A polyprotic acid is an acid which contains in its molecule more than one acid hydrogen, ionizable.
Take for example H 2 S, a 'diprotic acid:
The overall expression will be:
The time constant of the overall reaction is not more than the result of the product of the two dissociation constants:
When it has to do with a polyprotic acid usually the first acid dissociation constant is much larger (several orders of magnitude) of the successive and for this reason contributes more to the pH value. Considering all deriving from the first dissociation.
In principle, for the calculation of pH we must take into account two types of polybasic acids:
- 1 → Those whose first dissociation is complete (K a1 very large)
- 2 → Those whose first dissociation is partial (K relatively small a1)
case 1
0.1 M solution of H 2 SO 4. Since the first dissociation
is definitely complete, you can say that:
where C a is the initial molar concentration of acid. The pH will be:
case 2
0.1 M solution of H 2 S. As in "Case 1" we neglect the second dissociation. As regards the first dissociation instead, H 2 S behaves as an acid weak. Therefore:
the pH will be:
In both cases, we have resorted to approximate solutions, if you want to know how to calculate systematically the pH of polybasic acids → systematic calculation of pH of polybasic acids
