R.L. Wershaw
D.J. Pinckney
1973
<p>A procedure has been devised for the fractionation of humic acid samples from different environments. This procedure involves fractionation of the sample by adsorption chromatography on a Sephadex G-50 column followed by chromatography on either a G-25 or a G-100 column. The fractions of the solutions are then examined by small angle X-ray scattering. Three different types of behavior have been detected among the humic acid fractions: (1) Some fractions show very little change in aggregation at pH values above 3.5. (2) One fraction forms aggregates at pH values above and below pH 7, but at pH 7 it is completely dissociated. (3) In some fractions the degree of aggregation decreases with increasing pH. However, even at pH values as high as 11.5 some large particles are still present. These differences in<br />association behavior are due to the interaction of different attractive and repulsive forces. In many aggregating systems only one type of<br />attractive force is dominant; however in humic acid systems hydrogen bonding, π bonding between planar aromatic moieties, and other<br />coulombic interactions apparently all play a role in the formation of molecular aggregates.</p>
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U.S. Geological Survey
Determination of the association and dissociation of humic acid fractions by small angle X-ray scattering
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