Abstract: | 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 °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 (pKAPP) of weakly acidic carboxyl groups were a direct function of degree of dissociation (αL) in the mid-range of titration curves but were non-linear at high and low αL values. The stability constants for formation of Fe–FA complexes (log βFe) calculated from the titration data were in the range of 5.64–7.55, depending upon αL 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.
This revised version was published online in June 2006 with corrections to the Cover Date. |