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19.2 Slope of a pH Sensor
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 produces
+59mV/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 19-2).
-600
-400
-200
0
200
400
600
0
7
14
m
V
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 19-2