Electrophoresis of podzol soil humic acids

Podzol soil humic acids prepared by a gentle procedure were studied by electrophoresis in the pH range 3 to 7 at two ionic strengths; the positive counter ion concentrations related to the negatively charged buffers were 0.035 M and 0.1 M Na+. Phosphate, acetate, and citrate buffers were used. In the pH range studied, humic acids moved to the positive pole, indicating that the particles in solution are negatively charged. The distances moved were substantially larger for 0.035 M Na+ as compared to those, of 0.1 M Na+. When these latter data, in normalized form, were plotted as a function of pH, they compared well with proton dissociation data calculated for the pK-values 4.2 and 5.85, obtained via an acid-base titration. This indicates that the pH dependent net charge of humic acids can be essentially described by the dissociation of two groups forming negatively charged anionic groups, most likely carboxylate groups. (C) 1998 Elsevier Science Ltd.