### 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 (H_{2}Ox = HOOCCOOH)
is just the superposition for separate plots of the two acid-base systems
H_{2}Ox-HOx^{–} and HOx^{–}-Ox^{2–}
whose pKs are 1.2 and 4.2. The system points corresponding to 0.01M solutions
of pure H_{2}A (1) and A^{2–}
(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^{–} <==> H_{2}Ox
+ A^{2–}

which leads to the approximation

[HOx^{–}] ≈ [Ox^{2–}]

Thus fixing the pH at about 2.7. This same result can be obtained from
thewell known approximation pH = (pK_{1} + pK_{2})/2.