Influence of pH and of several organic acids on the interaction between esculetine and iron(III)

The availability of iron to plants is influenced by organic molecules of low molecular weight with complexing or reducing capacity such as organic acids and phenolic acids and their derivates. To provide information about the interactions between Fe(III) and esculetine (ESC) and the influence of organic acids on these interactions, the redox activity of ESC at different pH values in aqueous solution was investigated in the presence and in the absence of citric, malic, oxalic and pyruvic acid on systems with different Fe(III)/ESC molar ratios. At pH<4.0, all the systems showed the highest redox activity. At pH>=4, the redox activity decreased and was negligible at pH 6.0. This trend was attributed to the formation of hydrolyzed Fe(III) species, which interact with ESC by forming soluble complexes that subsequently transform into insoluble Fe(III)-ESC polymers. The malic, citric and oxalic acid compete with ESC in the Fe(III) co-ordination and form soluble complexes with the metal ion thus impeding its precipitation. The organic acids were found to compete with ESC in the iron co-ordination according to the following affinity order: citric > oxalic >malic >>pyruvic acid. © 2002 Elsevier B.V. All rights reserved.