Analysis of the green fraction of humic acids

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Abstract
Summary: The green fraction of humic acids (HAs), Pg, was fractionated by gel chromatography on Sephadex G‐50. Repeated chromatography of the crude Pg obtained by the first chromatography of HA yielded a sharp peak (G1) and two broad bands (G2) of purified Pg. The recovery of Pg in G1 and G2 as percentages of the total Pg content of the HA were 16.2 and 14.5%, respectively. Distinct peaks were detected at 615, 570, 450 and 280 nm in the UV‐visible spectra of both G1 and G2 in alkali solutions. Estimates of weight‐average molecular weight were 9.7 * 103 for G1 and 1.23 * 104 for G2. In the 1H‐NMR spectra the percentage of hydroxyl groups of 4,9‐dihydroxy‐perylene‐3, 10‐quinone (DHPQ) nuclei, which are known to be the chromophore of Pg, was 4.1 and 4.5% of the total H, respectively. Based on these values and the H/C ratios, DHPQ‐C was estimated as 28% of total C. 13C‐NMR spectra obtained using inverse‐gated decoupling yielded 40–41% aromatic C, which suggested the presence of aromatic rings other than DHPQ. All of the IR, 13C‐ and 1H‐NMR spectra indicated that the two purified Pg samples largely differed from each other in their contents of alkyl and polysaccharide components.