Thermo Fisher Scientific Dioxide Ion Selective Electrodes Carbon - Manual (Page 12)

Instruction Manual Carbon Dioxide Electrode
12
[H
+
] [HCO
3-
]
⎯⎯⎯⎯⎯⎯⎯ = constant
[CO
2
]
The bicarbonate ion level can be considered constant since the internal filling solution contains a
high level of sodium bicarbonate:
[H
+
] = [CO
2
] X constant
The electrode sensing element's potential, with respect to the internal reference element, varies in a
Nernstian manner with changes in the hydrogen level.
The Nernstian equation shows the relationship between the potential of the pH internal element and
the hydrogen ion concentration:
E = Eo + S log [H
+
]
where:
E = measured electrode potential
Eo = reference potential (a constant)
[H
+
] = hydrogen ion concentration
S = electrode slope (~56mV/decade)
Because the hydrogen ion concentration is directly related to the carbon dioxide concentration,
electrode response to carbon dioxide is also Nernstian:
E = E
1
+ S log [CO
2
]
Carbon Dioxide Chemistry
Carbon dioxide exists as bicarbonate and carbonate in basic solutions:
CO
2
+ OH-
⇒ HCO
3-
CO
2
+ 2OH-
⇔ CO
3
-2
+ H
2
O
The solution's pH governs the amount of carbon dioxide present in the form of carbonate and
bicarbonate ions. At a pH of 5, essentially all the carbon dioxide in solution is in the CO
2
form.
The pH is held between 4.8 and 5.2 by the carbon dioxide buffer used in carbon dioxide
determinations and converts the carbonate and bicarbonate to the CO
2
form:
2H
+
+ CO
3
-2 ⇒ H
2
O + CO
2
H
+
+ HCO
3-
⇔ H
2
O + CO
2
The total amount of carbon dioxide, carbonate, and bicarbonate is then measurable in the solution.