Amphoterism

In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used.

Etymology and terminology

Amphoteric is derived from the Greek word () meaning "both". Related words in acid-base chemistry are amphichromatic and amphichroic, both describing substances such as acid-base indicators which give one colour on reaction with an acid and another colour on reaction with a base.

Amphiprotism

Amphiprotism is exhibited by compounds with both Brønsted acidic and basic properties. A prime example is H₂O. Amphiprotic molecules can either donate or accept a proton (H⁺). Amino acids (and proteins) are amphiprotic molecules because of their amine (−NH₂) and carboxylic acid (−COOH) groups.

Ampholytes

Ampholytes are zwitterions ‒ molecules or ions that contain both acidic and basic functional groups. Amino acids H₂N−RCH−CO₂H have both a basic group −NH₂ and an acidic group −COOH. Often such species exists as several structures in chemical equilibrium:

H₂N−CRH−CO₂H + H₂O ⇌ H₂N−CRH−COO⁻ + H₃O⁺ ⇌ H₃N⁺−CRH−COOH + HO⁻ ⇌ H₃N⁺−CRH−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.

Amphiprotic molecules

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) HCO−3, dihydrogen phosphate ion H₂PO−4, and hydrogensulfate 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.

Examples

The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a hydronium ion H₃O⁺, or else lose a proton to form a hydroxide ion OH⁻.

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.

H₂O + H₂O ⇌ H₃O⁺ + HO⁻

The bicarbonate 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 the hydronium ion, H₃O⁺(aq), 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:

ZnO + 2 OH⁻ + H₂O → [Zn(OH)₄]²⁻

Zinc oxide can also act as a base:

ZnO + 2H⁺ + 5 H₂O → [Zn(H₂O)₆]²⁺

Oxides

Zinc oxide (ZnO) reacts both with acids and with bases:

• $\mathrm{Z}\mathrm{n}\mathrm{O}+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{\mathrm{H}{\phantom{A}}_2\mathrm{S}\mathrm{O}{\phantom{A}}_4}⟶\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{O}{\phantom{A}}_4+\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{Z}\mathrm{n}\mathrm{O}+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}+\mathrm{H}{\phantom{A}}_2\mathrm{O}⟶\mathrm{N}\mathrm{a}{\phantom{A}}_2[\mathrm{Z}\mathrm{n}(\mathrm{O}\mathrm{H}){\phantom{A}}_4]$

This reactivity can be used to separate different cations, for instance zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.

Lead oxide (PbO):

• $\mathrm{P}\mathrm{b}\mathrm{O}+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{2\mathrm{H}\mathrm{C}\mathrm{l}}⟶\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}{\phantom{A}}_2+\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{P}\mathrm{b}\mathrm{O}+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}+\mathrm{H}{\phantom{A}}_2\mathrm{O}⟶\mathrm{N}\mathrm{a}{\phantom{A}}_2[\mathrm{P}\mathrm{b}(\mathrm{O}\mathrm{H}){\phantom{A}}_4]$

Lead oxide (PbO₂):

• $\mathrm{P}\mathrm{b}\mathrm{O}{\phantom{A}}_2+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{4\mathrm{H}\mathrm{C}\mathrm{l}}⟶\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}{\phantom{A}}_4+2\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{P}\mathrm{b}\mathrm{O}{\phantom{A}}_2+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}+2\mathrm{H}{\phantom{A}}_2\mathrm{O}⟶\mathrm{N}\mathrm{a}{\phantom{A}}_2[\mathrm{P}\mathrm{b}(\mathrm{O}\mathrm{H}){\phantom{A}}_6]$

Aluminium oxide (Al₂O₃):

• $\mathrm{A}\mathrm{l}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_3+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{6\mathrm{H}\mathrm{C}\mathrm{l}}⟶2\mathrm{A}\mathrm{l}\mathrm{C}\mathrm{l}{\phantom{A}}_3+3\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{A}\mathrm{l}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_3+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}+3\mathrm{H}{\phantom{A}}_2\mathrm{O}⟶2\mathrm{N}\mathrm{a}[\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}){\phantom{A}}_4]$ (hydrated sodium aluminate)

Stannous oxide (SnO):

• $\mathrm{S}\mathrm{n}\mathrm{O}+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{2\mathrm{H}\mathrm{C}\mathrm{l}}\stackrel{-\rightharpoonup }{\leftharpoondown -}\mathrm{S}\mathrm{n}\mathrm{C}\mathrm{l}{\phantom{A}}_2+\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{S}\mathrm{n}\mathrm{O}+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}+\mathrm{H}{\phantom{A}}_2\mathrm{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}\mathrm{N}\mathrm{a}{\phantom{A}}_4[\mathrm{S}\mathrm{n}(\mathrm{O}\mathrm{H}){\phantom{A}}_6]$

Stannic oxide (SnO₂):

• $\mathrm{S}\mathrm{n}\mathrm{O}{\phantom{A}}_2+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{4\mathrm{H}\mathrm{C}\mathrm{l}}\stackrel{-\rightharpoonup }{\leftharpoondown -}\mathrm{S}\mathrm{n}\mathrm{C}\mathrm{l}{\phantom{A}}_4+2\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{S}\mathrm{n}\mathrm{O}{\phantom{A}}_2+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}+2\mathrm{H}{\phantom{A}}_2\mathrm{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}\mathrm{N}\mathrm{a}{\phantom{A}}_4[\mathrm{S}\mathrm{n}(\mathrm{O}\mathrm{H}){\phantom{A}}_8]$

Vanadium dioxide (VO₂):

• $\mathrm{V}\mathrm{O}{\phantom{A}}_2+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{2\mathrm{H}\mathrm{C}\mathrm{l}}⟶\mathrm{V}\mathrm{O}\mathrm{C}\mathrm{l}{\phantom{A}}_2+\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $4\mathrm{V}\mathrm{O}{\phantom{A}}_2+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}⟶\mathrm{N}\mathrm{a}{\phantom{A}}_2\mathrm{V}{\phantom{A}}_4\mathrm{O}{\phantom{A}}_9+\mathrm{H}{\phantom{A}}_2\mathrm{O}$

Some other elements which form amphoteric oxides are gallium, indium, scandium, titanium, zirconium, chromium, iron, cobalt, copper, silver, gold, germanium, antimony, bismuth, beryllium, and tellurium.

Hydroxides

Aluminium hydroxide is also amphoteric:

• $\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}){\phantom{A}}_3+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{3\mathrm{H}\mathrm{C}\mathrm{l}}⟶\mathrm{A}\mathrm{l}\mathrm{C}\mathrm{l}{\phantom{A}}_3+3\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}){\phantom{A}}_3+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}⟶\mathrm{N}\mathrm{a}[\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}){\phantom{A}}_4]$

Beryllium hydroxide:

• $\mathrm{B}\mathrm{e}(\mathrm{O}\mathrm{H}){\phantom{A}}_2+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{2\mathrm{H}\mathrm{C}\mathrm{l}}⟶\mathrm{B}\mathrm{e}\mathrm{C}\mathrm{l}{\phantom{A}}_2+2\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{B}\mathrm{e}(\mathrm{O}\mathrm{H}){\phantom{A}}_2+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}⟶\mathrm{N}\mathrm{a}{\phantom{A}}_2[\mathrm{B}\mathrm{e}(\mathrm{O}\mathrm{H}){\phantom{A}}_4]$

Chromium hydroxide:

• $\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H}){\phantom{A}}_3+\stackrel{\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}{3\mathrm{H}\mathrm{C}\mathrm{l}}⟶\mathrm{C}\mathrm{r}\mathrm{C}\mathrm{l}{\phantom{A}}_3+3\mathrm{H}{\phantom{A}}_2\mathrm{O}$
• $\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H}){\phantom{A}}_3+\stackrel{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}}{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}⟶\mathrm{N}\mathrm{a}[\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H}){\phantom{A}}_4]$

See also

• Ate complex
• Isoelectric point
• Zwitterion