the temperature and automatically
correct the pH for it using the Nernst
equation.
5. Know Your Tools
Perhaps most critical to ORP and
pH measurement is the use and
understanding of proper equipment.
There are many solution providers out
there with their own proprietary products
for water quality measurement.
ORP probes are extremely versatile
for monitoring biological reactions in
wastewater treatment plants. They can
indicate if acceptable or unacceptable
biological activity is occurring.
Knowing the ins and outs of the
treatment processes, operators should
utilize quick, reliable, and accurate
sensors and analyzers by a vendor they
can trust and that will be responsive in
cases of malfunction or other issues.