If you're using the Henderson-Hasselbalch approximation then activity coefficients are unlikely to be relevant - using the approximation will already take you 0.1-0.4 pH points away from a true value.
Care to elaborate? HH equation is not an approximation, it is just a rearranged acid dissociation constant. Problems start when it is used blindly and without understanding limitations of the typical approach.
The HH equation as I know it is of the form:
) where Cs
is the initial concentration of your salt = concentration of the cation B+ present from salt BA, where you also have some HA present (initial concentration Ca
) in the mixture
This is heavily approximated. An exact solution is first obtaining
After two approximations follows [H+]=Ka
I have also seen the equation Ka=([H+]*[salt])/[weak acid], but am unclear what this means - presumably [salt]=Cs as all salts are assumed to dissociate 100% in solution, or for salt Bb
*b. Thus in calculating buffer composition you would work to find out what mass of salt would produce this concentration [salt] for your desired pH. But what does [weak acid] refer to and how could you calculate it exactly? My issue with that is the reason I don't see this so far as an exact method.