Amphiprotic

In chemistry and physical sciences, a substance is described as amphiprotic if it can both donate or accept a proton, thus acting either like an acid or a base (according to Brønsted-Lowry theory of acids and bases: acids are proton donors and bases are proton acceptors. In Lewis theory of acids and bases; acids are electron pair acceptors and bases are electron pair donors). Water, amino acids, hydrogen carbonate ions and hydrogen sulfate ions are common examples of amphiprotic species. Since they can donate a proton, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric.

An amphoteric substance, however, is not necessarily amphiprotic. For example it may be a base and a Lewis acid, but not a Brønsted acid.

Example

A common example of an amphiprotic substance is the hydrogen carbonate ion, which can act as a base:

HCO₃^- + H₂O → H₂CO₃ + OH^-

Or as an acid:

HCO₃^- + H₂O → CO₃^2- + H₃O⁺

Thus, it can effectively accept or donate a proton.

Water is the most common example:

Basic: H₂O + HCl → H₃O⁺ + Cl^-

Acidic: H₂O + NH₃ → NH₄⁺ + OH^-