pKa

Every weak acid or base has a personal tipping-point pH where it sits exactly half charged and half neutral. That pH is its pKa. Think of it as the drug's own thermostat setting for ionisation: when the surrounding pH equals the pKa, the ionised and unionised forms are present in equal amounts.

Knowing the pKa lets you predict the drug's state anywhere. When the local pH moves one unit away from the pKa, the balance shifts to roughly ninety percent one form and ten percent the other; two units away, about ninety-nine to one. Combined with whether the drug is an acid or a base, the pKa tells a formulator how soluble and how absorbable the molecule will be at the pH of the stomach, the intestine, the blood or a formulation buffer.

A practical caution: many drugs have more than one ionisable group and therefore more than one pKa, and reported values can shift slightly with temperature and ionic strength. The pKa is a robust guide, not an exact constant, and it should be quoted alongside the conditions under which it was measured.