EFFECTS OF REACTION-TIME, CHEMICAL-REDUCTION, AND OXIDATION ON ESR INAQUEOUS-SOLUTIONS OF HUMIC ACIDS

Allophane-free and -adsorbed humic acids were extracted from Ando soil with aqueous ammonia, and three kinds of fractions were sequentially isolated with 90% aqueous acetonitrile, 90% aqueous dioxane, and aqueo us ammonia from the respective humic acids. The electron spin resonanc e spectra of each fraction was measured at pH 13.0 under argon atmosph ere in the presence of various concentrations of Na2S2O4 or K3[Fe(CN)6 ]. Spin concentrations of all the fractions increased as time elapsed after dissolution under argon. When air was bubbled into the solutions for 5 min, the concentrations were decreased by the oxidation with ox ygen molecules. However, in time, the original signals were recovered. The spin concentrations were increased by the reduction with a small amount of dithionite, but decreased on increasing the concentration of dithionite. The spin concentrations were decreased by the oxidation w ith hexacyanoferrate(III), and new signals were observed with a large amount of hexacyanoferrate(III). When the amount of hexacyanoferrate(I II) was not enough to oxidize all the electron donor moieties in the h umic acids, the original signals were recovered over time. The data su ggest that intramolecular or intermolecular electron transfer occurs i n humic acids.