Evaluation of pKa Values of Soil Humic Acids and their Complexation Properties

A study has been undertaken to determine the acidic, and metal complexing properties of humic acid extracted from tea garden soils of different depths. The dissociation constant of humic acid has been evaluated from the pH titration curves. The pKa values obtained in this study almost agree well with the data reported for humic acids from natural sources. The presence of carboxylic and/or hydroxyl groups in the humic acids, under study, have been confirmed. Metal complexation properties of the extracted humic acids have been studied with three metal ions (Zn2+, Ni2+, and Cu2+). The stability constants of the metal humic acid complexes have also been determined. One pKa value for humic acid extracted from the surface soil (pKa = 2.50 and the 30 cm depth soil (pKa = 5.2) has been obtained whereas two pKa values (3.1 and 7.5) for humic acid from the 15 cm depth soil have been found. These acids contain carboxylic and/or hydroxylic acid groups. Stability constants of metal-humic acid complexes (log K) do not differ much for humic acids from the surface soil (1.92, 2.09 and 2.07) and the 15 cm depth soil (2.01, 2.31 and 1.72) for Zn2+, Cu2+ and Ni2+ respectively. This work will contribute some knowledge to the nature of organic matter in tea garden soil.