I bubbled 5% CO2 through a needle into 30 mL of PBS open to atmosphere (25C) for approximately one hour and reduced the pH from 7.40 to 6.48. I was wondering if I can find the concentration of CO2 in the solution over time with one known (pH) using the Henderson-Haselbalch equation? I am pretty sure the pKa is 7.20 for the phosphate reaction. Since CO2 is an addition of a weak acid into the solution, how can I determine the CO2 concentration over time? Would I need the flow rate of CO2 infusion into the solution? Someone answered in another forum and said it is negligible. I know that the number of moles of CO2 in a 30 mL solution after bubbling is in the 10^-5 range, but I need to know, even if it is a small number. The partial pressure of CO2 above the solution should equilibrate to around 36 mmHg if that helps. The phosphate buffered saline consists of 138 mM of NaCl, 10mM phosphate, 2.7 mM KCl. I am pretty sure the buffer system dissolved Na2HPO4 and KH2PO4. Thanks.