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7.2.2
The Slope of a pH Sensor
As mentioned above, a pH sensor produces 0 mV output at around 7.00 pH. As
the pH goes up, an “ideal” pH sensor produces -59mV/pH unit at 25
o
C As the
pH goes down, an ideal pH sensor produces +59mV/pH unit. In practice, pH
sensors usually produce slightly less than this. The output of a pH sensor is
expressed as a percentage of an ideal sensor. For example, an ideal sensor that
produces 59mV/pH unit has “100% Slope”. An sensor that produces
50.15mV/pH unit has “85% Slope” (see Figure 7-2).
-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
E
le
c
tr
o
d
e
R
e
s
p
o
n
s
e
(
m
V
)
pH
Figure 7-2
