Henderson-Hasselbalch equation

The Henderson-Hasselbalch equation is the small piece of arithmetic that turns the idea of ionisation into a real number. Give it the drug's pKa and the pH of the surroundings, and it tells you the ratio of ionised to unionised drug present. It is the calculator behind the rule that pH near pKa means a roughly fifty-fifty split.

In its common form, pH equals pKa plus the logarithm of the ratio of the deprotonated form to the protonated form. The practical upshot is easy to remember: for a weak acid, raising the pH above the pKa pushes the equilibrium toward the ionised (charged) form; for a weak base, lowering the pH below the pKa does the same. Each whole pH unit away from the pKa shifts the ratio by a factor of ten.

Formulators use it constantly to choose buffers, predict solubility at a target pH, and estimate the fraction of drug available to cross membranes. It is reliable for dilute solutions of single ionisable groups; for concentrated solutions, multiple ionisable sites, or species that self-associate, it becomes an approximation that needs refinement.