Amphoteric Compounds and Ions

There are a number of compounds and ions that can act as either acids or bases. The term for such things is amphoteric. (The “am” prefix means “either” and is used in words like ambidextrous - someone who can use either hand to write and do other things, and amphibian - animals that spend some time on land and some in the water.)

Amphoteric Compounds

In order to be an acid, a compound must have an H to give away. Generally, in chemistry classes, any H’s are at the beginning of the formula. In order to be a base, a compound must have a lone pair of electrons that can bond with an H+1 ion (Lewis’s definition). So, compounds that start with an H and have an O or N atom (with lone pairs) are the most common examples in chemistry classes. In biology classes, the formulas are a little more cryptic, but generally they are compounds that have a COOH group (that’s a carbon double bonded to ONE oxygen and single bonded to an O-H) and also an amine group (that’s an N atom with 2 H’s and a lone pair). These are called amino acids and an example is shown here:

Amphoteric Ions

It is easy to spot amphoteric ions in chemistry class. These are ions that start with an H (which can be given away) and end with a negative charge (that can attract to an H+1 ion). Basically, the list is all of the “bi” ions, like bisulfate (HSO4-1) or biphosphate (HPO4-2).

How will an amphoteric compound or ion behave?

Whether an amphoteric compound or ion will act as an acid or a base is determined primarily by what it is with. Anything that is amphoteric will react as an acid if it is placed with a strong base and will act as a base if mixed with a strong acid. When mixed with a weak acid or base, only a mathematical comparison of the relevant Ka and Kb values will tell.