Figure 14-2 The fraction of local anesthetic in the protonated, cationic form in aqueous solution at physiologic pH (7.4) as a function of the pK_a of the drug. The drug with the lowest pK_a , lidocaine, has the smallest fraction of its molecules protonated and the largest in the neutral form, and vice versa for the local anesthetic with the highest pK_a , chloroprocaine. Individual drug molecules become protonated and deprotonated in thousandths of a second in solution.