Water absorbs CO2 from the atmosphere and forms H2CO3. Carbonic acid in turn dissociates because it's a weak acid. If we were to measure the pKa of another weak acid in water that contains H2CO3 using potentiometric titration with NaOH, how would this carbonate impurity influence the measured pKa?
It depends on the Ka of the acid you are titrating with. If the Ka of the weak acid is larger than the Ka of the carbonic acid, the effect is less and vice versa. To avoid (minimize) this effect, boil the water and keep in a closed environment prior to use to minimize the carbon dioxide contamination.
When carbon dioxide dissolves in water and disassociates, the products HCO3- and CO32- become a buffer to the effects of adding acid. So calculating the pH of the solution after adding an acid is not just a simple task of adding [H+] concentrations. Udaya's method would eliminate that problem.
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