pH of polyprotic acids

[Erika Patel 3I]

I am confused when to use this equation provided in the textbook: pH = 1/2 (pKa1 + pKa2) versus when to just assume that the second de-protonation is negligible to the pH value and only use Ka1. Does anyone know when specifically to use that equation?

[905987008]

use the pH = 1/2 (pKa1 + pKa2) equation when both ionization steps significantly contribute to the pH. If the second ionization is negligible, you can simplify by using only Ka1 for pH calculations.