Weak base

An uncharged drug that can accept a proton and become positively-charged. Weak bases dissociate incompletely at relatively neutral pH values and thus an equilibrium exists between uncharged and charged drug. B + H⁺ ⇋ BH⁺. If you add H⁺ to this equilibrium (i.e. lower the pH) then more of the drug will exist as charged BH⁺. 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 base, the proportion of uncharged drug decreases from 50% and moves towards 0%.