Amphoteric is derived from the Greek wordamphoteroi (ἀμφότεροι) meaning "both". Related words in acid-base chemistry areamphichromatic andamphichroic, both describing substances such asacid-base indicators which give one colour on reaction with an acid and another colour on reaction with a base.[2]
Amphiprotism is exhibited by compounds with both Brønsted acidic and basic properties.[3] A prime example is H2O.Amphiprotic molecules can eitherdonate oraccept aproton (H+).Amino acids (andproteins) are amphiprotic molecules because of theiramine (−NH2) andcarboxylic acid (−COOH) groups.
Ampholytes arezwitterions.[4] Molecules or ions that contain both acidic and basicfunctional groups.Amino acidsH2N−RCH−CO2H have both a basic group−NH2 and an acidic group−COOH. Often such species exists as several structures inchemical equilibrium:
In approximately neutralaqueous solution (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is thezwitterionH3N+−RCH−COO−. The pH at which the average charge is zero is known as the molecule'sisoelectric point. Ampholytes are used to establish a stable pH gradient for use inisoelectric focusing.
Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Many metals (such aszinc,tin,lead,aluminium, andberyllium) form amphoteric oxides or hydroxides.Aluminium oxide (Al2O3) is an example of an amphoteric oxide. Amphoterism depends on theoxidation states of the oxide. Amphoteric oxides includelead(II) oxide andzinc oxide, among many others.[5]
According to theBrønsted-Lowry theory of acids and bases, acids are proton donors and bases are proton acceptors.[6] An amphiprotic molecule (or ion) can either donate or accept aproton, thus acting either as anacid or abase.Water,amino acids,hydrogencarbonate ion (or bicarbonate ion)HCO−3,dihydrogen phosphate ionH2PO−4, andhydrogensulfate ion (or bisulfate ion)HSO−4 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.
The water molecule is amphoteric in aqueous solution. It can either gain a proton to form ahydronium ionH3O+, or else lose a proton to form ahydroxide ionOH−.[7]
Another possibility is themolecular autoionization reaction between two water molecules, in which one water molecule acts as an acid and another as a base.
H2O + H2O ⇌ H3O+ + HO−
Thebicarbonate ion,HCO−3, is amphoteric as it can act as either an acid or a base:
As an acid, losing a proton:HCO−3 + OH− ⇌ CO2−3 + H2O
As a base, accepting a proton:HCO−3 + H+ ⇌ H2CO3
Note: in dilute aqueous solution the formation of thehydronium ion,H3O+(aq), is effectively complete, so that hydration of the proton can be ignored in relation to the equilibria.
Although an amphiprotic species must be amphoteric, the converse is not true. For example, a metal oxide such aszinc 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 (ZnO) reacts both with acids and with bases:
This reactivity can be used to separate differentcations, for instance zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.
