I think you need to be bit more specific regarding your solution. You'd expect some difference between for example NaCl and CaCl2 solutions (and obviously a drastic one compared to HCl) for a given Cl concentration.

Thank you Risto. I updated the question now. I am looking for correlations between the pH and the NaCl concentrations in CO2-saturated solutions.

maybe this article can help you to simply calculate. It gives K values versus different NaCl concentrations fomr 0 to 50 Celcius. You can solve K values equations and carbon mass balance of your system simultaneously.

http://pubs.acs.org/doi/pdf/10.1021/ja01222a037