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Electron Spin Resonance Spectrometry of Humic Substances¹

ABSTRACT

Concentrations of free radicals, g-values, and line widths were determined for humic acids, fulvic acids, and humins originating from soils of widely differing geographical origins and pedological histories and for humic acids synthesized by fungi in the laboratory. Free radical content decreased in the following order: humins > humic acids > fulvic acids. Line widths decreased in the order: fulvic acids > humins > humic acids, while the g-values were more or less constant at 2.0030 ± 0.0003. Methylation of humic and fulvic acids greatly increased the free radical content, but decreased g-values and line widths. The spectrometric characteristics of the fungal humic acids were of the same order as those of fulvic acids. Plots of spin concentration vs. % H and of spin concentration vs. % O showed inverse and direct statistically significant correlation between the parameters, respectively. This is interpreted as an indication that the free radicals were formed via oxidative removal of H from -OH groups. Statistically significant correlations between spin concentration and absorbance at 465 mµ and between spin concentration and atomic C/H ratios were taken as evidence that the free radical content of humic substances is related to their dark color and increases as the molecular complexity increases.

¹ Contribution no. 386, Soil Research Institute, Canada Dept. of Agr., Ottawa, Ont.

² Visiting Scientist from the Agricultural Chemistry Inst., University of Pisa, Pisa, Italy, and Senior Research Scientist, respectively.

Received for publication July 27, 1971. Accepted for publication September 27, 1971.

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