Behavior of Antibacterial Ofloxacin; Hydronation Constants and Solubility in Aqueous Solutions of Sodium Chloride at Different Temperatures

The solubility and acid–base properties of the antibacterial Ofloxacin were studied in aqueous solutions of sodium chloride at a single ionic strength value (I = 0.15 mol·dm−3) and different temperatures. Hydronation constants were investigated at T = 288.15, 298.15, 310.15 and 318.15 K by potentiometric and UV-spectrophotometric titrations. From the distribution of the different hydronated/dehydronated Ofloxacin species, it was observed that at physiological pH, the neutral HL0 (zwitterion) species has a mole fraction percentage of ~ 85%. A lowering trend of the hydronation constants with increasing T/K was observed and the standard enthalpy, entropy and the Gibbs energy of hydronation were also calculated. The solubility (ST), investigated at T = 288.15, 295.15, 310.15 K, was determined by UV-spectrophotometric measurements by using the calibration straight line method. Absorbance values of the standard Ofloxacin solutions and of the saturated solutions were measured at two wavelengths (λ = 287 nm and 331.5 nm), and the concentration of Ofloxacin was calculated from the slope of the straight lines. The concentration of Ofloxacin neutral species (S0) was calculated from simple mass balance equations, by using the free hydrogen ion concentration of the saturated solutions and hydronation constants determined at the same ionic strength and temperature of the solubility measurements. From the experimental results, it was possible to observe a different effect of the temperature on solubility and hydronation constants. In the first case, the solubility increases almost linearly with the increase of the temperature, while the hydronation constants decrease by about 0.2 logarithmic units passing from T = 288.15 to 318.15 K. By using a Van’t Hoff equation as modified by Apelblat, the standard enthalpy, entropy and the Gibbs energy of solution were calculated by the slope and the intercept of the straight lines obtained by fitting ln x against 1/T, where x is the solubility of Ofloxacin expressed as mole fraction. By using an approach reported in the literature, calculated hydronation constants were obtained at different ionic strengths (up to 1.0 mol⋅dm−3) and at the same temperatures experimentally investigated. Successively, by means of a Debye–Hückel type equation, the corresponding hydronation constants at infinite dilution and the parameters for the dependence on the ionic strength were calculated, as well as the enthalpy change values of hydronation at infinite dilution. A very good agreement was obtained from the comparison of the hydronation constants and solubility of Ofloxacin here determined and those reported in pure water in the literature.