Hydrolysis of Amphiprotic Anion:

Let us consider hydrolysis of amphiprotic anion only, i.e., when counter cation is not hydrolysed, example of some salts of this category are NaHCO₃, NaHS, Na₂HPO₄, NaH₂PO₄.

Here, H₂PO^-₄ and HPO^2-₄  are amphiprotic anions. pH after their hydrolysis can be calculated as,

        pH of  H₂PO^-₄  in aqueous medium = (pk_a1 + pk_a2)/2

        pH of H₂PO^2-₄  in aqueous medium = (pk_a2 + pk_a3)/2

Here, H₂PO^2-₄  is conjugate base of H₂PO^-₄ and H₃PO₄ is conjugate acid of H₂PO^-₄.

Similarly, PO^3-_{4 i}s conjugate base of HPO^-2₄  and HPO^-₄  is conjugate acid of PO^3-₄ .

(iv)  Let us consider amphiprotic bicarbonate anion.

       pH HCO^-₃of  ion after hydrolysis in aqueous medium

                        = (pk_a1 + pk_a2)/2

(v)  Let us consider the hydrolysis of amphiprotic anion along with cation, e.g., NH₄HCO₃, NH₄HS.

In above examples both cations and anions are derived from weak base and weak acids respectively hence, both will undergo hydrolysis in aqueous medium.

When these salts are dissolved in water, [H₃O⁺] concentra­tion can be determined as,

[H₃O⁺] = √k_a1[k_w/k_b + k_a2]

pH = -log = √k_a1[k_w/k_b + k_a2]