# Can strong acids be amphoteric?

There are amphiprotic (or amphoteric) substances, like $\ce{H2O}$ and $\ce {HCO3-}$, which act both as an acid and base.

If you mix a strong acid like $\ce{HCl}$ ($\mathrm{p}K_{\mathrm{a}} = -6.3$) with an even stronger acid, such as perchloric acid ($\ce{HClO4},~\mathrm{p}K_{\mathrm{a}} \approx -10$) can it act as a base?

• I'm totally not sure what you mean by "If you mix a strong acid like HCl with an extremely strong acid, HCl, can it act as a base like them?". $\ce{HCl}$ is not an amphoteric acid. – Gaurang Tandon Mar 29 '18 at 9:50
• That said, it will be protonated, if you push really hard. – Ivan Neretin Mar 29 '18 at 13:14
• @GaurangTandon yes, HCl is amphoteric: aanda.org/articles/aa/ps/2010/13/aa14959-10/aa14959-10.ps.gz – DavePhD Mar 29 '18 at 20:42
• @DavePhD Oh! I didn't know that. Well, that paper seems to be using some pretty extreme conditions though, to forcibly protonate $\ce{HCl}$. It doesn't seem to be as "amphoteric" as the regular bicarbonate anion... – Gaurang Tandon Mar 30 '18 at 1:43
• @GaurangTandon That paper is reporting what occurs naturally in outer space. This table lists 12 common substances that are weaker bases than HCl. en.wikipedia.org/wiki/Proton_affinity_(data_page) – DavePhD Mar 30 '18 at 11:57

For example, $\ce{H2SO4}$ can be protonated to $\ce{H3SO4+}$
Chemists see this happen all the time when they use a mixture of nitric and sulfuric acids for nitration. The nitric acid acts as a Bronsted-Lowry base, but forms the powerful Lewis acid $\ce{NO_2^+}$, the nitryl ion, which then acts as the nitrating agent:
$\ce{NO_2(OH) + 2 H_2SO_4 -> NO_2^+ + H_3O^+ + 2 HSO_4^-}$