Siemens RAPIDLAB 248 Operator's Manual page 127

Operating Principles
Reference Sensor
Figure G-1. Reference sensor (cutaway view)
pH Sensor
The pH sensor is based on ion selective electrode (ISE) technology and
is a half-cell that forms a complete cell with the external reference
sensor. It contains a silver/silver chloride wire (Ag/AgCl) surrounded by
a buffer solution of fixed hydrogen ion concentration. A glass
membrane, highly sensitive and specific for hydrogen ions, separates
the sample from the solution.
As the sample comes into contact with the membrane of the pH sensor,
a potential develops due to the exchange of hydrogen ions in the
membrane. The silver/silver chloride wire conducts the potential to a
voltmeter where it is compared to the constant potential of the
reference sensor. The final measured potential reflects the hydrogen ion
concentration of the sample, and is used to calculate the pH value.
Figure G-2. pH sensor (cutaway view)
Vent Hole
Sensor Contact
Inner Electrode
O-Rings
Sample Path
Sensor Contact
Inner Electrode
Fill Solution
Sample Path
O-Rings
Reservoir
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