Quinolone Derivative\nPhysicochemical Studies: Phase\nEquilibria in Water and Alcoholic Solutions, Ionization Constants,\nand Calorimetric Analyses

The studies were conducted on commonly used or withdrawn\nfrom the\nmarket active ingredients of drugs used in the treatment of many bacterial\ninfections. In this research, solid–liquid phase equilibria\nwere determined in binary systems. The substances tested were the\nfollowing quinolone derivatives: cinoxacin, ciprofloxacin, flumequine,\nlomefloxacin hydrochloride, norfloxacin, and ofloxacin. Water, ethanol,\nand 1-octanol were used as solvents. Phase equilibria were determined\nby using dynamic and spectrophotometric methods. The results were\ncorrelated using the Wilson, NRTL, and UNIQUAC equations. Calorimetric\nanalyses were performed by using differential scanning calorimetry\nto determine the phase transitions of pure substances. Ionization\nconstants were determined spectrophotometrically at 298.2 and 310.2\nK using the Bates–Schwarzenbach method. Quinolone derivatives\nexhibited high melting points above 490 K. The substances showed the\nlowest solubility in water but were higher by an order of mole fraction\nin alcohols, in which solubility was comparable. These quantities\nwere below 1000th of mole fraction. Ionization constants corresponding\nto the deprotonation of the quinolone carboxyl group and also their\nzwitterionic form were determined.