Chromatography Forum

Hi, when calculating ionic strengths from first principles should one only use the apparent pka, and not the thermodynamic pka?
I've noticed some buffer calculation sites use apparent whilst some textbooks use thermodynamic. Which is correct.

It appears to be the concentration of ions (molality or molarity, which should not be intermixed except for concentrations where they coalesce) not activities that are used, I can´t go further than that as I am not sure of your terms "apparent" and "thermodynamic" in this context. In any case, since equilibrium can depend on ionic strength these should be neat calculations. I have never done this myself, and have never seen anybody present such. All material I have seen on this avoids it more or less skillfully or gives darnright wrong examples. So I am interested in seeing the refs to those that you mention to have done these calcs. (Beynon in his buffer calculator does this, apparently, but how?? Should buy his book?)

Hi, many thanks for your response, beynons book stipulates "the pka we get from textbooks is not the pka of our solutions - it is modified by both temperature and ionic strength. " The apparent(working pka) is calculated from the thermodynamic pka.
apparent pka = pka + ionic strength correction+Temperature correction.

I am confused by the choice of words to describe pKa. Tabulated pKa's should be thermodynamic, for example
[activitiy hydrogen ion][activity acid anion]/[acid] = K(thermo)

I don't know of any other pK that will yield a generally useful result. Apparent pKa, by definition I have seen, is the pKa one gets by making the incorrect assumpition that activity coefficients are unimportant, that is, using concentrations rather than activity. By measuring, one can calculate such an apparent pK but it will only be good for the exact solution conditions under which it was measured.

If you want to calculate an ionic strength of a weak acid solution it can be done iteratively. On first pass of calculation, calculate activity of hydrogen ion and acid anion using thermodynamic K and initial concentrations of acid. Then use these activites as concentrations to calculate ionic strength and then from ionic strength calculate an activity coefficient. From activity and activity coefficient calculate a refined ionic strength. Repeat until answer of desired accuracy is reached.