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17. Appendices
17.1 pH Electrode Fundamentals
A combination pH Electrode is two electrodes in one. The sensing membrane is the round or spear
shaped bulb at the tip of the electrode. This produces a voltage that changes with the pH of the
Solution. This voltage is measured with respect to the second part of the electrode, the reference
section. The reference section makes contact with the sample solution using a salt bridge, which is
referred to as the reference junction. A saturated solution of KCl is used to make contact with the
sample. It is vital that the KCl solution has an adequate flow rate in order to obtain stable, accurate
pH measurements.
17.1.1
Asymmetry of a pH Electrode
An “ideal” pH electrode produces 0 mV output at 7.00 pH. In practice, pH electrodes, generally
produce 0 mV output at slightly above or below 7.00 pH. The amount of variance from 7.00 pH is
called the asymmetry. Figure 17-1 illustrates how asymmetry is expressed.
-600
-400
-200
0
200
400
600
0
7
14
mV
+1.00 pH Asymmetry
0.00 pH Asymmetry
-1.00 pH Asymmetry
Response of pH Electrode, as a Function of Asymmetry
pH
Figure 17-1