Self-association of pyridine-2,6-dicarboxylic acid in aqueous solution as determined from ultraviolet hypochromic and...

The self-association of pyridine-2,6-dicarboxylic acid (dipicolinic acid), was studied in aqueous solution at the pH values of 0.2, 3.5, 6.9 and 13.0, by ultraviolet spectroscopy. The variation in molar absorptivity with the concentration of this compound was measured for the band in the mid-ultraviolet region. Deviations from Beer-Lambert law with increasing concentration were found at the pH values of 0.2, 3.5 and 6.9. Hypochromic effects were detected at pH 0.2 and 6.9, while a hyperchromic effect was observed at pH 3.5. These results were interpreted in terms of self-association leading to the formation of dimers. From the fitting of the experimental curves of hypochromic or hyperchromic effects, self-association constants were calculated. The occurrence of hypochromic and hyperchromic effects was analyzed in terms of vertical and horizontal interactions, respectively, to elucidate the nature of the self-association processes, stacking or hydrogen bonding, relevant to each of the protolytic species of dipicolinic acid in aqueous solution.