In this study, protonation equilibria for three amphoteric quinolone antibacterials, including grepafloxacin (GPFX), HSR-903 and ciprofloxacin (CPFX), were evaluated and compared. Then, pH-dependent behavior in both microspecies distribution and molecular electrical charge was described in detail. The UV spectrophotometry was employed to determine dissociation constants, with macro-dissociation constants describing integral acid-base property and micro-dissociation constants defining proton-binding ability ...