Diprotic acid

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A diprotic acid is an acid such as H2SO4 (sulfuric acid) that happens to contain within its molecular structure two hydrogen atoms capable of dissociation (i.e. ionizable) in water. The complete dissasociation of diprotic acids is of the same form as sulfuric acid:

H_2SO_4 \longrightarrow H^+ (aq) + HSO4^-(aq) Ka = 1x103

HSO_4^- \longrightarrow H^+(aq) + SO_4^{2-}(aq) Ka = 1x10 − 2

The dissocation does not happen all at once due to the two stages of dissasociation having different Ka values. The first dissasociation 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 ionizable hydrogen atoms on the acid molecule do not leave the acid at the same time.

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