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Fluorescence Excitation Spectra and Viscosity Behavior of a Fulvic Acid and its Copper and Iron Complexes¹

ABSTRACT

Fluorescence excitation spectra of fulvic acid (FA) and its Cu and Fe complexes showed that fluorophore groups participated in metal complexation with a concomitant reduction in fluorescence intensities. The two characteristic bands at 360 and 465 nm were affected by metal complexation but in addition, the 360-nm band shifted gradually toward longer wavelengths as more metal was complexed. Viscosity measurements indicated decreases in the molecular flexibility of FA with increasing metal complexing. This rigidity or strain in the structure was ascribed to metal complexing with carboxyl and phenolic hydroxyl groups and so bringing the macromolecular segments closer to each other. Viscosity-average molecular weights (_v) also increased as more metal was complexed possibly because of the formation of metal bridges between FA molecules. The two effects mentioned above were more prominent at pH 6.0 than at 4.0. The flexibility of FA molecules at pH 6.0 is greater than at pH 4.0, because of a reduction in intramolecular H-bonding at the higher pH, which allows the FA to interact with metal ions more favorably.

¹ Contribution no. 1154 from the Chemistry and Biology Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario, K1A 0C6, Canada.

² Visiting Research Scientist (National Research Council Canada) from the Department of Agricultural Chemistry and Soil Science, University of Calcutta, Calcutta 700 019, India, and Program Leader, Soil Chemistry and Biology Program, respectively.

Received for publication February 21, 1980. Accepted for publication September 17, 1980.

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