Page 39
12.2 pH Sensor Fundamentals
A combination pH sensor is two sensors in one. The sensing membrane
is the round or spear shaped bulb at the tip of the sensor. This produces
a voltage that changes with the pH of the Solution. This voltage is
measured with respect to the second part of the sensor, the reference
section. The reference section makes contact with the sample solution
using a salt bridge, which is referred to as the reference junction. A
saturated solution of KCl is used to make contact with the sample. It is
vital that the KCl solution has an adequate flow rate in order to obtain
stable, accurate pH measurements.
12.2.1 Asymmetry of a pH Sensor
An “ideal” pH sensor produces 0 mV output at 7.00 pH. In practice, pH
sensors generally produce 0 mV output at slightly above or below 7.00
pH. The amount of variance from 7.00 pH is called the asymmetry. Figure
12-1 illustrates how asymmetry is expressed.
Figure 12-1
-600
-400
-200
0
200
400
600
0
7
14
+1.00 pH Asymmetry
0.00 pH Asymmetry
-1.00 pH Asymmetry
Response of pH Electrode, as a Function of Asymmetry
E
lec
tr
o
d
e
Respo
n
se
pH
