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20. 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.
20.1 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 20-1 illustrates how
asymmetry is expressed.
-600
-400
-200
0
200
400
600
0
7
14
Millivolts
+1.00 pH Asymmetry
0.00 pH Asymmetry
-1.00 pH Asymmetry
Response of pH Electrode, as a Function of Asymmetry
pH
Figure 20-1