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Polyprotic acids and bases dissociation constants

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pH calculation lectures » 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:

polyprotic acid dissociation step 1
polyprotic acid dissociation step 2
polyprotic acid dissociation step 3

Then we have three stepwise dissociation constants:

1st acid dissociation constant, eq. 3.13.1
2nd acid dissociation constant, eq. 3.23.2
3rd acid dissociation constant, eq. 3.33.3

There are also overall dissociation constants defined, of which first is identical to the stepwise one, while the second and the third take forms:

polyprotic acid overall dissociation step 2
polyprotic acid overall dissociation step 3
2nd overall dissociation constant, eq. 3.43.4
3rd overall dissociation constant, eq. 3.53.5

It is not very difficult to find out relations:

eq. 3.63.6
eq. 3.73.7

or

3rd overall dissociation constant, eq. 3.83.8
3rd overall dissociation constant, eq. 3.93.9

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:

acid dissociation step 3acid
polyprotic conjugate base dissociation step 2base

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:

3rd overall dissociation constant, eq. 3.103.10
3rd overall dissociation constant, eq. 3.113.11
3rd overall dissociation constant, eq. 3.123.12

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