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Appendix B
B.1.3
Partial Pressure of Carbon Dioxide (PCO
)
2
Definition of PCO2
The partial pressure (tension) of carbon dioxide in solution
is defined as the partial pressure of carbon dioxide in the
gas phase in equilibrium with the blood.
Principle of PCO
Measurement
2
PCO
is measured with a modified pH sensor. Carbon dioxide
2
in the unknown solution makes contact with a hydrogen ion
selective membrane CO
diffuses across the membrane
2
into a thin layer of bicarbonate buffer in response to partial
pressure difference. This solution then becomes equilibrated
with the external gas pressure of the fluid in contact with
the outer surface of the membrane. CO
in the solution
2
becomes hydrated producing carbonic acid which results
in a change in hydrogen ion activity.
-
+
CO
+ H
0 <=> H
CO
<=> H
+ [HCO
]
Equation 8
2
2
2
3
3
The pH of this internal solution varies with the PCO
according
2
to the Henderson-Hasselbalch equation.
pH = pKa + log {HCO3- / PCO
* a}
2
The measured potential is related to the logarithm of PCO
2
content of the sample after compensation of the measured
potential of the pH sensor.
B-3
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