Triprotic Acid

[Chocolatebear89]

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Hi,I was wondering what's the relationship between pkA and pKb for a triprotic acid, like H3PO4.

I know for Carbonic acid (H2CO3), pKa1+ pKb2= 14
and pKa2 + pKb1= 14

 

[Rabolisk]

Likewise, pKa1 + pKb3 = 14
pKa2 + pKb2 = 14
pKa3 + pKb1 = 14

Ka1 of a triprotic acid is the equilibrium constant for H3A --> H2A- + H+
Kb3 of a triprotic acid is the equilibrium constant for H2A- + H2O --> H3A + OH-

Same logic applies to the rest.

 

[Chocolatebear89]

Wait, so kB3 does not correspond with the last conjugate base, but conjugate base of the first acid?

 

[Rabolisk]

Kb3 corresponds with the conjugate base of the "first" acid, because this is the "last" base. The three bases are, "in order", A3-, HA2-, and H2A-.

 

[Chocolatebear89]

in order of basicity?

 

[Rabolisk]

Right. Kb1 refers to A3-, Kb2 to HA2-, and so on. That's the convention.