Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: funtime on September 01, 2007, 01:41:11 AM
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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])
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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.