Correlating solution pH with pKa(conjugate pair)

Ka = [H3O+] × [conjugate base]
[acid]
pH = -log [H3O+]
pKa = -log Ka
For solutions prepared by dissolving an acid or its conjugate base or a mixture of the two, at equilibrium 
  • [acid], [conjugate base] and [H3O+] combined as shown equal Ka of the conjugate pair.
  • the pH relative to pKa depends on the ratio [conjugate base]/[acid]
Ka = [H3O+] ×[conjugate base]
[acid]
Ka = [H3O+] × [conjugate base]
[acid]
If [acid] is higher than [conjugate base]
the ratio [conjugate base]]/[acid] is smaller than Ka 
[H3O+] is greater than Ka
pH is lower than pKa (on the acid side).
Roll over the image to connect this with the titration curve.

If [acid] equals [conjugate base],
the ratio [conjugate base]]/[acid] equals 1.
[H3O+] equals Ka.
pH = pKa

If [conjugate base] is higher than [acid]
the ratio [conjugate base]]/[acid] is larger than Ka, [H3O+] must be smaller than Ka
pH is higher than pKa (on the base side).
Dr Sheila Woodgate
© The University of Auckland 2002