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AquaData 2000 Series ORP Sensors
ORP stands for oxidation-reduction potential, which is a measure, in millivolts, of the tendency of a chemical substance to oxidize or reduce another chemical substance.
Oxidation is the loss of electrons by an atom, molecule, or ion. It may or may not be accompanied by the addition of oxygen, which is the origin of the term. Familiar examples are iron rusting and wood burning.
Reduction is the net gain of electrons by an atom, molecule, or ion. When a chemical substance is reduced, its oxidation state is lowered. As is the case with oxidation, substances that can exhibit multiple oxidation states can also be sequentially reduced from one oxidation state to the next lower oxidation state.
How easily a substance is oxidized or reduced is given by the standard potential of its redox couple. The standard potential refers to half reaction written as a reduction. The negative of the tabulated standard potential gives the standard potential for the oxidation half-reaction.
Measurement of ORP
An ORP sensor consists of an ORP electrode and a reference electrode, in much the same fashion as pH measurement.
The ORP Electrode
The principle behind ORP measurement is the use of an inert metal electrode (platinum, sometimes gold), which, due to its low resistance, will give up reductant. The ORP electrode will continue to accept or give up electrons until it develops a potential, due to the build up charge, which is equal to the ORP of the solution. The typical accuracy of an ORP measurement is ±5 mV.
Sometimes the exchange of electrons between the ORP electrode and certain chemical substances is hampered by a low rate of electron exchange (exchange current density). In these cases, ORP may respond more strongly to a second redux couple in the solution (like dissolved oxygen). This leads to measurement errors, and it is recommended that new ORP applications be checked out in the laboratory before going on-line.
The Reference Electrode
The reference electrode used for ORP measurements is typically the same silver-silver chloride electrode used with pH measurements. In contrast with pH measurements, some offset in the reference is tolerable in ORP since, as seen, the mV changes measured in most ORP applications are large.
In certain specific applications (for example, bleach production), an ORP sensor may use a silver billet as a reference, or even a pH electrode.
The Application of ORP
When considering ORP for a particular application, it is necessary to know the half-reaction involved and the concentration range of all the substances appearing in the half-reaction. It is also necessary to use the Nernst equation to get an idea of the expected ORP behavior.
The oxidation-reduction potential of a solution is a measure of the oxidizing or reducing power of solution. Every oxidation or reduction can be characterized by a half-reaction, which gives all of the chemical substances participating in the reaction. The ORP of the solution depends upon the logarithm of the concentrations of the substances participating in the half- reaction. The ORP can be calculated using the Nernst equation. ORP is not a good method for measuring concentration and its dependence on multiple solution components. The best use of an ORP measurement is in monitoring and controlling oxidation-reduction reactions.