Discontinuous spectrocolorimetric titration method for determining stability constants of fulvic acid–iron complexes

Abstract

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 pK_INT, pK_m, 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.