Modification of the Spectral Absorbance Difference Method for Determining the Dissociation Constants of Polyprotic Acids with Delocalized π-Systems

Polyprotic organic acids with suitable molecular geometry and acidic properties are frequently employed in the development of novel nano- and macroscale materials. These acids can bind to inorganic surfaces in a bridging mode, effectively facilitating junctions at organic–inorganic interfaces. This study was motivated by the potential of triprotic 2-hydroxyisophthalic (tenacic) acid and its derivatives to fulfill this role. The absorbance spectra of the unsubstituted tenacic and 4-nitrotenacic acids were found to be strongly pH-dependent. This behavior is attributed to varying degrees of electron delocalization in the π-systems of the conjugate bases. The previously reported spectral absorbance difference method was applied in this work without modification to determine the first dissociation constants of both acids. To determine the second dissociation constants for both acids and the third for 4-nitrotenacic acid, we modified the method to improve the accuracy of the measured K_b of their conjugate bases. Utility of this method may be extended to other compounds that exhibit pH-dependent variations in π-electron delocalization.