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How does the hydrogen phosphate ion act as an acid or a base?

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sodium sulfate — acid–base reaction Sodium sulfate, also called Glauber's salt, is, like other salts, the product of an acid-base reaction.

Under the Brønsted–Lowry definition, an acid is a species that tends to lose a proton ( $H^{+}$ ), while a base is a species that tends to gain one. The hydrogen phosphate ion ( $H P O_{4}^{2 -}$ ) is considered amphoteric, meaning it can function as either an acid or a base depending on the chemical environment.1

It acts as an acid by losing a proton to form the phosphate ion, and it acts as a base by accepting a proton to form the dihydrogen phosphate ion. These processes are represented by the following equations:1

As an acid: $H P O_{4}^{2 -} ⇌ P O_{4}^{3 -} + H^{+}$ 1
As a base: $H P O_{4}^{2 -} + H^{+} ⇌ H_{2} P O_{4}^{-}$ 1

Because it can participate in both reactions, the hydrogen phosphate ion is a versatile species in acid–base chemistry, capable of adjusting its role to maintain equilibrium in a system.1