Chemical studies on soil humic acids

Abstract

The relationship between the ultraviolet and visible absorbance and the pH value of solution of each 3 samples of A, B, and Rp type humic acids and the Pg fraction of a P type humic acid was investigated. The absorbance of humic acids decreased stepwise with decreasing the pH value. The pK _a values of dissociative groups, which were deduced from the inflection points of the absorbance-pH curves, were distributed discontinuously over a very wide range from 3.5 to 13 in the humic acids with low degree of humification. The absorbance-pH curves of humic acids with high degree of humification were smoother, especially, in the pH region lower than 7, suggesting that the distribution of pK _a values are more continuous. This may be due to the electrostatic interaction among dissociative groups which is intensified with increasing content of dissociative groups. The pattern of dissociation of phenOlic hydroxyl groups in the Pg pigments was similar to that of polyelectrolytes. Therefore, Pg pigments were presumed to exist as polymers in humic acids.