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18.1.2 Slope of a pH or Specific Ion 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 –59.16mV/pH unit at 25
O
C As the pH goes down, an ideal pH
electrode produces +59.16mV/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 59.16mV/pH unit has “100% Slope”. An electrode that produces
50.15mV/pH unit has “85% Slope”.
Figure 18-2 illustrates the principle of electrode slope, using a pH sensor as an example.
Figure 18-2
-600
-400
-200
0
200
400
600
0 7 14
mV
85% Slope at 25 oC
(50.15mV/pH)
100% Slope at 25 oC
(59.16mV/pH)
Response of pH Electrode, as a Function of Slope
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