Amphoteric

In chemistry, an amphoteric substance is one that can react as either an acid or base.^[1] The word is derived from the Greek word amphoteroi (ἀμφότεροι) meaning "both". Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloids have amphoteric oxides or hydroxides.

Another class of amphoteric substances are amphiprotic molecules which can either donate or accept a proton. Examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water and ammonia.

Amphoteric oxides and hydroxides^[2]

Zinc oxide (ZnO) reacts differently depending on the pH of the solution:

In acids: ZnO + 2H⁺ → Zn²⁺ + H₂O

In bases: ZnO + H₂O + 2OH^- → [Zn(OH)₄]^2-

This effect can be used to separate different cations, such as zinc from manganese.

Aluminium hydroxide is as well:

Base (neutralizing an acid): Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

Acid (neutralizing a base): Al(OH)₃ + NaOH → Na[Al(OH)₄]

Some other examples include:

• Beryllium hydroxide
  • with Acid: Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O
  • with Base: Be(OH)₂ + 2NaOH → Na₂Be(OH)₄

• Lead oxide
  • with acid: PbO + 2HCl → PbCl₂ + H₂O
  • with base: PbO + Ca(OH)₂ +H₂O → Ca²⁺[Pb(OH)₄]^2-

• Zinc oxide
  • with acid: ZnO + 2HCl → ZnCl₂ + H₂O
  • with base: ZnO + 2NaOH + H₂O → Na₂²⁺[Zn(OH)₄]^2-

Some other elements which form amphoteric oxides: Si, Ti, V, Fe, Co, Ge, Zr, Ag, Sn, Au^[3]

Amphiprotic molecules

According to the Br��nsted-Lowry theory of acids and bases: acids are proton donors and bases are proton acceptors.^[4] An amphiprotic molecule (or ion) can either donate or accept a proton, thus acting either as an acid or a base. Water, amino acids, hydrogen carbonate ions and hydrogen sulfate ions 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

A common example of an amphiprotic substance is the hydrogen carbonate ion, which can act as a base:

HCO₃^- + H₂O → H₂CO₃ + OH^-

or as an acid:

HCO₃^- + H₂O → CO₃^2- + H₃O⁺

Thus, it can effectively accept or donate a proton.

Water is the most common example, acting as a base when reacting with an acid such as hydrogen chloride

H₂O + HCl → H₃O⁺ + Cl^-,

and acting as an acid when reacting with a base such as ammonia:

H₂O + NH₃ → NH₄⁺ + OH^-

References

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