According to the
Brønsted-Lowry theory of acids and bases, acids are proton donors and bases are proton acceptors. An amphiprotic molecule (or ion) can either donate or accept a
proton, thus acting either as an
acid or a
base.
Water,
amino acids,
hydrogencarbonate ion (or bicarbonate ion) ,
dihydrogen phosphate ion , and
hydrogensulfate ion (or bisulfate ion) 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.
Examples The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a
hydronium ion , or else lose a proton to form a
hydroxide ion . Another possibility is the
molecular autoionization reaction between two water molecules, in which one water molecule acts as an acid and another as a base. : The
bicarbonate ion, , is amphoteric as it can act as either an acid or a base: :As an acid, losing a proton: :As a base, accepting a proton: Note: in dilute aqueous solution the formation of the
hydronium ion, , is effectively complete, so that hydration of the proton can be ignored in relation to the equilibria. Other examples of inorganic polyprotic acids include anions of
sulfuric acid,
phosphoric acid and
hydrogen sulfide that have lost one or more protons. In organic chemistry and biochemistry, important examples include
amino acids and derivatives of
citric acid. Although an amphiprotic species must be amphoteric, the converse is not true. For example, a metal oxide such as
zinc oxide, ZnO, contains no hydrogen and so cannot donate a proton. Nevertheless, it can act as an acid by reacting with the hydroxide ion, a base: : Zinc oxide can also act as a base: : ==Oxides==