Polyprotic Acid and Bases Approximation

[Amy Xia 1J]

Hi,

For polyprotic acid and bases, what does it mean by ignoring second deprotonations only when the approximation is justified? I'm confused about under what situation we are supposed to do the Ka2 calculation.

Thank you!

[Noah Ho 1F]

Often times, the second deprotonation won't provide a significant number of H+ ions. This applies to weak acids. However, for acids like H2SO4 or oxalic acid, which are strong acids or have low pka values, the 2nd deprotonation can affect the pH and H+ ion concentration. I don't know what cutoff we'll be using to determine whether a polyprotic acid is strong enough to do a second protonation or not.

[Henry_2A]

Hi,

I believe the only acid that requires taking into account the second deprotonation is H2SO4. The rest have a minimal effect.

[Yuhan Sun]

We should provide that Ka2 is less than about Ka1/1000. In this way, they do not affect the pH significantly and can be ignored.
This justification requirement is written in the book of page 484.

[Matthew Li 1B]

we ignore the second steps of all polyprotic acid and bases unless it is H2SO4