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The Determination of Quinone Groups in Humic Substances¹

ABSTRACT

The reduction of quinone groups in humic substances by acid SnCl₂ solution was found to be unsatisfactory. The experimental conditions are drastic, the systems are heterogeneous, favoring adsorption or complexing of the reducing agent by the humic materials, and so leading to excessively high quinone values.

Reduction of humic substances by SnCl₂ and Fe²⁺-triethanolamine in alkaline solution at room temperature and atmospheric pressure appeared to be more suitable for the determination of quinone groups in these materials. There was good agreement between quinone groups determined by the two methods on seven humic and one fulvic acid. The quinone group content of the humic acids analyzed ranged from 1.4 to 2.2 meq/g, that of the fulvic acid was < 1.0 meq/g.

No correlation was found between quinone groups and the free radical content of the humic materials. The results suggest that quinone groups in humic substances form part of complex chemical structures and not of simple compounds.

The determination of quinone groups in humic substances is difficult because of the presence of substantial amounts of CO₂H, OH, and ketonic C=0 groups, some or all of which may interfere. Thus, it is advisable to check each analysis by at least two different methods.

¹ Contribution no. 406. Soil Research Institute, Canada Dep. of Agr., Ottawa, Ontario.

² Senior Research Scientist and visiting Scientist from the Agr. Chemistry Institute, Univ. of Pisa, Pisa, Italy, respectively.

Received for publication January 26, 1972. Accepted for publication May 9, 1972.