Buffer capacity of humic acid: thermodynamic approach.

Commercial humic acid was dialyzed and characterized by infrared, UV/vis spectroscopy, (13)C NMR spectrometry, thermogravimetry, and elemental analysis. The dialyzed humic acid was titrated with HNO(3) and NaOH in order to obtain the buffer capacity value (beta). The humic acid presented buffer behavior by base and acid addition, and moreover, an excellent buffer capacity by addition of NaOH. Humic acid showed buffer action between pH 5.5 and 8.0, and a maximum buffer capacity at pH 6.0. The same study was followed calorimetrically to determinate the enthalpy of interaction between H(+)/OH(-) and buffer, which resulted in a maximum enthalpy of -38.49 kJ mol(-1) at pH 6.0. This value suggests that the buffer activity is based on chemisorption of proton and hydroxyl.