An interpretation of the conductometric titration curve of humic acid

Methods of fractionating the functional groups of humic acid by conductometric titration were examined. Replacement titration, in which a sample solution is titrated with a standard acid solution after adding an excess amount of a standard base, seems to be a suitable method. By this method, the functional groups of humic acid were fractionated into three clusters of different dissociation constants, designated as VW (very weak), W (weak), and S (strong) clusters. The VW cluster was presumed to include phenolic hydroxyl residues, carboxyl residues and nitrogenous bases of very high pK's. The W cluster was presumed to include various aliphatic and aromatic carboxyl residues and nitrogenous bases of medium pK's, whereas the S cluster was presumed to include carboxyl residues of low pK₁ of salicylate or phthalate. A humic acid sample obtained from the A horizon of Shitara “kuroboku” soil had 0.78 me/g of the VW cluster, 3.24 me/g of the W cluster and 1.90 me/g of the S cluster.