OSCILLATING, NONEQUILIBRIUM REDOX PROPERTIES OF HUMIC ACIDS

Humic acids in solution behave as a kinetic system that is characteriz ed by steady-states rather than by equilibria. This is shown by redox measurements at 0.1 M NaCl and 24 degrees C under carefully controlled anaerobic conditions. When the humic acids were titrated with HCl fro m pH 6.4 to 3, the redox potential, after relaxation to a steady-state , showed a linear pH dependence with a slope of -42 mV/pH unit. (A sup plementary study in a 0.1 M NaClO4 medium, using humic acids prepared from another source, agrees within the limits of experimental errors o f +/-1 mV giving a slope of -44 mV/pH unit). In each titration point t here was an initial. relatively fast relaxation with a tau-value withi n the range 150-400 s, followed by further slow relaxation (tau > 2.5 h) and/or oscillations, followed by arrival at a steady-state. This no nequilibrium behaviour implies the presence of polyphenolic residues, such as resorcinol. within the humic acid particles and their free rad icals. In a redox diagram, the pH dependence of the apparent redox pot ential of humic acids is compared with the redox potential of various other redox pairs present in soil. Copyright (C) 1997 Elsevier Science Ltd.