[ionic81]

Hi,

I have a solution containing acetic acid and NaCl. The solution is adjusted to a particular pH value by adding NaOH. For the calculation of the undissociated acetic acid concentration, I use the correction suggested by Sortwell 2001 (J. Food Prot. 63(2): 222-230 (2000)), which says that I have to account for the ionic strength in order to calculate pKa properly.

But my question is: how do I calculate this ionic strength? Do I only take NaCl into account? Or do I also account for the presence of NaOH?

Thanks!

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

Edit/shameless advertising: note that in most systems you will have troubles calculating corrections, as ionic strength modifies activity coefficients which in turn shifts the equilibrium, which changes ionic strength and so on. For reasonably accurate results you need to do the calculations iteratively. It can be done manually (usually about two iterations are enough), but it is a waste of time. There are programs that do such calculations much better. See my signature.

[Pradeep]

you  should consider the presence of NaOH.

[AWK]

you  should consider the presence of NaOH.

There is no NaOH in the presence of acetic acid (if acid is in excess). Ionic strength in this case is a sum of ionic strengths of NaCl and CH₃COONa

[Pradeep]

Of cause. That is what I tried to say. But sorry for poor language. That should be CH3COONa and NaCl.