The basic instrumentation for ORP measurement closely parallels that for pH measurement. In fact, many instrument suppliers use slightly modified pH analyzers, with a changed sensitivity and an mV scale in place of a pH scale. The hardware used to install the electrodes in the process stream is generally the same as that used in pH systems (see Figure 7.7.9).
Two major differences exist between an ORP system and a pH system. One difference is the sensing electrode, which is normally a noble metal, typically platinum or gold, although other metals and carbon have been used.
The second major difference is in temperature compensation. Process pH systems are typically temperature-compensated, whereas ORP systems are almost never temperature-compensated.
Basic thermodynamics apply to pH and ORP as expressed by the classic Nernst equation. For oxidation-reduction half-cell reactions, this equation can be represented as:
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