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Effects of pH and Neutral Electrolyte Concentration on Free Radicals in Humic Substances¹

ABSTRACT

Electron spin resonance (ESR) investigations on humic substances were carried out at different pH's and at different neutral salt concentrations, two of the main factors controlling chemical and biological reactions in soils. With increase in neutral electrolyte concentration, free radical concentrations of humic materials decreased. With both fulvic acid (FA) and humic acid (HA), spin contents increased after raising the pH from the near neutral to the alkaline range, because of the stabilization of semiquinone radicals. With FA, a minimum spin concentration region was observed from pH 5.0 to 6.5. Potentiometric titrations revealed that activities of both electron donor and electron acceptor groups in humic molecules were at a minimum around that pH range. Between pH 5.0 and 6.5, a range into which many agricultural soils fall, humic materials appear to be chemically least reactive, which may explain, at least in part, their stability in soils.

¹ Contribution no. 1147 from Chemistry and Biology Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6, Canada.

² Visiting Research Scientist (National Research Council of Canada) from the Dep. of Agric. Chemistry and Soil Science, Univ. of Calcutta, Calcutta 700 019, India, and Program Leader, Soil Chemistry and Biology Program, respectively.

Received for publication January 21, 1980. Accepted for publication May 21, 1980.