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What Is "ORP"?
ORP is
an abbreviation for for Oxidation Reduction Potential, also know
as REDOX and is a useful measurement for monitoring and controlling
chemical reactions.
Oxidation:
addition of oxygen / reduction of electrons
Reduction: reduction of oxygen/ addition
of electrons
Characterisitics
of ORP:
- Non-specific measurement of total activity
- mV output allows automated control of chemical reactions
Typical
examples of ORP uses include:
- Ozone or chlorine control
- Chromate reduction or cyanide destruction
Units of
Measure for ORP = mV
How
Does an Electrode Work?
An ORP measuring electrode is identical to a pH measuring electrodes
except a Nobel metal is used in place of the pH glass as the measuring
element. Nobel metals are used because they will not enter into
the chemical reaction taking place. Other Nobel metals such as
gold or silver can be used, but platinum is the most commonly
used.
The reference
is identical to the one used in the pH electrode. It is a Ag/AgCl
(silver/silver chloride) wire in 3.5M KCl saturated with AgCl
(silver chloride). A second junction for protection of the
reference wire is common in industrial electrodes and is referred
to as a "Double Junction". A combination ORP electrode
works the same as a combination pH electrode. The measuring
electrode generates a millivolt output based on the oxidizing
or reducing reactions taking place while the reference electrode
generates a constant millivolt output The working range of an
ORP electrode is +/- 2000mV. A pH meter with a millivolt
scale or ORP transmitter or controller is used to display readings.
Temperature
compensation is not used for ORP measurements. The correction
factors are system and chemical dependent and are not easily determined.
ORP mV vs. Ozone ppm
| 200-400 |
Aquaculture, cooling towers |
<0.06 |
| 500-600 |
Swimming pools, hot tubes |
<0.15 |
| 600-800 |
Water disinfection |
<0.4 |
| 800+ |
Water sterilization |
>0.4 |
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