Some of the acids like oxalic acid, sulphuric acid and phosphoric acids have more than one ionizable proton per molecule of the acid. Such acids are known as polybasic or polyprotic acids.
The ionization reactions for example for a dibasic acid H2X are represented by the equations:
H2X(aq) ⇔ H+(aq) + HX-(aq)
HX-(aq) ⇔ H+(aq) + X2-(aq)
And the corresponding equilibrium constants are given below:
Ka1 = [H+][HX-]/[H2X] and
Ka2 = [H+][X2-]/[HX-]
Here, Ka1 and Ka2 are called the first and second ionization constants respectively of the acid H2X. Similarly, for tribasic acids like H3PO4 we have three ionization constants.
It can be seen that higher order ionization constants (Ka2 , Ka3) are smaller than the lower order ionization constant (Ka1 ) of a polyprotic acid. The reason for this is that it is more difficult to remove a positively charged proton from a negative ion due to electrostatic forces.
This can be seen in the case of removing a proton from the uncharged H2CO3 as compared from a negatively charged HCO3–. Similarly, it is more difficult to remove a proton from a doubly charged HPO42– anion as compared to H2PO4–.
Polyprotic acid solutions contain a mixture of acids like H2A, HA– and A2– in case of a diprotic acid. H2A being a strong acid, the primary reaction involves the dissociation of H2A, and H3O+ in the solution comes mainly from the first dissociation step.