Determination of pKa values by capillary zone electrophoresis with a dynamic coating procedure

CZE allows to measure the acidic dissociation constant (pK(a)) of many drug substances. However, determining the EOF intensity may be time-consuming, especially at a low pH. In order to overcome this drawback, a dynamic coating procedure of the capillary was carried out to increase mu(EOF), and thus to reduce the analysis time. In addition, this coating procedure enhanced migration time stability. The effective mobilities of 15 compounds were measured at different pH, producing pKa values dependent on BGE ionic strength. The latter values were corrected with the activity coefficient to obtain a true pK(a), value. The 15 investigated compounds were (i) five acids: namely, salicylic acid, benzoic acid, ketoprofen, phenobarbital, and phenol, (ii) four bases: lidocaine, propafenone, propranolol, and quinine, (iii), five ampholytes: sulfanilamide, sulfabenzamide, sulfadimethoxine, sulfadoxine, and sulfisoxazole, and (iv) one zwitterion: cetirizine. The range of determined pK(a), values was between 1.2 and 11.2, and close to the pK(a), values available from the literature.