Kinetics and mechanism of autocatalytic oxidation of Fe(phen)2+3 and Fe(bpy)2+3 by nitric acid

Abstract

The kinetics of the title reactions has been studied and the investigation of the oxidation of the iron(II) aqua ion by nitric acid has been extended. All three reactions are autocatalytic in nature. Oxidation of Fe(aq)²⁺ is practically complete at [HNO₃] > 0.5 mol dm^–3. In the case of the two other complexes the process is reversible; the conversion in the equilibrium state depends on the initial concentrations of reactants. Values of the equilibrium constants have been determined from experimental data and compared to those calculated from standard electrochemical potentials. The autocatalytic S-shaped kinetic curves have been measured by means of spectrophotometry and potentiometry and are characterised using the time coordinate (t_i) and the slope (r_max) of the concentration vs. time curves at the inflection point. The general mechanism of autocatalytic nitric acid oxidations has been applied in order to fit single experimental curves and all concentration dependences. The validity of the proposed mechanism is supported by the agreement between experiments and the results of the numerical integration of differential rate equations derived from the mechanism. An interesting local maximum and minimum are found in the t_ivs.[Fe²⁺]₀ curves both experimentally and numerically.