[funtime]

We recently completed a buffer capacity prac using 0.1M H2PO4- as the weak acid and 0.1M HPO4-- as its conjugate base.
For the results my pH values were consistantly less than the H-H estimate for what they should be. Apparently this has something to do with the ionic strength and pKa (pH).
here are my results :

Buffer solution     pH              Henderson-Hasselbalch estimate
1               5.95                                6.25
2               6.47                                6.83
3               6.74                                7.20
4               7.32                                7.57
5               7.77                                8.15

Anyone have any idea as to why my pH is less than the H-H estimate?

*H-H = pH = pKa + log(/[A])

[Borek]

You have already answered your own question.

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

Note that ionic strength of 0.2 is above 0.1, which is usually cited as the limit of Debye-Huckel theory applicability, but it still gives reasonable results in this case.