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 (H2Ox = HOOCCOOH) is just the superposition for separate plots of the two acid-base systems H2Ox-HOx and HOx-Ox2– whose pKs are 1.2 and 4.2. The system points corresponding to 0.01M solutions of pure H2A (1) and A2– (2) are determined just as in the monoprotic case. For a solution of the ampholyte  NaHOx, several equilibria can be written, but the one that dominates is

2 HOx <==> H2Ox + A2–

which leads to the approximation

[HOx] ≈ [Ox2–]

Thus fixing the pH at about 2.7. This same result can be obtained from thewell known approximation pH = (pK1 + pK2)/2.