RADIOMETER ABL90 FLEX - Measuring Process

ABL90 FLEX reference manual 5. Sensors and optical system
5-25
The following is an account of the measuring process in the
pCO
2
sensor.
Part Function
Transport of CO
2
CO
2
from the sample permeates the membrane.
Dissolution of CO
2
The CO
2
dissolves in the electrolyte solution.
This produces carbonic acid:
H
2
O + CO
2
 H
2
CO
3
Dissociation of
carbonic acid
Carbonic acid dissociates according to the following
equilibrium reaction:
H
2
CO
3
 H
+
+ HCO
3
–
pH change The release of H
+
ions changes the H
+
concentration, and
thus the pH of the inner buffer solution on one side of
the pH-sensitive membrane.
Measurement of
potential
The concentration gradient of H
+
ions across the
membrane creates a potential difference across the
membrane.
This change in potential across the membrane is
measured by the voltmeter.
Relation of pH to
pCO
2
The pH value is related to the partial pressure of CO
2
in
the sample by the following equation:


2
-
3
a
*
HCO
log +pK = pH
pCO 
where:
a
pK = log K
a
, the equilibrium constant for the
dissociation of carbonic acid in water
 = solubility coefficient for CO
2
in water
The structure of the
pCO
2
sensor is similar to the pH
sensor, including the presence of a pH-sensitive
membrane. The major difference is in the internal
electrolyte solution present in the
pCO
2
sensor which
allows the dissolution and ultimate dissociation of
carbonic acid mentioned above.
If [
cHCO
3
–
] and  in the electrolyte solution is constant
this results in the following:
2
pH = K- lo
g
COp
Where
K contains the equilibrium constant pK
a
, the
solubility coefficient  and the concentration of
bicarbonate [
cHCO
3
–
].
E = E'
0
- 61.5 × pH = E
0
+ 61.5 × log pCO
2
Measuring
process