Fulvic Acid Metal Complexes

The interactions between metallic cations and fulvic acid fraction (F.A.) of an alluvial soil from West Bengal (Chinsura sub-soil) were studied by electrometric titrations and absorption spectroscopy. Conductometric studies show one type of complex with trivalent metallic cations, viz., Fe³⁺ and Al³⁺ and two to three different types of complexes with bhalent metal ions, viz, Cu²⁺+, Zn²⁺+ and Ni³⁺. Potemiometric titrations in presence of metallic cations indicate the formation of hydroxylated F.A. metal complex where the metal ions form electrovalent bond with the negative carboxylic group of the ligand. The stabilities of these complexes follow the order of Fe³⁺ < Al³⁺ for trivalent ions. For bivalent metal ions, the order is Ni²⁺ < Zn ²⁺ < Cu²⁺ at pH < 7.0 and Cu²⁺ < Ni²⁺ at pH > 8 for F. A. low metal complexes. In case of F. A. high metal complexes, the order is Zn²⁺ < Ni²⁺ < Cu²⁺ at pH < 6.8 and Ni²⁺ < Zn²⁺ < Cu²⁺ at pH > 6.8. These results were confirmed from absorption spectroscopy.