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20.3.2
The 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 20-2).
-600
-400
-200
0
200
400
600
0
7
14
85% Slope at 25 oC
(50.15mV/pH)
100% Slope at 25 oC
(59mV/pH)
Response of pH Electrode, as a Function of Slope
Electrode Response (mV)
pH
Figure 20-2