Oxygenation of Fe(II) in the presence of citrate in aqueous solutions at pH 6.0-8.0 and 25 degrees C: interpretation from an Fe(II)/citrate speciation perspective

The kinetics of Fe(II) oxidation in the presence of various citrate concentrations have been investigated in aqueous solutions over the pH range 6.0-8.0 using colorimetry and speciation modeling. Oxidation of Fe(II) was interpreted and quantitatively modeled in terms of oxidation of various Fe(II)-citrate species. Using the model, it is possible to predict whether the presence of citrate would dominate the Fe(II) oxidation and thus enhance/retard the oxidation rate of Fe(II) and vice versa. The study also supports the presence of other Fe(II)-citrate species rather than just the monomeric species at circumneutral pH. At low pH and in a system where complexation of Fe(II) by citrate is dominant, oxidation of Fe(II) is controlled by the oxidation of both Fecit- and Fecit24-. As the pH increases, the oxidation of Fe(OH)cit25- becomes increasingly important and dominates the oxidation of Fe(II) at pH 8.0. Rate constants for the oxidation of all five suggested Fe(II)-citrate species have been estimated and may be used to predict the rate of Fe(II) oxidation at any combination of pH and citrate concentration.