28
Theory of Operation
The carbon dioxide electrode uses a gas-permeable membrane to
separate the sample solution from the electrode internal solution.
Dissolved carbon dioxide in the sample solution diffuses through the
membrane until an equilibrium is reached between the partial
pressure of CO
2
in the sample solution and the CO
2
in the internal
filling solution. In any given sample the partial pressure of carbon
dioxide will be proportional to the concentration of carbon dioxide.
The diffusion across the membrane affects the level of hydrogen ions
in the internal filling solution:
CO
2
+ H
2
O H
+
+ HCO
3
-
The hydrogen level of the internal filling solution is measured by the
pH electrode (inner body) located behind the membrane.
The relationship between carbon dioxide, water, bicarbonate, and
hydrogen ion is given by the following equation:
[H
+
] [HCO
3
-] = constant
[CO
2
]
The internal filling solution contains a high level of sodium
bicarbonate so that the bicarbonate level can be considered constant:
[H
+
] = [CO
2
] constant
The potential of the pH sensing element is related to the hydrogen ion
concentration by the Nersnst equation:
E = E
O
+ S log [H
+
]
where:
E = measured electrode potential
E
O
= reference potential (a constant)
[H
+
] = hydrogen ion concentration
S = electrode slope
Since the hydrogen ion concentration is directly related to carbon
dioxide concentration, electrode response to carbon dioxide is also
Nernstian.
E = E
O
+ S log [CO
2
]
The reference potential, E
O
is partly determined by the internal
reference element that responds to the fixed level of chloride in the
internal filling solution.
