The stability constants for the complexes formed between iron species existing
in ambient soil environment and fulvic acids (FA) extracted from organic
wastes like sewage sludge, farm yard manure (FYM), poultry manure, and press
mud were determined in aqueous media of pH 5·0 and 8·5 by
discontinuous spectrocolorimetric titration method. The values of stability
constant (log K) of Fe–FA complexes estimated at pH 5·0 were
6·026, 6·212, 6·270, and 6·342 for FYM, sludge,
poultry manure, and press mud, respectively. The respective values at pH
8·5 were 6·145, 6·276, 6·350, and 6·940.
The order of the values of log K for different preparations of fulvic acids
was press mud > poultry manure > sludge > FYM. The functional group
contents, their pH of neutralisation, and electrostatic properties of the FA
such as pKINT, pKm, and
0·868 nW, were found to be the determining factors for maximum binding
ability of FA for metal cations and the stability constant of Fe–FA for
different FA preparations. The basic assumptions and the limitations of the
discontinuous spectrocolorimetric estimation of stability constants for
Fe–FA are discussed.