Uv and Visible Absorption Spectroscopic Investigations in Relation to Macromolecular Characteristics of Humic Substances

Summary Humic substances were investigated by both UV and visible absorption spectroscopic methods with special emphasis on their macromolecular characteristics. The spectral ranges examined were 200–350 nm and 350–700 nm. Conclusions were drawn on the basis of absorption intensities in optical density (O.D.) vs. wavelength (λ) curves, slopes of log O.D. vs. log λ plots, nature of shoulders in the 250–270 nm region, and E 4 /E 6 ratios. As macromolecular characteristics are expected to be dependent on the medium, studies were carried out at different pH values as well as at different neutral salt concentrations, the two major parameters influencing the soil environment. It was noted that with increasing neutral electrolyte concentrations, the particle size decreased, probably due to the coiling of the macromolecule. On increasing the pH from 6.5 to 9.5, the particle size increased slightly, possibly because of partial association through homolytic bonding. At pH 3.5 (with FA only), the degree of association was greater than that at pH 9.5. However, here association occurred by a different mechanism, that of intermolecular hydrogen bonding. At pH 2.0 (with FA only), association was markedly pronounced because of the very dominant role played by intermolecular hydrogen bonding.