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An Infrared and Chemical Investigation of the Acid Tin Chloride Method of Determining Quinones in Humus¹

ABSTRACT

Observations of infrared spectral changes accompanying an acid SnCl₂ treatment of soil humic preparations and a known quinone confirmed the effectiveness of SnCl₂-HCl for reducing quinones. The treatment did not reduce the nonquinoid but conjugated carbonyls of p-hydroxy-benzaldehyde, benzophenone, o-benzoylbenzoic acid, chalcone, and curcumin. Recent criticisms of the technique were examined. It was proposed that due to the diversity in carbonyl frequencies, the variety of quinone groups and their derivatives, perhaps no single species of quinoid carbonyl was present in amounts adequate enough to be represented by a distinct peak in the spectra of unmodified humic compounds. Instead, there may have been several small peaks, all masked by the broad absorption bands due to other chromophores in the 1600- and 1700-cm^-1 regions. Spectra of blends of fulvic acids (FA) with certain quinones, and naphtharesorcinol, supported the above concept. The 1600-cm^-1 region of the FA spectra may have represented aromatic –C = C as well as quinones H-bonded to -OH groups. In an indirect manner, this study furnished additional evidence for the presence of quinones in soil humus.

¹ Contribution no. 399 from the Soil Research Institute, Canada Dep. of Agr., Ottawa, Ontario, Canada K1A OC6.

² Soil Biochemist. The author thanks Mr. R. B. Sanderson for his skillful technical assistance, Dr. H. Morita for the valuable discussions held with him, and Dr. C. Steelink of the University of Arizona for the EPR measurements made by him.

Received for publication November 11, 1971. Accepted for publication April 7, 1972.