= the potential developed across the membrane a1 = the activity of the measured ion in the sample or process atat = the activity of the same ion in the internal solution

2.3 RT/nF = the Nernst slope, or the slope of the calibration curve, and is a function of the absolute temperature T and the charge on the ion being measured n R = the gas law constant

With few exceptions—notably the silver-billet electrode for halide measurements and the sodium-glass electrode—the pH electrode was the only satisfactory electrode available to the process industry prior to 1966. Currently, more than two dozen electrodes are suitable for industrial use. Table 7.7.6 lists several electrodes commercially available for

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