amphiprotic

Amphiprotic refers to a substance that can both donate and accept a proton in aqueous solution. In Brønsted-Lowry terms, such a species can act as an acid (proton donor) or as a base (proton acceptor) depending on the reaction partners and the pH of the solution. Amphiprotic species are typically conjugate to two different species, a conjugate acid and a conjugate base, enabling two related acid–base equilibria.

Common examples include water (H2O), bicarbonate (HCO3−), dihydrogen phosphate (H2PO4−), and bisulfite (HSO3−). For instance, water can donate a proton to become hydroxide (H2O ⇌ H+ + OH−) or accept a proton to form hydronium (H2O + H+ ⇌ H3O+). Bicarbonate can donate a proton to form carbonate (HCO3− ⇌ CO3^2− + H+) or accept a proton to form carbonic acid (HCO3− + H+ ⇌ H2CO3). These dual roles enable amphiprotic species to participate in multiple acid–base reactions.

In solution, amphiprotic substances often function as buffers, particularly in the pH range between their two pKa values. They help moderate changes in pH by neutralizing added acids or bases. The concept is central to physiological buffering systems, such as the bicarbonate buffer in blood, as well as to environmental buffering in natural waters and to the behavior of amino acids and other zwitterionic species in solution.