Siemens Rapidlab 1200, Operator'S Manual, Page: 55 / 448

Rapidlab 1200 Operator’s Guide: System Overview and Intended Use 1-35
02087462 Rev. V
pH Sensor
The pH sensor, which is based on ISE technology, is a half-cell that forms a
complete electrochemical cell when combined with the external reference sensor.
It contains a silver/silver chloride wire surrounded by a buffer solution. A glass
membrane that is highly sensitive and specific for hydrogen ions separates the
sample from the solution.
Figure 1-30 pH Sensor
As the sample comes in contact with the membrane of the pH sensor, a membrane
potential develops due to the exchange of hydrogen ions in the membrane. The
silver/silver chloride inner conductor transmits 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
report the pH value of the sample.
Carbon Dioxide Tension ( p CO
2
)
Carbon dioxide (CO
2
) is produced during normal cell metabolism and is released
into the blood stream where it is transported to the kidneys and lungs for
excretion. CO
2
is transported through the blood as bicarbonate (HCO
3
–
),
dissolved CO
2
, and carbonic acid (H
2
CO
3
). CO
2
exists in a dynamic state in the
blood as seen in the following equation:
+ −
+ ↔ ↔ +
H HCO CO H O H CO
3 3 2 2 2
1 Buffer solution
2 Silver/silver chloride electrode
3 Sample path