60
25.3 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 and
accurate pH measurements.
25.3.1 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 25-1 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 25-1
25.3.2 Slope of a pH Electrode
As mentioned above, a pH electrode produces 0 mV output at around 7.00 pH. As the pH goes up,
an “ideal” pH electrode produces -59mV/pH unit at 25
O
C. As the pH goes down, an ideal pH
electrode pr59mV/pH unit. In practice, pH electrodes usually produce slightly less than
this. The output of a pH electrode is expressed as a percentage of an ideal electrode. For example,
an ideal electrode that produces 59mV/pH unit has “100% Slope”. An electrode that produces
50.15mV/pH unit has “85% Slope” (see Figure 25-2).
-600
-400
-200
0
200
400
600
0
7
14
mV
85% Slope at 25 oC
(50.15mV/pH)
100% Slope at 25 oC
(59mV/pH)
Response of pH Electrode, as a Function of Slope
pH
Figure 25-2
