Abstract
Acid-base properties of molecular weight fractionated humic acids (HAs) were investigated by the acid-base potentiometric titration. The acidic group contents ( C A t ) and the average values of apparent p K (p K app) were evaluated by applying a modified Henderson-Hasselbalch equation to the experimental titration curves. The average values of p K app of the fractionated and unfractionated HAs were about 4.1–4.4, and the distribution of p K app values could be represented by the relationships between α and p K app plots in the range 2–8. The C A t 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.
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