Carboxylic Content of Humic Acid Determined by Modeling, Calcium Acetate, and Precipitation Methods

The key to understanding proton equilibria and cation binding on the surface of humic acid is the quantification of the total carboxylic acid contents. Such quantification via titration alone is highly ambiguous and not straightforward due to the complexity and interference of noncarboxylic acids (phenolic acid, thiols, etc.). To gain a greater understanding of such analysis, potentiometric titration data was incorporated into modeling approaches, and they were compared with two other methods: conventional Ca(OAc)₂ exchange and the precipitation titration method. Each of these three methods was used to analyze six different humic samples. The humic samples were collected from various sources, such as upland, paddy fields, sediments, and peat. The carboxylic acid contents ranged from 263 to 487 cmol(c) kg⁻¹. Furthermore, the carboxylic acid content in the humic acid from the Namwon series showed significantly higher values. This was attributed to the relatively higher percentage of organic C in the mother material. Two modeling approaches using titration data were performed. The first was a simple electrostatic model with limited functional group heterogeneity (Model A). The second was our implemented version of Model V with four carboxylic acid group sites and four phenolic hydroxyl group sites. The relative difference in total carboxylic acid contents between the Ca(OAc)₂ method and Model A prediction ranged from 0.21 to 7.43%, and that between the Ca(OAc)₂ method and Model V prediction was 1.90 to 35.71%. The predicted carboxylic acid content from Model A was more comparable to the results of the Ca(OAc)₂ method and the cetyltrimethylammonium method than the Model V prediction. The modeling approaches with the simple ligand model showed a better prediction of the total carboxylic acid content from the potentiometric titration data.