Before any outside acid or base is added, the pH of a buffer mixture
of a weak acid and its salt is given by the same expression that
we derived previously from the equilibrium constant:
The pH of the buffer solution is determined by the pKa of the acid
used and the ratio of basic form (salt, A-) to the acidic
form (acid, HA). To prepare the buffer mixture, [A-]
can be assumed to be equal to the total concentration of the salt,
and [HA] equal to the total concentration of the weak acid used.
If the ratio of salt to acid is 1: 1, then the pH will be the
same as the pKa, but one obviously can obtain any desired
pH close to pKa by altering the salt-acid ratio of the
solution as it is made up.
Example. The second dissociation of phosphoric acid has
a pKa near 7.0: