ABSTRACT
Key Notes
pH is a measure of the concentration of H+in a solution. 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.
An acid–base conjugate pair can act as a buffer, resisting changes in pH. From a titration curve of an acid the inflexion point indicates the pK value. The buffering capacity of the acid–base pair is the pK 1 pH unit. In biological fluids the phosphate and carbonate ions act as buffers. Amino acids, proteins, nucleic acids and lipids also have some buffering capacity. In the laboratory other compounds, such as TRIS, are used to buffer solutions at the appropriate pH.
The α-amino and α-carboxyl groups on amino acids act as acid–base groups, donating or accepting a proton as the pH is altered. At low pH, both groups are fully protonated, but as the pH is increased first the carboxyl group and then the amino group loses a hydrogen ion. For the standard 20 amino acids, the pK is in the range 1.8–2.9 for the α-carboxyl group and 8.8–10.8 for the α-amino group. Those amino acids with an ionizable side-chain have an additional acid–base group with a distinctive pK.
Related topic
Amino acids (B1)
