Application of thermometric titrations to the study of soil organic matter. II. Humic acids

The technique of thermometric titrimetry was applied to the study of humic acid and its interaction with metal ions. The nature of the thermometric titration curve of humic acid with sodium hydroxide is consistent with the presence in humic acid of many classes of ionizable groups. La³⁺ and Cu²⁺ reacted endothermically with sodium humate and in each case 1 g organic matter reacted with approximately 1.7 mequiv. metal ion. The end point of these titrations was characterized by an abrupt “bump”, attributed to conformational changes in the macromolecular network of the organic matter accompanying precipitation of the metal-humate complexes. The thermometric method is also used to determine chloride in the presence of humic acid or sodium humate.