Dissolved Carbon Dioxide
A River in New Hampshire
These approximate calculations work for water buffered by acids/bases and when the water is not in contact with limestone. Keep in mind that the water is probably not at equilibrium.
The maximum total amount of CO2 that may dissolve in water is a function of pH.
This is because as pH changes, the fraction of the total dissolved CO2 as CO2 (aq) also changes.
On the other hand, the concentration of CO2 as CO2 (aq) does not change because of the Henrys Law equilibrium between the large reservoir of gaseous CO2, i.e., the atmosphere, and the relatively finite body of water.
It is relatively easy to calculate the fractional amount of CO2 (aq) as a function of pH and the Henrys Law concentration of CO2 (aq).
As shown in the overhead for the Carbon Dioxide and Carbonic Acid, the Henrys Law concentration (@ 25 oC) is
[CO2 (aq)]=1.2´ 10-5 mole/L
Also, the fraction of carbonate species as CO2 (aq) is
One may solve this equation to obtain total CO2 (aq) as a function of [CO2 (aq)], [H+] and the acid equilibrium constants
The total CO2 in moles per liter is plotted as a function of pH below.
Some points to keep in mind;
This page edited Thursday, December 21, 2006
This page was last edited Thursday, December 21, 2006