Amphoteric Species

An amphoteric, or amphiprotic, species is one that can act either as an acid or a base, depending on
its chemical environment. In the Brønsted-Lowry sense, an amphoteric species can either gain or
lose a proton. Water is the most common example. When water reacts with a base, it behaves as an
acid:

H 2 O   +   B−  HB  +   OH−

When water reacts with an acid, it behaves as a base:

HA  +   H 2 O       H 3 O+  +   A−

The partially dissociated conjugate base of a polyprotic acid is usually amphoteric (e.g., HSO 4 − can
either gain an H+ to form H 2 SO 4 , or lose an H+ to form SO 4 2–). The hydroxides of certain metals, such
as Al, Zn, Pb, and Cr, are also amphoteric.

THINGS TO REMEMBER

Arrhenius   Definition

Brønsted-Lowry  Definition

Lewis   Definition

Nomenclature    of  Acids

Hydrogen    Ion Equilibria  (pH and pOH)

Strong  Acids   and Bases

Weak    Acids   and Bases

Conjugate   Acids   and Bases

Applications    of  Ka  and Kb

Polyvalence and Normality