Absorption - Introduction to Carbon Capture and Sequestration - page 186

Question 5.3.1 Raise or lower pH?
To increase the solubility of CO 2 do we need to increase or decrease the pH?
Do the principles guiding CCS also apply to soda (carbonated) water? The
burning feel of soda is caused by its acidity (pH = 3). Are the conditions the
same?
certainly makes the absorption of water more complicated than we previ-
ously discussed, the equilibrium constants for all of these reactions when
CO 2 is dissolved in water have been measured (see Table 5.3.1 [5.4]).
One point to think about is the impact of pH on the solubility of CO 2 . We
can use the equilibrium equations to determine the concentrations of the
various carbonate compounds as a function of the pH of the solution.
The lower the pH, the more H + ions we have in solution, and the more the
chemical equilibrium will shift toward carbonic acid (H 2 CO 3 ). Increasing
the pH causes the equilibrium to shift toward bicarbonate (HCO 3 − ), and
for the highest pH, toward carbonate (CO 3 2 − ). Using the equilibrium con-
stants we can calculate the concentrations and see that this is indeed the
trend (see Figure 5.3.3 ).
This brings us back to the idea of adding something to water to
increase the solubility.
Bases
Remember that CO 2 is slightly acidic when dissolved in water, so with the
addition of a base the solution will be able to take up even more CO 2 . A
Table 5.3.1 Reaction constants of CO 2 in water (see Figure 5.3.2 )
Reaction K i
1 1.15 × 10 − 3
2 3.21 × 10 − 3 M − 1
3 5.95 × 10 10 M − 1
4 2.46 × 10 3 M − 1
log(K 5 ) = − 5839.48/T-22.473 log(T) + 61.2060-log(c 0 )
Data from McCann et al . [5.4].