B.1.3 Partial Pressure of Carbon Dioxide (PCO
2
)
Definition of PCO2
The partial pressure (tension) of carbon dioxide in solution
is dened as the partial pressure of carbon dioxide in the
gas phase in equilibrium with the blood.
Principle of PCO
2
Measurement
PCO
2
is measured with a modied pH sensor. Carbon dioxide
in the unknown solution makes contact with a hydrogen ion
selective membrane CO
2
diffuses across the membrane
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 uid in contact with
the outer surface of the membrane. CO
2
in the solution
becomes hydrated producing carbonic acid which results
in a change in hydrogen ion activity.
CO
2
+ H
2
0 <=> H
2
CO
3
<=> H
+
+ [HCO
-
3
] Equation 8
The pH of this internal solution varies with the PCO
2
according
to the Henderson-Hasselbalch equation.
pH = pKa + log {HCO3- / PCO
2
*
a}
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.
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