Amphoterism

Acids and bases
Acceptor number Acid Acid–base reaction Acid–base homeostasis Acid strength Acidity function Amphoterism Base Buffer solutions Dissociation constant Donor number Equilibrium chemistry Extraction Hammett acidity function pH Proton affinity Self-ionization of water Titration Lewis acid catalysis Frustrated Lewis pair Chiral Lewis acid
Acid types
Brønsted–Lowry Lewis Mineral Organic Oxide Strong Superacids Weak Solid
Base types
Brønsted–Lowry Lewis Organic Oxide Strong Superbases Non-nucleophilic Weak
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In chemistry, an amphoteric compound (from Greek amphoteros 'both') is a molecule or ion that can react both as an acid and as a base.^[1] What exactly this can mean depends on which definitions of acids and bases are being used.

One type of amphoteric species are amphiprotic molecules, which can either donate or accept a proton (H⁺). This is what "amphoteric" means in Brønsted–Lowry acid–base theory. For example, amino acids and proteins are amphiprotic molecules because of their amine (−NH₂) and carboxylic acid (−COOH) groups. Self-ionizable compounds like water are also amphiprotic.

Ampholytes are amphoteric molecules that contain both acidic and basic functional groups. For example, an amino acid H₂N−RCH−CO₂H has both a basic group −NH₂ and an acidic group −COOH, and exists as several structures in chemical equilibrium:

${\displaystyle {\ce {H2N-RCH-CO2H + H2O}}}$${\displaystyle {\ce {<=> H2N-RCH-COO- + H3O+}}}$${\displaystyle {\ce {<=> H3N+-RCH-COOH + OH-}}}$${\displaystyle {\ce {<=> H3N+-RCH-COO- + H2O}}}$

In approximately neutral aqueous solution (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is the zwitterion H₃N⁺−RCH−COO⁻. The pH at which the average charge is zero is known as the molecule's isoelectric point. Ampholytes are used to establish a stable pH gradient for use in isoelectric 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 as zinc, tin, lead, aluminium, and beryllium) form amphoteric oxides or hydroxides. Aluminium oxide (Al₂O₃) is an example of an amphoteric oxide. Amphoterism depends on the oxidation states of the oxide. Amphoteric oxides include lead(II) oxide and zinc oxide, among many others.^[2]