buffer

A buffer is an excipient that keeps a formulation's acidity (its pH) steady, like a shock absorber for acid and base. Add a little acid or base to plain water and the pH lurches; add the same amount to a buffered solution and it barely moves. The buffer soaks up stray hydrogen ions so the medicine stays at the pH where it is most stable, even as small amounts of acid or base appear from degradation, the container, or dissolved gases.

A buffer is a pair: a weak acid and its matching base form (for example, acetic acid with acetate, or one of the phosphate or histidine forms). When acid is added the base half mops it up; when base is added the acid half neutralizes it. This teamwork is described by the Henderson-Hasselbalch equation, and a buffer works best within about one pH unit of its characteristic pKa, which is why the buffer is chosen to match the target pH of the product.

pH control is one of the most important formulation decisions because pH governs a drug's stability, solubility, and — for a protein — its very fold. The wrong pH speeds hydrolysis or oxidation, can precipitate the drug, or can unfold and aggregate a protein. So nearly every injectable, eye drop, and protein product contains a buffer holding it at a deliberately chosen pH, often confirmed through stability studies.

An honest caveat: a buffer is not free of side effects. It contributes to the solution's tonicity and ionic strength, some buffers (such as certain phosphates) can interact badly with freezing by shifting pH as they crystallize, and too strong a buffer can be irritating on injection. The choice of buffer species, pH, and concentration is therefore a deliberate balance, not an afterthought.