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Hydrogen Ion Affinity in Humic Acids: A Coordination Approach

Potentiometric titrations of humic acids (HA) with 0.250 M Ba(OH)2/BaCl2 titrant (0.750 M constant ionic strength), have been performed at 25 ± 0.1°C with a calibrated glass electrode for measuring p[H+]. The pK′w for water in this BaCl2 medium is 12.899 ± 0.006 and 13.712 ± 0.006 in 0.500 M NaCl. Divalent Ba2+ cations force the ionization of the acid groups and improve solubility. Under such conditions derivative potentiometric curves show diverse equivalent peaks of the acidic sites of HA. The presence of 0.05% of a neutral detergent such as Triton X-100 is essential for effective dispersion of HA in the working solutions and to obtain very stable potentiometric measurements. Computer programs were used in the treatment of the potentiometric data in order to solve a number of simultaneous equations to obtain overall conditional β i H formation constants, which come from a coordination model of hydrogen ions to the organic matrix and permit calculation of conditional pK data. Conductimetric titrations with Ba(OH)2 or NaOH give the total acidity. A typical result in BaCl2 medium for a peat HA presents seven acid groups with the following pK data: pK 1 = 3.80 ± 0.3, pK 2 = 4.67 ± 0.02, pK 3 = 7.57 ± 0.01, pK 4 = 8.190 ± 0.005, pK 5 = 8.80 ± 0.01, pK 6 = 8.91 ± 0.02, and pK 7 = 8.93 ± 0.01.