Dimethylformamide , humic acid

Controlled electrolytic reduction of humic acid and hydroxyquinones in dimethylformamide in our laboratory (27) have shown small initial decreases in spin content followed by an increase up to the first half-wave potential. Since this was carried out in the absence of base, it would correspond to the portion of the curve in Figure 7 after the first maximum. The probable series of competing reactions is illustrated in Figure 8. Austen and co-workers (1) have observed similar behavior in coal extracts that were electrolytically reduced. 

Figure 1 gives a typical infrared spectrum of lake humic acid. The interpretation of infrared spectra of humic substances is discussed in depth by -MacCarthy and Rice in Chapter 21. The similarity of infrared spectra of humic acids from different lakes suggests a similarity of the aspects of chemical structure that are related to their infrared absorptions. However, infrared spectroscopy is not sensitive enough to uncover minor structural differences among humic acids. In fact, humic acids were separated by organic solvents (chloroform, methylethylketone, methanol, dimethylformamide) into various fractions (Ishiwatari, 1969b, 1973). Infrared spectra of two of these fractions, the chloroform-soluble fraction and the methylethylketone-... 

Sears, P. G., Wolford, R. K., and Dawson, L. R. (1956). Conductances of some acids, bromides, and picrates in dimethylformamide at 25°C. J. Electrochem. Soc. 103, 633-636. Senesi, N. (1981). Free radicals in electron donor-acceptor reactions between a soil humic acid and photosynthesis inhibitor herbicides. Z. Pflanzenernahr Dung. Bodenkd. 144, 580-586. Senesi, N., Chen, Y., and Schnitzer, M. (1977a). Hyperfine splitting in electron spin resonance spectra of fulvic acid. Soil Biol. Biochem. 9, 371-372. 

Except for extracts with anhydrous EDA, data in Table 4 were obtained for humic substances isolated from an air-dried H+-exchanged humic histosol soil. For extractions with pyridine, A,A-dimethylformamide (DMF), di-methylsulfoxide (DMSO), and sulfolane, soils (60 g) were thoroughly mixed with the appropriate solvent (250 cm ). After centrifugation the residues were repeatedly extracted with water until the supernatants were only faintly colored. Supernatants for each of the solvent systems were combined and the pH values of the solutions were adjusted to 1.0 using 5M hydrochloric acid. Humic and fulvic acids were separated by centrifugation. 

Nonaqueous titrations have been used to estimate total acidity (e.g., Wright and Schnitzer, 1959). This approach utilizes an aprotic solvent such as pyridine or dimethylformamide, in which a very strong base such as ethoxide ion can be used to react with the acidic functional groups of the humic substance sample. The comparison of nonaqueous total acidity and aqueous total acidity should be tempered with caution and any efforts to subdivide nonaqueous total acidity into functional group classes by analogy with pKa values of acidic groups in aqueous solution are theoretically unfounded. Some reasons for concern are presented in the next paragraph. 

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