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J. Phys. Chem. 1996, 100, 7072-7077
Oxidation of Ferrous and Ferrocyanide Ions by Peroxyl Radicals G. I. Khaikin,† Z. B. Alfassi,‡ R. E. Huie, and P. Neta* Chemical Kinetics and Thermodynamics DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: NoVember 13, 1995; In Final Form: January 2, 1996X
Alkylperoxyl and arylperoxyl radicals were produced by pulse radiolysis in aqueous solutions, and their reactions with ferrous and ferrocyanide ions were studied by kinetic spectrophotometry. Oxidation of Fe(CN)64took place with rate constants that varied from 330 nm are similar but at λ < 320 nm the spectrum at the low [Feaq2+] has higher relative intensity. The difference in intensity in the visible range suggests that the complex is produced with different yields. The increased yield at high [Feaq2+] may be due to two effects: (a) more complete scavenging of the peroxyl radicals by reaction 4 (in competition with their self decay) and (b) more rapid formation before significant decay of the complex. To correct for the latter effect, we used the kinetic fits described above (for two consecutive reactions) to derive the absorbance of the complex, essentially correcting for its decay, and plotted the spectra in Figure 5b. Even after this correction, the spectra at both high and low [Feaq2+] are found to be more intense, but there is still
Oxidation of Ferrous and Ferrocyanide Ions
J. Phys. Chem., Vol. 100, No. 17, 1996 7075
Figure 5. Absorption spectra monitored by pulse radiolysis of aerated aqueous solutions containing 1.3 mol L-1 2-PrOH, 0.1 mol L-1 HClO4, and Fe(ClO4)2 (5 × 10-3 (O) or 0.1 (b) mol L-1). (a) Absorbance from the kinetic trace at its maximum; (b) maximum absorbance calculated from the kinetic fits. The triangles are the spectra at the high concentration normalized to fit those at low concentration (divided by 2.5 and 2.3 for a and b, respectively).
a significant difference between the intensities. To assess the effects of all the reactions involved and because of the overlap of some of these reactions, we modeled the systems by using the ACUCHEM software.24 The initial model involved mainly reactions 3-7 and the selfdecay of the peroxyl radicals. The initial concentrations of
(CH3)2C˙ OH + O2 f (CH3)2C(OH)O2• ()RO2•)
(3)
RO2• + Feaq2+ f RO2-Fe3+
(4)
RO2-Fe3+ + H2O f RO2H + OH- + Feaq3+
(5)
RO2-Fe3+ + H+ f RO2H + Feaq3+
(6)
RO2-Fe3+ + Feaq2+ + H+ f RO• + OH- + 2Feaq3+ (7) Feaq2+, H+, and O2 are given. The initial concentration of the (CH3)2C˙ OH radicals is calculated from the dose per pulse and the known radiation yield. The rate constant for reaction 3 is taken as k3 ) 4 × 109 L mol-1 s-1.6 The peroxyl radicals are assumed to either react with Feaq2+, with k4 ) 5.2 × 105 L mol-1 s-1 or decay by self-reactions. The self-decay of these peroxyl radicals is taken either as the second-order and firstorder processes described in the literature25 or from the intercept of Figure 4. The rate of decay of the RO2-Fe3+ complex is assumed to be either the sum of reactions 5-7 or the actual experimental value determined at the specific conditions used. This modeling reveals several points.
First, at low [Feaq2+], the rate of reaction of RO2• with Feaq2+ is of the same order of magnitude as the rate of loss of the RO2-Fe3+ complex. For example, at [Feaq2+] ) 5 × 10-3 mol L-1, when the concentration of the RO2-Fe3+ is at its maximum, the concentration of RO2• radicals remaining is nearly equal to that of the RO2-Fe3+. At [Feaq2+] ) 0.1 mol L-1, however, the amount of RO2• remaining is
