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Humic acid concentration could not be determined using the acid-base precipitation-gravity method of Kousen Bunsekihou Shishin (Standard Methods of Analysis for Mineral Springs, Japan) for spring water from central Hokkaido, Japan, although the precipitate of humic acid formed. This reason was that the precipitate passed through a membrane filter with a pore size of 0.45 μm while washing with water. We examined the modification of standard method and also surveyed the characteristics of humic acid. The detection of humic acid was possible when washing the membrane filter with a solution of 2 % HCl. The carbon concentration of humic acid, estimated by the difference between organic carbon concentrations before and after acid-base assays, revealed that the weight of humic acid appears to be underestimated by the acid-base precipitation-gravity method. The influences of washing media on precipitation on the membrane filter as well as the pore size of membrane filters were surveyed to elucidate the best method for the determination of humic acid concentrations. The results show that for filters with a pore size of 0.6 μm or less and filters with a pore size of 0.2 μm, the loss of precipitate should be reduced when washing with 2 % HCl solution and with Milli-Q water, respectively. In addition, the carbon concentrations of humic and fulvic acid were determined via DAX-8 resin analyses. These concentrations comprised 59.4 % of whole organic concentrations as determined by the DAX-8 resin analysis. This value as well as carbon concentrations of humic acid, estimated from the difference between carbon concentrations before and after acid-base precipitation, showed that carbon concentrations of fulvic acid were higher than those of humic acid. However, the proportion of humic acid was higher than those that have been previously reported in the literature. The molecular size distribution of humic acid fractionated by ultrafiltration also showed that humic acids above 25 kDa in molecular weight dominated the total humic acids. The characteristics for the proportion of humic acid concentration to whole humus concentration and the molecular size distribution of humic acid in the spring water tested were different from those of surface waters.

Analysis and characteristics of humic acid in spring water from central Hokkaido, Japan.