Lecture 5

pH and pK

The Henderson-Hasselbalch equation

The dissociation of a weak acid, AH to form A- and H+ can be represented as:

AH <==> A- + H+

The equilibrium constant for this reaction is given by:

K = -----------

Taking logarithms for both sides we get:

logK = log[H+] + log ------

since pH = -log[H+], it is convenient to define a term, pK = -logK. Then, substituting in the above equation, and rearranging, we have:

pH = pK + log ------

The pK will have the same value as the pH at which the dissociated and undissociated forms have the same activity in a titration,- it defines the midpoint of the titration curve.

©Copyright 1996, Antony Crofts, University of Illinois at Urbana-Champaign, a-crofts@uiuc.edu