Determination of the Dissociation Constants of Urocanic Acid Isomers in Aqueous Solutions

Abstract

Summary. (E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation constants (pKa1 and pKa2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation (pKa1 ¼ 3.43 � 0.12 and pKa2 ¼ 5.80 � 0.04) and (pKa1 ¼ 2.7 � 0.3 and pKa2 ¼ 6.65 � 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride solutions up to the isotonic salt concentration (0.154 mol dm � 3 ), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at temperature 25 � Ca nd for (E)urocanic acid also at 37 � C. The obtained pKa2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived. The obtained pKa1 and pKa2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature.