Diprotic acids are no more complicated to treat graphically than monoprotic acids-- something that definitely cannot be said for exact algebraic treatment! The above example for oxalic acid (H2A = HOOCCOOH) is just the superposition for separate plots of the two acid-base systems H2A-HA and HA-A2 whose pKs are 1.2 and 4.2. The system points corresponding to 0.01M solutions of pure H2A; and A2 are determined just as in the monoprotic case. For a solution of the ampholyte, several equilibria can be written, but the one that dominates is
2 HA <==> H2A + A2
which leads to the approximation
[HA] = [A2]
Thus fixing the pH at about 2.7. This same result can be obtained from thewell known approximation pH = (pK1 + pK2)/2.