What is the equation for the base ionization constant of PO4^3-

Chemical reaction of PO₄³⁻ ion in water:

PO₄³⁻ (aq) + H₂O (l) → HPO₄²⁻ (aq) + OH⁻ (aq).

Kb = [HPO₄²⁻] · [OH⁻] / [PO₄³⁻]; base ionization constant.

Base ionization constant is the equilibrium constant for the ionization of a base.

According to Bronsted-Lowry theory acid are donor of protons and bases are acceptors of protons (the hydrogen cation or H⁺).

PO₄³⁻ is Bronsted base and it can accept proton and become conjugate acid HPO₄²⁻.