The pKa, values are especially convenient, because the
pKa of a dissociation reaction is just the pH at which
the undissociated and dissociated forms are present in equal amounts:
If the pH exactly equals the pKa for the acid under
and A- and HA are present in equal amounts. If the pH
falls below pKa (more acidic), then log10 [A-]/[HA]
will be negative, the ratio [A-]/[HA] will be less than
1.00, and the HA form will be favoured.
In physical terms, if the pH is lowered by the addition of H+,
some of the excess H+ combines with A- to
make more of the undissociated acid. The dissociation equilibrium
is shifted to the left.