Ronald L. Malcolm, 1972. "Comparison of Conditional Stability Constants of North Carolina Humic and Fulvic Acids with Co(II) and Fe(III)", Environmental Framework of Coastal Plain Estuaries, Bruce W. Nelson
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Both humic and fulvic acid fractions of soil and sediment organic matter form stable water-soluble complexes and perhaps chelates with ions of Co(II) and Fe (III). The conditional stability constants (log10 K values) for these complexes were determined at a constant ionic strength of 1.0 in KC1 by the resin exchange procedure of Schubert using radioisotopic methods. The conditional stability constants were 4.0 (pH 4.5) and 6.6 (pH 6.0) for fulvic acid—Co(II) complexes; 6.0 (pH 4.5) and 8.3 (pH 6.0) for humic acid—Co(II) complexes; 5.4 (pH 4.5) and 5.6 (pH 6.0) for fulvic acid–Fe(III) complexes; and 6.8 (pH 4.5) and 7.2 (pH 6.0) for humic acid—Fe(III) complexes. Humic acid-metal complexes may be as important as fulvic acid-metal complexes in geochemical transport of certain metal ions because (l) they form water-soluble complexes with metal ions, (2) they are more abundant in nature than fulvic acids, and (3) they form more stable metal-organic complexes than corresponding fulvic acid-metal complexes.
Fulvic acids form stable complexes with Co(II) in marine and simulated estuarine waters with conditional stability constants of 4.9 and 5.3, respectively; therefore, the formation of metal-organic complexes may be an important factor in considering natural processes in estuarine and marine environments.