Weak acid

An uncharged drug that can donate a proton and become negatively-charged. Weak acids dissociate incompletely at relatively neutral pH values and thus an equilibrium exists between uncharged and charged drug. AH ⇋ A^– + H⁺. If you add H⁺ to this equilibrium (i.e. lower the pH) then more of the drug will exist as uncharged AH. The pH at which the drug is 50% charged is the drug’s pK_a. So as you lower the pH away from the pK_a of a weak acid, the proportion of uncharged drug increases from 50% and moves towards 100%.