Kinetics and Equilibrium of Binding of Fe3+ by a Fulvic Acid: A Study by Stopped Flow Methods

Abstract

The first report of a rate of binding of a metal ion (Fe³⁺) by a soluble fulvic acid is derived from stopped flow measurements. The rate of complex formation is normal in Wilkins' sense and similar to that for sulfosalicylic acid. Dissociation is slow (t_1/2 > 10 s). The binding of Fe³⁺ by the fulvic acid in acid solution, pH = 1–2.5, was investigated by kinetic analysis in which the reaction of free Fe³⁺ with sulfosalicylic acid was followed by stopped flow spectrophotometry on a time scale short compared to release of Fe³⁺ by fulvic acid. Conditional equilibrium constants found were 1.5 ± 0.3 × 10⁴ at pH = 1.5 and 2.5, and 2.8 ± 0.3 × 10³ at pH = 1.0 at 25 °C (ionic strength 0.1).