CO2 bubbling in phosphate buffered saline (pH 7.4)

[wils0645]

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.

 

[wils0645]

The tricky part is not all of the CO2 dissolves. CO2 in saline at 25C has a henry constant of 0.0317. Any help would be appreciated.

 

[ravenprp]

The Henderson-Haselbalch equation can be used to calculate the concentration of CO2 in solution, given the pH and pKa values. However, it is important to note that this equation assumes that the solution is at equilibrium, which may not be the case in your experiment due to the continuous bubbling of CO2. Additionally, the equation does not take into account the changes in volume and ionic strength of the solution, which can also affect the equilibrium. Therefore, it may not accurately reflect the actual concentration of CO2 in your solution.

To determine the concentration of CO2 over time, you would need to take into account the flow rate of CO2 infusion into the solution, as well as the volume and ionic strength changes. This can be done by measuring the flow rate and calculating the amount of CO2 added to the solution over time. You can also measure the pH of the solution at different time points and use the Henderson-Haselbalch equation to estimate the concentration of CO2 at each point.

It is also important to consider the partial pressure of CO2 above the solution, as this can affect the equilibrium and therefore the concentration of CO2 in solution. The partial pressure should equilibrate to around 36 mmHg, but this may vary depending on factors such as temperature and atmospheric pressure.

In summary, while the Henderson-Haselbalch equation can provide an estimate of the CO2 concentration in solution, it may not accurately reflect the actual concentration in your experimental setup. It is important to take into account the flow rate, volume and ionic strength changes, and partial pressure of CO2 to accurately determine the concentration over time.