pH and pK
The equilibrium constant for this reaction is given by:
Taking logarithms for both sides we get:
since pH = -log[H+], it is convenient to define a term, pK = -logK. Then, substituting in the above equation, and rearranging, we have:
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.