POTENTIOMETRIC titration was used for obtaining detailed ……information on the acid-base properties of humic acids. The studied humic acids were isolated from different four sources, i.e., alluvial soil (HAs), farmyard manure (HAf), compost (HAc) and poudrette (HAp). The isolated humic acids were purified and performed. Back titration using 0.1M HCl as the titrant and direct titration using 0.1M NaOH as the titrant of humic acid solutions (2 mg/ml) at the ionic strength of 0.1 were executed. Moreover, total acidity and functional groups of humic acids were estimated.
The studied humic acids were similar in their acid-base properties: i) the data of direct and back titration curves which limited the position and composition of dissociation or ionization of functional groups of the studied humic acids were more similar, ii) total functional groups of HA determined from back and direct potentiometric titration was ranged between 5.40-6.54 and 4.20-5.00 mmolc/g HA, respectively, iii) total functional groups of humic acids determined using potentiometric titration were varied from those determined using normal titration methods, where total functional groups at normal titration methods ranged between 11.32 and 19.58 mmolc/g HA and iv) humic acid extracted from poudrette has the highest value of total acidity followed by compost, farmyard manure, while the soil came late.