Mobility and ionization constant of basic drugs in methanol. Application of nonaqueous background electrolytes for capillary zone electrophoresis based on a conventional pH scale

The ionization constants of nine basic drugs (ethylmorphine, thebaine, code ine, levorphanol, dihydrocodeine, cocaine, alprenolol. propranolol and meth amphetamine) in methanol were determined from the dependence of their mobil ities on the pH* of the background electrolyte (BGE), based on a convention al pH* scale. Mobilities in the BGEs with acetate as counter ion were seemi ngly too low, an effect that was related to ion pair formation of the catio nic analytes with acetate. After numerical correction of this effect the ap plication of the Henderson-Hasselbalch equation enabled the calculation of the pK(a)* values, which were found to be between 9.3 and 11.2. These pK(a) * values are only 1 to 2.5 pK units higher than in pure water, which is in marked contrast to the changes observable for neutral acids, (C) 2001 John Wiley & Sons, Inc.