### Fig 5 - a diprotic acid: oxalic acid

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.