A diprotic acid is an acid such as H2SO4 (sulfuric acid) that contains within its molecular structure two hydrogen atoms per molecule capable of dissociating (i.e. ionizable) in water. The complete dissociation of diprotic acids is of the same form as sulfuric acid:
- H2SO4 → H+(aq) + HSO4−(aq) Ka = 1 × 103
- HSO4− → H+(aq) + SO42−(aq) Ka = 1 × 10−2
The dissociation does not happen all at once due to the two stages of dissociation having different Ka values. The first dissociation will, in the case of sulfuric acid, occur completely, but the second one will not. Diprotic acids are of particular note in regards to titration experiments, where a pH versus titrant volume curve will clearly show two equivalence points for the acid. This occurs because the two ionization capable hydrogen atoms on the acid molecule do not leave the acid at the same time.
Common diprotic acids include malic acid, found in apples and cherries, and tartaric acid, found in grapes and pineapples.