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Viscosity Measurements on Soil Humic Substances¹

ABSTRACT

Viscosity measurements were done on a humic acid and a fulvic acid to obtain information on particle shapes and dimensions, particle weights and polyelectroytic behavior in aqueous solutions. The pH of the humic acid solutions ranged from 7.0 to 10.5, that of the fulvic acid solutions from 1.0 to 10.0.

Humic acid at pH 7.0 and fulvic acid at pH 1.0 and 1.5 behaved like uncharged polymers. At higher pH levels, both humic and fulvic acids exhibited strong polyelectrolytic characteristics. The viscosity data fitted an equation developed by Fuoss and Strauss for linear, flexible polyelectrolytes.

In the case of fulvic acid, it was possible to assess the effect of a wide range of pH values. At very low pH, fulvic acid had the highest particle weight and particle volume. With increase in pH, the two parameters decreased to a minimum at pH 3, and then began to increase moderately. An analysis of the data for particle shapes and dimensions showed that the most likely particle configuration was rods. Viscometry offers an almost unique opportunity to study important structural characteristics of humic substances in aqueous solutions. Molecular modifications associated with drying, heating, and exposure to high vacuum can so be avoided.

¹ Contribution no. 596. Soil Research Instit., Agriculture Canada, Ottawa, Ont. KIA 0C6.

² Visiting Scientist from the Dep. of Soil and Water, Faculty of Agriculture, Hebrew Univ., Rehovot, Israel, and Principal Research Scientist, respectively.

Received for publication May 10, 1976. Accepted for publication August 13, 1976.

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