Lowering pH with CO2 bubbling in phosphate buffered saline (pH 7.4)?

Question

I have a 30 mL solution of PBS at a pH of 7.40 open to atmosphere (at 25C). I bubble 5% CO2 through a tube until the pH stabalizes to 6.48. Does anybody know the Ka value of adding a weak acid (ie CO2) to a buffered system? How would I incorporate the Henderson-Hasselbalch equation to find changes in CO2 concentration over time with one known (pH)?

Answer 1

Its negligible

Form a time dependent form.

Answer 2

Sometimes I have to use NaOH to bring the pH up when I'm making PBS.