Amphoteric

In chemistry, an amphoteric substance is one that can react as either an acid or a base. The word derives from the Greek prefix amphi- (αμφu-), which means 'both'. Many metals (such as zinc, tin, lead, aluminum, and beryllium) and most metalloids have amphoteric oxides or hydroxides.

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

Amphoteric oxides and hydroxides

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.

Aluminum hydroxide is like this:

• Base (neutralizing an acid): Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O
• Acid (neutralizing a base): Al(OH)₃ + NaOH → Na[Al(OH)₄]4.

Other examples include:

• Beryllium hydroxide
  • with acid: Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O
  • based: 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)₆]^2-

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

Other elements that form amphoteric oxides are: Si, Ti, V, Fe, Co, Ge, Zr,Cr, Ag, Sn, Au

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, acting as either 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 hydrogen atoms. Furthermore, since they can act as either an acid or a base, they are amphoteric.

Examples

A common example of an amphiprotic substance is the hydrogencarbonate (or bicarbonate) ion, which can act as a base:

HCO₃^- + H₂O H₂CO₃ + OH^-

or as a lubric acid:

HCO₃^- + H₂O CO₃^2- + H₃O⁺

Therefore, you can accept or donate a proton.

Water is the most common example, acting as a base by reacting with an acid such as hydrochloric acid:

H₂O + HCl H₃O⁺ + Cl^-,

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

H₂O + NH₃ NH₄⁺ + OH^-

Substances classified as amphoteric have the particularity that the electrical charge of the hydrophilic part balances depending on the pH of the medium. They act as acids in acidic media and as bases in basic media, to counteract the pH of the medium. Amphoteric surfactants have a positive charge in strongly acidic environments, have a negative charge in strongly basic environments, and in neutral media have a hybrid intermediate form (mixed ion).

By applying a current flow in the medium where they are, they will move towards the positive charge if they act as acids (and therefore negatively) or towards the negative charge if they act as bases (and therefore positively). If they are in the form of a mixed ion they will remain immobile. This process is called electrophoresis.