10. Polyprotic Acids

Acids that are able to donate more than one proton per molecule are called polyprotic acids.

H3PO4(aq) H+(aq) + H2PO4-(aq)	Ka1 = 7.5 x 10-3
H2PO4-(aq) H+(aq) + HPO42-(aq)	Ka2 = 6.2 x 10-8
HPO42-(aq) H+(aq) + PO43-(aq)	Ka3 = 2.2 x 10-13

Note that K_a1 >> K_a2 >> K_a3: this means that only the first ionization step is important. The numbers of protons donated in the second and third step are negligible.

H2CO3(aq) H+(aq) + HCO3-(aq)	Ka1 = 4.3 x 10-7
HCO3-(aq) H+(aq) + CO32-(aq)	Ka2 = 5.6 x 10-11

Identical remark: K_a1 >> K_a2. This means that only the first ionization step is important. The number of protons donated in the second step is negligible.

H2SO4(aq) H+(aq) + HSO4-(aq)	Ka1 >> 1
HSO4-(aq) H+(aq) + SO42-(aq)	Ka2 = 1.2 x 10-2

Sulfuric acid is a strong acid, but only for the first ionization step. For the second step on the other hand, the equilibrium state is rather positioned at the left.

If we want to calculate the pH of a polyprotic acid, we only have to take into account the first ionization step due to the fact that K_a1 >> K_a2 >> K_a3. Every following step can be neglected.

Summary

Polyprotic acids

can donate more than one proton per molecule first dissociation step is important next steps can be neglected