Abstract

Annals of Environmental Science
Volume 3, June 2009, Pages 131-138
www.aes.northeastern.edu, ISSN 1939-2621

Spectrophotometric Analysis of Fulvic Acid Solutions - A Second Look

Elham A. Ghabbour and Geoffrey Davies, Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA 02115, USA

Received April 7, 2009; in final form June 8, 2009; Accepted June 11, 2009.

This paper re-evaluates and modifies a previously proposed, simple method of estimating the concentration of aqueous fulvic acid solutions. Spectrophotometric measurements of aqueous solutions of three International Humic Substances Society (IHSS) standard fulvic acids (FAs) at pH 1.0 and pH 4.0 in the wavelength range 350-500 nm at 20^oC indicate that a) the optical absorbances in this wavelength range decrease exponentially with increasing wave-length; b) the spectrum of each standard FA is independent of pH in the range 1.0 to 4.0; c) plots of absorbance vs. FA concentration at fixed wavelength are linear up to at least 135 mg FA/L; d) the absorption coefficients at each wavelength derived from the linear plots cover a small range for the three standard FAs and average to 5.3(0.3), 3.4(0.3), 1.9(0.2) and 0.89(0.10) Lcm^-1g^-1 at 350, 370, 400 and 450 nm, respectively. The averages at 350, 370 and 400 nm are close to the actual absorption coefficients for IHSS standard FA 1S101F and are substantially lower than the absorption coefficients of IHSS standard FA 2S103F employed in the previous work; and e) substitution of the absorption coefficients of this study in the previously observed linear correlation of measured carbon concentrations and spectrophotometrically estimated FA concentrations results in a predicted average carbon content of 52% for five commercial FA samples. This estimate is similar to the average measured % C values of the standard FAs used to develop this simple analytical method, which has potential value for the certification and regulation of humic substances