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Structural Arrangement of Functional Groups in Soil Humic Acid as Revealed by Infrared Analyses¹

ABSTRACT

Humic acid from a brunizem soil was subjected to a series of treatments (methylation, saponification, acetylation) to determine the number and arrangement of functional groups when analyzed by a combination of chemical and infrared techniques. One-third of the COOH groups (total COOH, 390 meq/100 g) occurred in positions close enough to form cyclic anhydrides; two-thirds of the OH groups (total OH, 290 meq/100 g) were phenolic. Some phenolic OH groups in the samples failed to methylate wth diazomethane. Potassium bromide catalyzed the formation of cyclic anhydrides by mild thermal treatment of acid-treated and methylated-saponified humic acids, but not of untreated humic acid. Acetylation gave a small amount of brown material (acetyl content, 20%) whose infrared pattern was characteristic of an acetylated carbohydrate. Tests for quinone and H-bonded phenolic OH groups were negative.

¹ Contribution from the Department of Soils, Missouri Agr. Exp. Sta., Columbia, and the Department of Agronomy, Illinois Agr. Exp. Sta., Urbana. Missouri Journal Series No. 2739. Approved by the Directors. The research was supported in part by Regional Research Funds under North Central Cooperative Regional Project NC-55 on Soil Organic Matter. Presented before Div. S-3, Soil Sci. Soc. Amer., Denver, Colo., November 19, 1963.

² Assistant Professor of Soils, University of Missouri, Columbia, and Professor of Soil Chemistry, University of Illinois, Urbana.

Received for publication May 6, 1964. Accepted for publication June 17, 1964.