Solubilities of Naringin and Naringenin in Different Solvents and Dissociation Constants of Naringenin

The solubilities of naringin and naringenin in methanol, isopropanol, ethyl acetate, petroleum ether, n-butanol, and hexane were measured by using UV spectrophotometric methods at the maximum absorption wavelength of (283 and 372) nm from (288.15 to 328.15) K. The solubilities of naringin and naringenin in different solvents increase with an increase of temperature. The solubility of naringin in the six solvents was in the order methanol > ethyl acetate > n-butanol > isopropanol > petroleum ether > hexane, and the solubility of naringenin in these solvents follows the order ethyl acetate > isopropanol > methanol > n-butanol > petroleum ether > hexane. The solubility of naringenin was significantly higher than naringin in all selected solvents because of the more similar polarity of naringenin with solvents. The solubilities of naringin and naringenin are calculated by using the modified Apelblat equation model, ideal model, and the ?H equation model. From solubility data, the changes of dissolution enthalpy, entropy, and the free Gibbs energy of naringin and naringenin in six solvents were also calculated using the vant Hoff equation. The course of naringin and naringenin dissolving in the selected solvents in the experimental temperature range was endothermic. The dissociation constants (pKa) of naringenin were determined at 298.2 K by ultraviolet (UV) spectroscopy method. The pKa1 and pKa2 values of naringenin are 7.05 +/- 0.06 and 8.84 +/- 0.08.