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21.3.2 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 21-2).
-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 21-2