Second deprotonation

[JaesalSoma1E]

How do you determine when to ignore the second Ka in polyprotic acids when calculating pH?

[Abril Guanes 2A]

I had the same question. When I was doing the textbook problems, I read that you ignore the second protonation when the Ka value of the second protonation is less than the first. So basically when KA2 < than KA1.

[MaiVyDang2I]

The textbook says that the second Ka is typically less than about the first Ka/1000, so they do not affect the pH significantly and can be ignored. I think for this course since we're not calculating pH and equilibrium of weak acids and bases, so we just focus on recognizing whether an acid is polyproctic or not, but not the actual value when calculating.