ABSTRACT
Drugs are organic compounds, and as a result, their activity, their solubility in plasma, and their distribution to various tissues are all dependent on their physicochemical properties. According to the Bronsted–Lowry theory, an acid is a substance, charged or uncharged, that is capable of donating a proton, and a base is a substance, charged or uncharged, that is capable of accepting a proton. Water can act as both a weak base and a weak acid, which might be another reason for the deviations. According to the pH partition hypothesis, the un-ionized form of a weakly basic or a weakly acidic drug partitions into the lipid bilayer of the membrane. Thus, it is important to be familiar with an expression that relates the percent or fraction of the drug in the ionized and the un-ionized state because of the regional differences in the pH of the gastrointestinal tract. This equation is called the Henderson–Hasselbalch equation.
