Norfloxacin (NX) again nearly doubled CF permeability, but the change of CF partition coefficient depends on the method of addition. At pH 7.4, NX is primarily a zwitterion, but since both the basic and acidic pKa is close to the used pH (7.4), there is a non-negligible amount of both anionic and cationic form. An approximate ratio of the three forms is zwitterion : anion : cation = 89 : 7 : 4. The anion can play the same role in increasing CF permeability as in the case of ASC described above. The difference in the partition coefficient for both ways of addition remains unclear.