Potentiometric measurement of stability constants of complexes between fulvic acid carboxylate and Fe3+

The stability constants of complexes formed between iron (III) and fulvic acid extracted from organic manures and wastes such as urban domestic sewage sludge, farmyard manure, poultry manure and sulfitation pressmud were investigated by the potentiometric titration method in an ionic medium of 0.1 M KNO3 at 25 +/- 1 degrees C. A modification of the Katchalsky's model was employed for the estimation of stability constants. The displacement of the titration curves due to presence of Fe3+ in FA solutions formed the basis of calculations. The weak acidic property of fulvic acids due to carboxyl groups resulted in buffering over a wide range of pH; fulvic acids were completely neutralized in the pH range of 7.00-8.85. Apparent dissociation constants (pK(APP)) of weakly acidic carboxyl groups were a direct function of degree of dissociation (alpha(L)) in the mid-range of titration curves but were non-linear at high and low alpha(L) values. The stability constants for formation of Fe-FA complexes (log beta(Fe)) calculated from the titration data were in the range of 5.64-7.55, depending upon alpha(L) and electrostatic properties of fulvic acids. The relatively high stability constants of Fe-FA complexes in comparison to those with other competing cations suggest that the Fe-FA complexes are relatively stable in a soil environment.