Chemical studies on soil humic acids

Abstract

The pH-dependent nature of the ultraviolet and visible absorption spectra of 39 humic acids (HAs) was investigated. Two kinds of difference absorbance, △E(pH 12.4-7) and JE(pH 7-3.5), were compared for these HAs and related substances, and their characteristics are discussed. The wavelengths at the peaks of △E(pH 12.4-7) were common to Rp type HAs and the milled wood lignin of rice straw, but not to A type HAs. The △E(pH 12.4-7) values of HAs at 600 run had no linear correlation with the phenol contents but these were approximately proportional to the absorbance of HAs at 600 run. This result suggests that the △E(pH 12.4-7) of HAs at 600 run does not depend on the phenolic content but on the amount of conjugate systems in HAs. The 4E(pH 7-3.5) of HAs at 285 and 360 nm increased linearly with increasinll absorbance at 285 and 360 nm. The chemical structures in HAs responsible for the change in absorbance with change in pH from 3.5 to 7 was presumed to increase with increasing degree of humification.