The first ion will break off in water by the dissociation
H2SO4 → H+ + HSO4-
The second H+ comes from the dissociation of HSO4- by
HSO4- → H+ + SO42-
This is essentially titrating two acids at once. The curve shows the same trend as a weak acid titration where the pH does not change for a while, spikes up and levels off again. The difference occurs when the second acid reaction is taking place. The same curve happens again where a slow change in pH is followed by a spike and leveling off.
Each 'hump' has it's own half-equivalence point. The first hump's point occurs when just enough base is added to the solution to convert half the H+ ions from the first dissociation to its conjugate base, or it's Ka value.
The second hump's half-equivalence point occurs at the point where half the secondary acid is converted to the secondary conjugate base or that acid's Ka value.
On many tables of Ka for acids, these will be listed as K1 and K2. Other tables will list only the Ka for each acid in the dissociation.
This graph illustrates a diprotic acid. For an acid with more hydrogen ions to donate [e.g., citric acid (H3C6H5O7) with 3 hydrogen ions] the graph will have a third hump with a half-equivalence point at pH = pK3.
