* Brønsted-Lowry Definition: In the Brønsted-Lowry theory, an acid is a proton (H⁺) donor, and a base is a proton acceptor.

* PO₄³⁻ Behavior: PO₄³⁻ (phosphate ion) can accept a proton (H⁺) to form HPO₄²⁻ (hydrogen phosphate ion):

PO₄³⁻(aq) + H⁺(aq) ⇌ HPO₄²⁻(aq)

* Conclusion: Since PO₄³⁻ accepts a proton, it acts as a Brønsted base.