4
III.III.
III.III.
III.
Theory of OperationTheory of Operation
Theory of OperationTheory of Operation
Theory of Operation
::
::
:
The Carbon Dioxide electrode is a complete potentiometric
cell that contains both a silver/silver chloride (Ag/AgCl)
reference and a pH measurement element. These elements
are housed within a thermoplastic body in a chloride ion-
containing electrolyte, and are isolated from the sample by
a PTFE membrane.
ISA addition changes the pH of the sample to approxi-
mately 4.7 pH and Bicarbonate (HCO
3
-
) and carbonate
(CO
3
2-
) ions in the sample are converted to carbon dioxide
(CO
2
). The CO
2
in the sample solution diffuses through the
PTFE membrane where it dissolves into the thin film of fill
solution found between the membrane and the internal pH
membrane. Here it converts back into bicarbonate and
hydrogen ions. The pH changes proportionally with the
concentration of dissolved gas in the sample solution. Dif-
fusion of CO
2
continues until the partial pressures of the gas
in the sample and thin film are equal.
The Nernst expression for an Carbon Dioxide sensor is ex-
pressed in the equation below. Note that the potential is a
function of the Carbon Dioxide gas, which in turn is related
to the hydrogen ion concentration. The glass internal,
Ag/AgCl reference, equilibrium constant and Henry’s law
constant are rolled into the E’
and E
o
terms. The Nernst
equation for the sensor becomes the equation noted below:
E = E’+2.3RT/nF log [CO
2
]= E
o
+0.059 log [H
+
]
E = observed potential
E’ = Reference and fixed internal voltages
R = gas constant (8.314 J/K Mol)
n= Charge on ion (equivalents/mol)
T = absolute temperature in K
F = Faraday constant (9.648 x 10
4
C/equivalent)
The mV should increase in a Nernstian manner as the
carbon dioxide partial pressure increases in the sample.
