A Kinetics Study of the Oxidation of Iron(II) by Nitric Acid

Article abstract of DOI:10.1021/ja00531a015

The kinetics of the ferrous-nitrate clock reaction have been studied by spectrophotometric and potentiometric techniques at high acid concentration (0.6-2.0 M), ionic strength 2.1 M, and temperature 23+/-1 deg C.Our results as well as those of earlier thermodynamic and kinetics investigations on related systems are used to construct a reaction scheme, the rate equations of which are numerically integrated.The overall reaction 3Fe²⁺ + 4H⁺ + NO3^- <*> 3Fe³⁺ + 2H2O + NO can be followed by monitoring the absorbance of the FeNO²⁺ intermediate which is formed, or the potential of the Fe(III)/Fe(II) couple.The observed time, t_max, to the absorbance peak at 450 nm varies inversely as +>₀²->₀/2+>₀^0.6.Added oxynitrogen intermediates accelerate the reaction, while dissolved oxygen is an inhibitor.The reaction scheme consists of seven principal reactions, three of which describe the reduction of nitrate to nitric oxide by ferrous ion, one for the formation of the FeNO⁺ complex, and three additional reactions which summarize the chemistry of oxynitrogen species in nitric acid solutions.The seven rate equations were integrated numerically, which permitted a computer simulation of the reaction profile.Each calulation required the specification of a set of rate constants and a set of initial concentrations for the seven independent species.All but two of the rate constants were available from the literature; in addition the calculation required the specification of the NO2 solubility β and a parameter C₀ determining the initial HNO2 (always present) concentration.The simulations revealed that the major factor which determines t_max is the rate of the reaction between nitric oxide and nitrate, which is the rate-determining step (with the rate constant k₇) in a sequience of reactions which leads to the oxidation of Fe²⁺ and which is autocatalytic in NO.The model is moderately sensitive to the rate constant for the initial attack of nitric acid on ferrous and to the free parameters C₀ and β, but it is extremely sensitive to the value of k₇.Thus, the date should determine k₇ rather accurately (to a factor of 2), if the basic features of the model are correct.Autocatalytic shutdown occurs when becomes large enough so that nitrous acid is consumed more rapidly by second-order disproportionation than by reaction with ferrous.Thus, addition of oxynitrogen intermediates to the initial reaction mixture provides a quicker way to build up to a level where autocatalysis is shut down.