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Fulvic Acid Ultraviolet-Visible Spectra: Influence of Solvent and pH

Baes, A.U., Bloom, P. R.
Soil Science Society of America journal 1990 v.54 no.5 pp. 1248-1254
Histosols, Mollisols, Spodosols, absorbance, absorptivity, electron transfer, ethanol, fulvic acids, light scattering, milk, pH, quinones, sodium bicarbonate, solvents, wavelengths
The ultraviolet-visible spectra of three fulvic acids from a Mollisol (FA1), a Histosol (FA2), and a Spodosol (FA3) were determined using four solvents: 0.05 M NaHCO₃, distilled water, 95% (v/v) ethanol, and absolute ethanol. Light scattering was evaluated by varying the distance of the sample cell from the detector. This test, which gave positive results for a dilute milk sample, showed no light scattering effects for the fulvic acids. The ratios of 465 to 665 nm absorbances (E₄/E₆ ratios) determined in 0.05 M NaHCO₃ were 16.7 for FA1, 16.8 for FA2, and 18.5 for FA3. The ultraviolet spectra in water showed only slight inflections for FA1 and FA3, but the FA2 spectrum had a pronounced shoulder centered at about 280 nm. In ethanol, the shoulder was shifted towards the red and absorbance decreased. Similar red shifts were shown to occur in 2,3-; 2,4-; and 2,6-dihydroxybenzoic acids (DHB). The absorbance of bands in the range of 26 to 290 nm obeyed the Beer-Lambert law. Estimates of molar absorptivities ranged from 2300 to 4500 L mol⁻¹ cm⁻¹, which is within the range of molar absorptivities of the electron transfer (ET) bands of carboxyphenols. Shifts in the position of the shoulders with pH, in both water and 95% ethanol, resembled that of carboxyphenols. Absorbance at visible wavelengths was not consistent with the behavior of simple quinones but might be due to chromophores with extended conjugation, possibly polyaromatic structures. Another possibility considered was intramolecular electron donor-acceptor complexes. Scientific Journal Series, Paper no. 16599, Minnesota Agric. Exp. Stn.