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 Henry’s 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 Henry’s Law concentration of CO2 (aq).
As shown in the overhead for the Carbon Dioxide and Carbonic Acid, the Henry’s Law concentration (@ 25 °C) is
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;
The pH is related to proton concentration by [H+]=10-pH
The acid constants for H2CO3 are Ka1=4.45x10-7 and Ka2=4.69x10-11
The first acid constant accounts for the relatively small fraction (~0.1%) of CO2 (aq) as H2CO3
The CO2 (aq) concentration is a function of the Henry's Law solubility and the partial pressure of CO2 (g) in the air above the water
CO2 (g) makes up ~0.0355% of the atmosphere (on the average) and has the Henry's Law constant KCO2 =2x10-3 @ 25oC.
While CO2 (aq) concentration is independent of pH, concentrations of the other species, i.e., HCO3- and CO32-, vary with pH
Subsequently, total CO2 (aq) concentration increases with pH due to an increase in HCO3- and CO32-
The total CO2 (aq) concentration is stable at pH below 5.5 because the major species is CO2 (aq)
The total CO2 (aq) concentration increases the pH range between 6 and 11 due to an increase in HCO3-