Polyprotic acids and bases dissociation constants
First, and the simplest approach to defining equilibriums constants for polyprotic acid is to treat every step of dissociation separately:
Then we have three stepwise dissociation constants:
There are also overall dissociation constants defined, of which first is identical to the stepwise one, while the second and the third take forms:
It is not very difficult to find out relations:
For polyprotic acids with greater number of protons analogous constants are used.
Same holds for polyprotic bases.
In case of polyprotic substances the idea of conjugate acids and bases becomes a little bit confusing, as the same substance can play both roles:
and the numeration of dissociation steps is reversed, so that first acid dissociation step described by Ka1:
creates conjugate base for the last dissociation step:
described by Kb3. So for the substance with three protons relations analogous to 2.5 take forms: