pKa values of three quinolone compounds (ciprofloxacin. norfloxacin and enoxacin) in methanol-water (MeOH-water) and acetonitrile-water (ACN-water) mixtures were determined in potentiometric measurements Standardization of the electrode system in MeOH-water and ACN-water mixtures was carried out potentiometrically by Gran's method The obtained results were compared to the literature pKa values determined by various techniques and to the values predicted by the SPARC on-line pK(a) calculator