I'm a bit confused about amphiprotic species – specifically why some species can be amphiprotic and others can't.
For instance, $\ce{SH-}$ is an amphiprotic species (in aqueous solution).
$$ \begin{align} &\text{Acid:} &\ce{SH- + H2O &<=> S^2- + H3O+}\\ &\text{Base:} &\ce{SH- + H2O &<=> H2S + OH-} \end{align} $$
On the other hand, isn't $\ce{HNO3}$ an amphiprotic species too? I don't understand why $\ce{HNO3}$ isn't amphiprotic whilst $\ce{HS-}$ is; they seem to both be able to have an acid and a base reaction:
$$ \begin{align} &\text{Acid:} &\ce{HNO3 + H2O &<=> NO3- + H3O+}\\ &\text{Base:} &\ce{HNO3 + H2O &<=> H2NO3+ +OH-} \end{align} $$
I hope this makes sense.