Reply to “Comment on ‘Kinetics and Mechanism of Nitrite Oxidation by HOBr/BrO- in Atmospheric Water and Comparison with Oxidation by HOCI/CIO-' ”

Abstract

Becker et al. claim that aqueous chloride media containing bromide contains at least 13 species under conditions comparable to those used in our study and that these equilibria may complicate the analysis of kinetic and equilibrium data. However, under the relevant pH conditions (>6.5) of almost all of our experiments, their Figure 1 shows that only five species have appreciable concentrations: HOBr, OBr-, Br-, HOCl, and OCl-. Of these five, interference from the two Cl(I) species is of concern. Fortunately, the formation of both HOCl and OClis effectively suppressed in the presence of even small amounts of added bromide. For kinetics experiments at pH g 7 (corresponding to 34 of our 36 independent kinetics experiments in chloride media), we prepared Br(I) solutions by the addition of a 10-fold excess of Brto a 1 mM solution of OCl-.1 Under these conditions, the concentrations of both Cl(I) species are negligible, Figure 1a; therefore, we may neglect their contributions in our kinetic analysis. Although excess bromide was not present in the remaining two kinetics experiments at 6.5 e pH e 7.0 in chloride media, their rate constants are consistent with the rest of our data. (See Figure 6a in ref 2.) Even in the absence of added Br-, the formation of other halogen-containing species is suppressed at high Br(I) concentrations. Figure 1b shows the speciation for 20 mM Br(I) in 1 M NaCl, representative of conditions used in our acidimetric titration experiments. HOCl and OClmake minor contributions. However, Becker et al. correctly point out that BrCl2 contributes to the UV spectrum shown in ref 2, captioned “UV-vis spectra of Br(I) in 1.0 M NaCl: (a) pH ) 6”. Because of its much larger extinction coefficient, BrCl2 dominates the spectrum even though its concentration is much smaller than that of HOBr. We used kinetics data across the entire range of pH used (6.5 e pH e 10.0) and [NO2] (0.01-0.04 M) to obtain fitted values of k1, K1, Kw, and Ka, where Ka corresponds to the acid dissociation constant for HOBr. For the reasons described above, we maintain that HOBrand OBrare the only significant X(I) species present in our solutions under these conditions. The fitted values for pKa, 8.27 and 8.50 in 0.10 M NaCl and 1.0 M NaCl, respectively (Table 1)2, are slightly lower than those determined independently by acidimetric titration (8.58 and 8.80) in the range of 7 e pH e 11 with and without added bromide. The differences may indeed be caused by a slightly more complex speciation of X(I) in the titration experiments. Becker et al. assert that the rates of the HOBr/NO2 and Br2/NO2 reactions are mathematically the same and that large differences in the reaction rates “represent the differences in the species distribution of HOBr and Br2 at the pH values shown”. We agree that both reactions proceed via the same ratedetermining step (the reaction of BrNO2 with NO2). The reaction rate is a function of both the rate constant for this step and the concentration of the BrNO2 intermediate. Because the latter varies with Br(I) speciation, the rates are not mathematically the same, indeed, this is the point of our Figure 8.2 Finally, we made no attempt to apply these solution phase rates to gasphase chemistry.2 We regret that the dominant equilibria involving Br(I) in chloride media were not presented as explicitly as they could have been in our original paper.3 The calculated speciation is identical to that determined experimentally by Barcellos da Rosa and Zetzsch.4 Nevertheless, we believe our kinetic experiments and analyses are valid.3 * Corresponding author. E-mail: Nazafarin.Lahoutifard@ science.uottawa.ca. † University of Ottawa. ‡ University of California, Santa Barbara. Figure 1. Speciation of (a) 1.0 mM Br(I) in the presence of 9.0 mM Br-; and (b) 20 mM Br(I) in the absence of added bromide. Calculations were performed with Mathematica, using the same published equilibrium constants as Becker et al. (25 °C, I ) 1.0 M).5 Frame a corresponds to conditions used in all but two of our kinetics experiments in chloride media. Frame b corresponds to the conditions used in our acidimetric titrations. 10617 J. Phys. Chem. A 2004, 108, 10617-10618