# Do amphoteric cations exist?

I know that anions such as $$\ce{CH3COO-}, \ce{OH-}, \ce{CN-}$$ are basic in nature, since they are conjugate bases of weak acids. Similarly, cations such as $$\ce{NH4+}, \ce{H3O+}$$ are acidic in nature , since they conjugate acids of weak bases.

However, we also have another case whereby, anions such as $$\ce{HSO3-}, \ce{HS-}$$ are amphoteric in nature. This is due to the fact that their conjugate acids are polyprotic acids. Thus, one can have the two reactions:

$$\ce{HS- + H2O <=> S^2- + H3O+}$$ and $$\ce{HS- + H2O <=> H2S + OH-}$$

Since, for this case, we got amphoteric anions from polyprotic acids, is there a possibility that we can do the same for polyprotic bases so as to get amphoteric cations? i.e. Can we have a conjugate acid of a polyprotic base, which can be regarded as an amphoteric cation? I haven't seen any such examples of amphoteric cations yet, hence I am asking this question.

• Polyamines. Or basic aminoacids. – Poutnik Jan 9 at 14:16
• $\ce{H3O+}$ and $\ce{H3S+}$ are technically amphoteric (as well as likely many others), though they're (always?) extremely weak bases. I wonder if there are more natural examples which are limited to few atoms. – Nicolau Saker Neto Jan 9 at 14:27
• Or CaOH+, as Ca(OH)2 has 2 dissociation constants. – Poutnik Jan 9 at 15:04
• What about singly protonated DABCO? (ie 1,4-diazabicyclo[2.2.2]octane). It is cationic, and the neutral conjugate base has no additional acidic protons. With one proton, it is weakly basic (pKa of conjugate acid is ~3) and weakly acidic (pKa ~ 9). – Andrew Jan 9 at 15:27
• Thing is, almost any cation can be protonated. Only the smallest ones may be to unstable to have minimum on PES. Another thing is that "amphoteric" tends to be used for stuff easily / commonly acting as acid and base, so what exactly you'd like to get here? – Mithoron Jan 9 at 18:15

Lysine has two amino groups and one carboxyl group. When dissolved in aprotic solvents, all of these will be uncharged so the carboxyl group will be $$\ce{-COOH}$$ and the two amino groups $$\ce{-NH2}$$. When dissolved in water, these three groups have distinct $$\mathrm pK_\mathrm a$$ values of $$2.15, 9.16$$ and $$10.67$$. Depending on the acidity of the solution, the following (acid-sided) equilibriums occur: