The term amphiprotic in modern acid-base chemistry is the replacement for the older term amphoteric. An amphiprotic substance is a substance which can act both as an acid and as a base, participating in two acid-base equilibria, because it contains at least one proton which can be given up and at least one site at which a proton can be acquired.
Most polyprotic acids have at least one amphiprotic ion.
Using phosphoric acid as an example, the monohydrogen phosphate ion and the dihydrogen phosphate ion are both amphiprotic while phosphoric acid itself can only be an acid and the phosphate ion can only be a base. Organic compounds which contain both a carboxylic acid group and an amine group on the same molecule are called amino acids. Many amino acids are essential to living organisms and will be discussed later in this course. When an amino acid such as glycine, H 2 NCH 2 COOH, is dissolved in water. This produces an ionic structure with opposite charges on both ends, a zwitterion .