Question

For a diprotic acid, why does the autoionization of water have a greater impact on the pH at the second equivalence point?

Answer 1

for example H2A is diprotic acid

H2A + H2O -----------> HA^- + H3O⁺

now for second dissociation.

HA^- + H2O -------> A^-2 + H3O⁺

Ka of HA^- is lower compared to HA

so second dissociation is slower compared to first.

and already solution contaion H3O⁺ ions from first dissociation so second dissociation becomes much weaker due to common ion effect.

so H3O⁺ from second dissociation is very less.

so autoionization of H2O has greater impact.

while in first dissociation more H3O⁺ comes from H2A

so autoionization do nothave much impact.