Acid-base characterization of molecular weight fractionated humic acid

Acid-base properties of molecular weight fractionated humic acids (HAs) were investigated by the acid-base potentiometric titration. The acidic group contents (C-A1) and the average values of apparent pK (pK(app)) were evaluated by applying a modified Henderson-Hasselbalch equation to the experimental titration curves. The average values of pK(app) of the fractionated and unfractionated HAs were about 4.1-4.3, and the distribution of pK(app) values could be represented by the relationships between alpha and pK(app) plots in the range 2-8. The C-A1 values increased with a decrease in molecular size, as did the aromaticity. This suggests that the acidic group contents are related to the aromaticity of the HA.