Chemistry: The Molecular Science (5th Edition)

$H_2P{O_4}^-$ is an amphiprotic substance.
- $H_2{PO_4}^-$ is capable of donating a proton, and becoming $HP{O_4}^{2-}$ or receiving a proton, becoming $H_3PO_4$. Therefore it is considered amphiprotic. - You can also analyze the table $14.2$ (Page 624), and notice that $H_2P{O_4}^-$ has a $K_a$ value $(6.3 \times 10^{-8})$ and a $K_b$ value $(1.4 \times 10^{-12})$. Since both values are $> 10^{-14}$, we consider $H_2{PO_4}^-$ an amphiprotic substance.