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 aci dic group contents (C-A1) and the average values of apparent pK (pK(ap p)) were evaluated by applying a modified Henderson-Hasselbalch equati on to the experimental titration curves. The average values of pK(app) of the fractionated and unfractionated HAs were about 4.1-4.3, and th e distribution of pK(app) values could be represented by the relations hips 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 aromat icity of the HA.