Monitoring ORP leads to successful
treatment
By Mike Ross
Oxidation reduction potential (ORP), also known as
redox, is the measurement of a solution's oxidizing
and reducing activity. The ORP process can be likened
to stock-market activity: Whenever one material is
oxidized, another material is reduced (for someone
to purchase a share, another person must sell that
share).
Fire is an example of rapid oxidation and reduction.
The carbon from a hydrocarbon combines with oxygen
from the air to make CO2 while hydrogen from the hydrocarbon
combines with oxygen to make H2O. The carbon and hydrogen
have been oxidized while the oxygen has been reduced.
Rust is a slower example of
an oxidation/reduction reaction. Oxygen combines with
iron to form iron oxides. In this process, the iron
is oxidized and, once again, the oxygen has been reduced.
Fire and rust illustrate the
basic characteristics of oxidation/reduction processes;
namely, that materials involved undergo chemical changes.
More pertinent to water treatment is the oxidation/reduction
potential of chlorine reacting with bacteria or algae.
Bacteria and algae essentially are hydrocarbons, and
chlorine is a powerful oxidizing reagent. Even though
it can't really be seen, chlorine destroys bacteria
and algae by literally burning their carbon and hydrocarbon
into CO2 and H2O. When all of the oxidizing and reducing
materials have reacted, an equilibrium is reached
and there is usually a surplus It is this surplus
material that creates the oxidation or reduction potential
of a solution.
Measuring ORP
ORP can be measured by colorimetric or potentiometric
means. Colorimetric techniques take advantage of the
fact that certain chemicals can change their color
as for example, the amount of chlorine in water changes.
Colorimetric kits are inexpensive but subject to errors
from the color of the water. They are not well-suited
for monitoring or control applications.
The principle of potentiometric operation is that whenever
a metal is exposed to varying concentrations of chemicals,
a millivolt level(mV) electrical potential is generated.
The millivolts generated are a function of the type
of metal used, the type and concentration of the chemicals
in solution and the solution's temperature. By selecting
a particular metal, a correlation to the chemical
type and concentration can be made and useful information
obtained.
In actual practice, a noble metal (a pure, elemental
metal) is always used in ORP electrodes because it
will not enter into unwanted chemical reactions that
can lead to measurement errors. The use of a noble
metal is important because the ORP value is a function
of both the solution's chemicals and the type of metal
in contact with that solution (even different noble
metals can give different readings in the same solution).
Platinum is normally the metal of choice; however,
gold and other noble metals can also be used.
The ORP potential generated at the platinum electrode
varies as the chemicals in the solution change. This
signal is compared to that of a reference electrode
(one so constructed that its potential remains constant
even when the chemicals in the solution change). The
most commonly used reference electrode is a silver
or silver chloride (Ag/AgCl) type.
Unlike the pH electrode which responds only to hydrogen
ion activity, an ORP electrode responds to chemical
reaction activity in which material is converted from
one oxidized state to another through electron transfer.
There are many similarities between pH and ORP measurements
as shown below.
- Both are examples of electrochemical measurements
- Both are forms of batteries and have limited lives
- Both require a special high impedance mV input
circuit
- Both use the same Ag/AgCl reference electrode
design
- pH electrodes are designed to respond to hydrogen
ion activity, while ORP electrodes respond to
all ions that have oxidizing or reducing activities
- pH measuring electrodes are constructed of hydrogen
ion sensitive glass. ORP electrodes are constructed
of a noble metal.
- pH measuring electrodes can be made to automatically
compensate for temperature changes, but the effect
of temperature on ORP is not known
Why Measure?
Many industries can benefit from the use of ORP measurements,
including waste water treatment applications and the
pulp and paper industry. From a water treatment perspective,
use of ORP for controlling water disinfection or the
growth of algae with chlorine, chlorine dioxide, bromine
and ozone in applications such as cooling towers,
swimming pools, potable water supplies and a multitude
of other sterile water applications is of prime interest.
Other oxidizers include fluorine and hydrogen dioxide.
ORP measurement can be done in a variety of ways. A
pH meter with a mV scale can be used simply by connecting
an ORP electrode in place of the pH electrode. The
mV signal generated by the electrode is representative
of, for example, the residual chlorine in solution.
This ORP potential is temperature-dependent; however,
temperature compensation is not used because the compensation
would vary for each different oxidation/reduction
reaction occurring, and it is likely that several
reactions are taking place at the same time.
Another factor to consider when making ORP measurements
is that they can be pH- dependent (remember that pH
is a measure of hydrogen ions). For example, chlorine
exists in solution as hypochlorous acid (OCl- ). Depending
on the pH, this hypochlorous acid will shift its equilibrium
to provide more or less free chlorine (this accounts
for chlorine reacting more strongly at low pH values-as
pH is lowered, more free chlorine is generated).
Even though the concentration of chlorine remains constant,
its oxidizing power is pH-dependent. To obtain accurate
residual chlorine information, the pH must either
be constant or adjusted.
ORP measurement is slow when compared to a pH measurement.
Whereas a pH electrode will respond in seconds, a
new or cleaned ORP electrode can take several hours
to initially equilibrate or re-equilibrate to a sample.
Once equilibrated, an electrode's response time is
measured in minutes, not seconds.
ORP measurements can be defined as a measurement of
oxidant demand relative to whatever ORP value needed
to accomplish a particular disinfection goal. Actual
ORP levels required for bacteriological control will
vary with use of different oxidizers and makeup waters.
Both concentration and activity of the oxidizer will
affect the ORP levels. In addition, water chemistry-which
may inhibit an oxidizer's performance-can affect the
ORP levels and affect the choice of oxidizing agents.
For example, cyanuric acid is used in swimming pools
to minimize the loss of chlorine. The cyanuric acid
reacts with the hypochlorous acid to bind it in a
form that reduces the free available chlorine. This
chemical binding has the net effect of lowering the
concentration of chlorine detected by the ORP electrode.
Verifying Electrode Operation
Calibration is not normally required. In fact many
ORP meters do not have calibration adjustments. However,
measurement error can occur due to contamination or
coatings on the electrode. Even though the meter cannot
be adjusted, calibration verification can be helpful.
To verify the operation of an ORP electrode, quinhydrone
is added to pH buffers 4.0 and 7.0. When added to
these buffers, two known, stable ORP solutions will
be created. A 7.0 buffer with quinhydrone will produce
a solution which will generate 90mV with a platinum
ORP electrode. A 4.0 buffer with quinhydrone will
produce a solution of 265mV.
If the electrode responds
correctly in the samples, no further steps are required.
If the values are incorrect, clean the electrodes
measuring surface and reference junction with 5% hydrochloric
acid. Scratches to the metal surface should be avoided.
However, if acid treatment is not effective, very
lightly abrade the metal measuring surface with a
600-grit wet silicon carbide sandpaper using a circular
polishing motion. After such abrasive cleaning the
electrode may require several hours of soaking in
a quinhydrone solution before providing stable readings. If these cleaning procedures do not restore the electrode's
calibration, the electrode will need to be replaced.
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