Acids and Bases

An acid is a proton donor, a base is a proton acceptor. Ionization of an acid yields its conjugate base, and the two are termed a conjugate acid–base pair, for example acetic acid (CH₃COOH) and acetate (CH₃COO^-). The pK of an acid is the pH at which it is half dissociated. The Henderson–Hasselbach equation expresses the relationship between pH, pK and the ratio of acid to base, and can be used to calculate these values. From a titration curve of an acid the inflexion point indicates the pK value. An acid–base conjugate pair, such as acetic acid and acetate, is able to resist changes in the pH of a solution. That is, it can act as a buffer. The 20 standard amino acids have two acid–base groups: the α-amino and α-carboxyl groups attached to the C_α atom. Those amino acids with an ionizable side-chain have an additional acid–base group.