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:

[A^-][H⁺]
K = -----------
[AH]

Taking logarithms for both sides we get:

[A^-]
logK = log[H⁺] + log ------
[AH]

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

[A^-]
pH = pK + log ------
[AH]

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