30
Chemistry of Carbon Dioxide
Carbon dioxide reacts with water to form a weak carbonic acid
solution:
CO
2 +
H
2
O H
2
CO
3
In basic solutions carbon dioxide exists as bicarbonate and carbonate:
CO
2
+ OH-
HCO
3
-
CO
2
+ 2OH-
CO
3
= + H
2
O
The amount of carbon dioxide in the form of carbonate and
bicarbonate depends on the pH of the solution. See
Figure 6.
At pH 5, virtually all the carbon dioxide in the solution is in the CO
2
form. Below pH 5 carbon dioxide exists in the CO
2
form, but acetic
acid, formic acid, nitrogen dioxide, and sulfur dioxide interfere
significantly with the electrode measurement.
The carbon dioxide buffer used in carbon dioxide determinations
keeps the pH between 4.8 and 5.2 and converts the carbonate and
bicarbonate to the CO
2
form:
H
+
+ HCO
3
-
H
2
O + CO
2
2H
+
+ CO
3
= H
2
O + CO
2
The electrode then can be used to measure the total amount of carbon
dioxide in solution.
The concentration of carbon dioxide in solution is directly proportional
to the partial pressure of carbon dioxide over the solution. This
relationship is described by Henry’s law:
Henry Law Constant for CO
2
PCO
2
in mmHg
Kh = 1.25 x 106 =
at 25°C
[CO
2
] in mole fractions
*Daniels & Alberty, Physical Chemistry, 2nd Ed. Wiley
Where:
[CO
2
] is the concentration of CO
2
in solution, PCO
2
is the partial
pressure of CO
2
, and Kh is Henry’s constant, which varies with
solution temperature.
