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Oxidation reduction potential or redox potential is a measure of the chemical exchange of electrons between multiple species in a solution. ORP sensors are based upon an electrochemical measuring principle similar to pH, and contain both measurement and reference half-cell electrodes. In place of a pH sensitive glass membrane, typical ORP probes contain a measurement electrode usually made from a noble metal such as platinum.

To offset electric potential of the sensor derived from individual components of the reference and measuring electrodes, a calibration is performed. In ORP, this is done by standardizing a sensor against a solution of known oxidation-reduction potential.

A single point calibration is performed using ORP liquid buffer of a known value. Temperature is an important consideration when calibrating and measuring ORP, as temperature can influence the availability of free protons. Hamilton ORP buffers come with a standard table of ORP values at various temperatures. If an Arc ORP sensor is used, the ORP buffer value is automatically recognized within ArcAir software. Any correction due to temperature is compensated for with the built in RTD. With non-Arc ORP sensors, temperature should be compensated for manually at the transmitter.

Hamilton Redox (ORP) Buffer 475 ± 5 mV

Celsius
mV*
mV**
Celsius
mV*
mV**
Celsius
mV*
mV**
5
456
677
20
470
681
35
484
684
10
461
678
22
472
681
40
488
684
15
465
679
25
475
682
45
492
684
18
468
680
30
479
683
50
496
684

* E(Reference Potential based on Ag/AgCl element with 3M KCl electrolyte)

** E(Pt, Hydrogen)

Hamilton Duracal ORP Buffers come in 271 and 475 mV standards