Chlorine and ORP
ORP or Oxidation-Reduction
Potential is a measure of the oxidizing properties of the sanitizer in
water. ClOH being more active that ClO- has a higher ORP value. As the
pH increases, an ORP sensor shows a decrease in value which reflects the
decrease in ClOH. Scientific and field studies in Germany and elsewhere
have shown that ORP readings are better indication of bactericidal properties
of chlorine than PPM Free Chlorine values.
ORP or "Redox" stands for Oxidation-Reduction Potential. The
two chemical actions involved, "oxidation" and "reduction",
always occur together. Although originally referred only to the action
of oxygen, the term "oxidation" now refers to any chemical action
in which electrons are transferred between atoms. The atom that loses
an electron is said to be "oxidized". The atom that gains an
electron is said to be "reduced," because in picking up that
extra electron, it loses the electrical energy that makes it "hungry"
for more electrons.
Effect of Oxidizers:
Chemicals like chlorine, bromine and ozone are all strong oxidizers. It
is their ability to oxidize or to steal electrons from other substances,
that makes them good sanitizers. The sanitizing action is caused by the
alteration of the chemical makeup of unwanted organisms. Oxidizers literally
burn off germs, bacteria and other organic material in water leaving as
a by-product a few harmless chemicals. Of course, in the process of oxidizing,
all of these oxidizers are reduced, thus losing their ability to continue
oxidizing other things; eventually, they may combine with other substances
in the water, or their electrical charge may be simply used up. To make
sure that the chemical process continues to the very end, it is necessary
to have a high enough concentration of oxidizer in the water to do the
"Potential" is a word that refers to ability rather than action.
Potential energy is energy that is stored and ready to be put to work.
It's not actually working, but we know that the energy is there if and
when we need it. Another word for potential might be electrochemical pressure.
In electrical terms, potential energy is measured in volts. Actual energy
(current flow) is measured in amps. When you put a voltmeter across the
leads of a battery, the reading you get is the difference in electrical
pressure, the potential energy between the two poles. This pressure represents
the excess electrons present at one pole of the battery (caused, incidentally,
by a chemical reaction within the battery) ready to flow to the opposite
pole. When we use the term potential in describing ORP, we are actually
talking about electrochemical potential or voltage.
ORP meter readings are very tiny voltages generated when a metal is placed
in water in the presence of oxidizing and reducing agents. These voltage
readings give us an indication of the ability of the oxidizers present
in the water to keep it free from contaminants. An ORP role is really
a millivoltmeter, measuring the voltage across a circuit formed by a measuring
electrode (the positive pole of the circuit), and a reference electrode
(the negative pole), with the water in between. The measuring electrode
(+) of the probe, is usually made of platinum, although other noble metals
(which do not oxidize easily), such as gold, could be used. When this
platinum electrode is placed in water in the presence of oxidizing agents,
electrons are constantly transferred back-and-forth on its surface, generating
a tiny voltage. The reference electrode (-), usually made of silver is
surrounded by a saline (electrolyte) solution that produces another tiny
voltage. The voltage is the reference against which the voltage generated
by the platinum and the oxidizers in the water is compared. The difference
in voltage between the two electrodes is what is actually measured by
the meter. As an oxidizer is added to the water, it "steals"
electrons from the surface of the platinum measuring electrode, causing
the electrode to become more and more positively charged. As you continue
to add oxidizer to the water, the electrode generates a higher and higher
positive voltage. When used with a chlorine-based sanitation system, an
ORP measuring device will not specifically indicate the chlorine concentration
in parts per million. It will however, indicate the effectiveness of the
chlorine as an oxidizer. Also, ORP readings will vary as pH fluctuates.
As the pH goes up, the millivolt reading on an ORP meter will go down,
indicating that the sanitizer is not as effective. Bringing the pH down
or adding more sanitizers raises the millivolt reading. That is why most
ORP instruments also incorporate an electronic pH meter. With water, the
meter measures the difference in electrical potential between the water
sample and a sample of known pH that is contained in the meter in a small
Once the instruments and methods for measuring ORP were developed in the
1960s, researchers began working towards setting standards under which
ORP measurements could be used as an accurate gauge of water quality.
In 1968, a laboratory study by Carlson, Hasselbarth and Mecke of the Water
Hygiene Institute of the German Federal Health Office showed that the
rate of killing of E. Coli organisms in swimming pool water is dependent
on ORP and not on the free residual chlorine level. The kill time is just
a fraction of a second at a Redox level of 650 mV, but increases rapidly
to several hours at lower ORP values. In 1971, the World Health Organization
adopted an ORP standard for drinking water disinfection of 650 millivolts.
That is, the WHO stated that when Oxidation-Reduction Potential (ORP)
in a body of water measures 650/1000 (about 2/3) of a volt, the sanitizer
in the water is active enough to destroy harmful organisms almost instantaneously.
In Germany, which has about the strictest water-quality standards in the
world, an ORP level of 750 millivolts was established by the Deutsche
Institut fur Normung (DIN) Standard 19643, as the minimum standard for
public pools in 1982 and DIN Standard 19644 for public spas in 1984. France
and most other European countries have since adopted these standards.
The results of an extensive study on 30 public and semi-public spas in
metropolitan Portland, Oregon, were presented at the 1985 meeting of the
National Environmental Health Association (NEHA) by James C. Brown, of
the Oregon Health Department and Professor Eric W. Mood of the University
School of Medicine. Their conclusions should convince the most incredulous.
"ORP has been shown to be monitoring parameter which takes into account
the numerous water chemistry constituents that can affect overall bactericidal
efficacy (i.e. pH, free chlorine residual, cyanuric acid concentration,
organic and nitrogenous material loading, etc.) and converts them into
a single value (i.e. millivolts) which can continuously and reliably indicate
acceptable bacterial quality. A review of the data shows that whenever
the ORP is 650 mV or more, the water is well within bacteriological standards.
However, whenever the ORP is below 650 mV bacteriological contamination
is evident. Brown (et.al.) found that: "Total and fecal coliform
parameters proved to be unreliable indicators of bacteriological water
quality (but) the oxidation reduction potential (ORP) was found to be
a reliable indicator of bacteriological water quality." "Public
Health officials should adopt a requirement for the maintenance of an
ORP reading of at least 650 mV for all chlorinated or brominated spas
of a free chlorine residual or 2.0 ppm or more does not assure safe spa
water." In its 1988 standards for commercial pools and spas, the
National Spa and Pool Institute (NSPI), stated that ORP can be used as
"supplemental measurement of proper sanitizer activity" when
chlorine or bromine are used as a primary disinfectant. The recommended
minimum reading under the NSPI standards is 650 millivolts, with no ideal
and no maximum.
The NSPI also stated
that "the use of ORP testing does not eliminate or supersede the
need for testing the sanitizer level with standard test kits."
The above statement
is not necessarily a matter of the NSPI being cautious about endorsing
ORP standards. It was issued in recognition of the fact that most health
codes still specify that a measurable free, available residual - usually
1.0 part per million (ppm) - be present in the water of public pools and
spas, as measured with a DPD test kit.