Dissociation Constants and Solubilities of Dalbergin and Nordalbergin in Different Solvents

The dissociation constants (pK(a)) of dalbergin and nordalbergin were measured at 298.2 K using ultraviolet (UV) spectroscopy method. The solubilities of dalbergin and nordalbergin in water, methanol, propanone, ethyl ethanoate, trichloromethane, and hexane have been determined by the UV spectrophotometric method from 283.2 to 308.2 K at atmospheric pressure. The experimental solubility values were correlated with a modified Apelblat equation, lambda h model, and ideal model. The pKa value of dalbergin is 8.91 +/- 0.12, and the pK(a1) and pK(a2) values of nordalbergin are 7.31 +/- 0.29 and 9.79 +/- 0.39, respectively. The solubilities of dalbergin and nordalbergin in six solvents increase with an increase in temperature. The solubility order of dalbergin in six pure solvents was trichloromethane > propanone > ethyl ethanoate > methanol > hexane > water, whereas that of nordalbergin was propanone > methanol > ethyl ethanoate > trichloromethane > water > hexane. A comparison of the solubility of dalbergin with that of nordalbergin shows that the 7-methoxyl moiety of dalbergin leads to a significantly higher solubility in trichloromethane and hexane, a little higher solubility in ethyl ethanoate, an approximately equal solubility in propanone, and a little lower solubility in methanol and water (exception 283.2 K). For dalbergin, the modified Apelblat equation shows the best correlation for all solvents, and for nordalbergin, correlation results by three models are similar.