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

H₂A is termed as the diprotic acid.
H₂A ₊ H₂O →HA⁻ ₊ H₃O⁺.
The ka of HA⁻ is low if it is compared with HA. Due to the common ion effect, the second dissociation becomes weaker.
Ionization of water has great results. Most of H₃O⁺ they come from HA and in this dissociation ionization has no impact at all.

Answer 2

For H₂A as a diprotic acid

H₂A + H₂O → HA⁻ + H₃O⁺

And for the second dissociation:

HA⁻ + H₂O → A⁻² + H₃O⁺

Ka of HA⁻ is much lower compared to HA

so second dissociation is slower compared to first.

and already solution contains 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 auto-ionization of H₂O has greater impact.

while in first dissociation more H₃O⁺ comes from H₂A

so auto-ionization do not have much impact.