Question

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

Answer 1

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⁻.