pH of a 1 mM CO2 Solution
Problem
What is the pH value and the equilibrium composition of a pure 1 millimolar CO2 solution?
Procedure
Step 1. As always, we begin with the “empty” input window of pure water (by clicking the lower menu button H2O). After switching to molar units (checkbox mol), enter the value 1 mM for DIC — see right image. Then button Start.
[Optional: In the first line, Filename, you can enter the name under which the water is to be saved. Here “dic” is chosen.]
Step 2. The first information you get is that the solution is not charge balanced. There are no cations in the solution to balance the anions.
Choose “pH” (i.e., the cation H+) as the parameter for charge-balance adjustment, then button next ≫.
A new window pops up showing the result of the adjustment from pH = 7 to 4.68.
Step 3. The button next ≫ opens the main output table with three columns: the input water and the equilibrium compositions Output 1 (without mineral precipitation) and Output 2 (with mineral precipitation and redox). In this example, there is no mineral precipitation and therefore both outputs are the same (except the pe value, which is irrelevant here).
Step 4. The main output table (shown above) contains only the total concentrations (in this case it’s DIC alone). To obtain the equilibrium composition of each carbonate species, click the button Ions. The result is shown on the right.
The equilibrium composition is in Output 1. The only cation is H+ (= 0.021 mM) and the anion is HCO3- (= 0.021 mM), whereas CO2 is uncharged and the amount of CO3-2 is almost zero. The charge balance is fulfilled.
[Note that the input solution has a CBE of 100% and is therefore absolutely wrong; the data in the column “Input” should be ignored.]
Answer
The pH value and equilibrium composition of a pure 1 mM CO2 solution is given by:
| pH | 4.68 | |
| CO2 | 0.979 mM | |
| HCO3- | 0.021 mM | |
| CO3-2 | 4.8 10-8 mM |
The sum of CO2, HCO3-, and CO3-2 is DIC = 1 mM.