Determination of molecular weights of humic acids by osmotic pressure measurement and by permeation chromatography on controlled pore glass

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Abstract
Number-average molecular weights (M̄ n) of humic acids (HAs) were determined by osmometry and by permeation chromatography on controlled pore glass with a pore size of 500 A. M̄ n value of dialyzed HAs tended to decrease in the order of Rp(2)>Rp(l)>P0 type, and increase in the order of P0M̄ n value of non-dialyzed HAs did not show any remarkable difference among the HA types, which was considered to be due to the predominance in the number of species with lower molecular weight (< 10,000) in HAs. M̄ n value calculated from the permeation chromatograms showed a similar trend to that of M̄ n obtained by osmometry, but the values differed depending on the detection methods. Ultraviolet absorption gave a smaller value for M̄ n while the differential refractive index gave a larger value for M̄ n than that by osmometry. It was shown that both the decrease and increase in the molecular weight take place during the humiflcation process of HAs, where the decrease was considered to be associated with the decomposition of partially decayed bio-polymers and the increase with the formation of a dark colored high polymer intrinsic to soils.

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