R F T

the potential under standard conditions of unit activity referred to the SHE = the gas constant, 1.986 cal per mol degree = Faraday's constant = temperature in °K

= number of electrons exchanged in the reaction

Table 7.7.5 shows how the n values change from reaction to reaction. These changes, plus the fact that a given ORP reaction can encompass side reactions, reveal why it is difficult, if not impossible, to temperature-compensate an ORP reaction. In the Nernstian representation of pH, n always equals 1.

In Equation 7.7(8), the standard potential EOx/Red is found in tables in handbooks and in the value relative to the standard hydrogen electrode. Therefore, the Ecell for the prevailing concentration is also relative to the SHE.

Often, ORP and pH electrodes can be mounted in the same tees or flow-through chambers and can have identi

Ecell E

n cal appearances. The electrode shown in Figure 7.7.9 is a ruggedized flat-glass electrode with identical dimensions for both pH and ORP services; the only difference is the gold or platinum wire tip in the ORP version.

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