Correspondence Comment on “Redox Interactions of Cr(VI) and Substituted Phenols: Products and Mechanism” SIR: Recently, Elovitz and Fish have reported detailed kinetic studies on the oxidation reactions of phenols by Cr(VI) (1, 2). According to these authors, the initial step of the overall reaction is a fast adduct formation between the reactants, which is followed by considerably slower electron-transfer steps. The reported UV/visible difference spectra show absorbance bands in the 350-450 nm region with maxima around 415 and 397 nm for the Cr(VI)-4-methylphenol (pH 5.0) and -4-chlorophenol (pH 2.0) systems, respectively (2). It was suggested that the “discernible” spectral changes are due to the formation of Cr(VI)-oxy esters. To our surprise, we could not confirm the results of Elovitz and Fish. In our measurements, the spectra were recorded on a HP-8453 diode array spectrophotometer at 2 mm cell path length and by using the same reactant concentrations as in ref 2. The spectra were measured 1 min after initiating the reaction, and each trace was taken as the average of several scans. A few experiments were made by using a Carry 1E double-beam instrument. The results obtained with the two spectrophotometers were fully consistent. Spectra are shown for the 4-chlorophenol (4ClP) system in Figure 1. In the visible region and around the characteristic absorbance band of chromium(VI) (λmax ) 353 nm), the spectra are practically identical and do not depend on the phenol concentration. At shorter wavelengths (λ < 300 nm) phenol absorbs significantly, and the spectra show the corresponding concentration dependence. These observations do not indicate any kind of adduct formation.
The same conclusion can be drawn on the basis of the difference spectra obtained by subtracting the spectra of Cr(VI) and 4ClP from that of the corresponding reaction mixture. The differences show some sort of structured feature below 425 nm. However, the absorbance maximum reported earlier at 397 nm was not observed. Furthermore, the difference spectra do not follow the anticipated trend as a function of phenol concentration. In a regular case, the spectra should change in the order of 0.02, 0.025, 0.0375, and 0.05 M 4ClP. Such inconsistency is also noticed in the spectra for the Cr(VI)-4-methylphenol system (Figure 1 in ref 2). The largest variation in the absorbance is about 0.02 AU, which corresponds to less than 3.0% absorbance change. By considering the experimental limitations of the spectrophotometric method (3), such a small change clearly cannot be taken as evidence for the formation of a new species. In fact, our observations are consistent with the experimental data of Elovitz and Fish in that the spectral difference found in their studies is probably also negligibly small, in particular, when their spectra are also corrected for the slight absorbance contribution from the phenol. Such correction was not made in that study. According to our measurements, 4ClP solutions have a small but measurable absorbance in the near-UV region. On the basis of the observations, we believe that the absorbance maximum at ∼397 nm reported by Elovitz and Fish is an artifact due to the actual experimental design and reflects the characteristics of the applied instrumentation rather than real chemical changes. It is important to note that the spectrophotometer was zeroed to a 0.005 M chromium(VI) solution in ref 2. The absorbance of that reference solution is substantial in the wavelength range where the largest spectral differences were observed. The operation under such conditions is far from the optimum for most spectrophotometers, and the error level can be
FIGURE 1. Difference spectra of chromium(VI)-4-chlorophenol reaction mixtures obtained by subtracting the sum of the spectra of Cr(VI) and 4-chlorophenol solutions from the spectra of the corresponding reaction mixtures at 2 mm cell path length, pH 2.00, and 25 °C. Inset: The spectra of the Cr(VI) solution and the reaction mixtures. [Cr(VI)] ) 0.005 M; [4-ClP] ) 0.02 M (a), 0.025 M (b), 0.0375 M (c), and 0.05 M (d). S0013-936X(97)01059-6 CCC: $15.00 Published on Web 06/04/1998
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higher than 1.5-2.0% (3). Finally, it should be emphasized that our findings do not allow for critical evaluation of the kinetic results of Elovitz and Fish (2). The validity of their rate law and the proposed mechanism were not tested. Even the formation of an adduct between the reactants cannot be excluded. However, in our opinion, direct experimental evidence is not available to confirm the formation of Cr(VI)-oxy esters with substituted phenols. Until rigorous proof of their existence, such species should be considered highly hypothetical.
Literature Cited (1) Elovitz, M. S.; Fish, W. Environ. Sci. Technol. 1994, 28, 2161.
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(2) Elovitz, M. S.; Fish, W. Environ. Sci. Technol. 1995, 29, 1933. (3) Fifield, F. W.; Kealey, D. Principles and Practice of AnalyticalChemistry, 4th ed.; Blackie Academic & Professional: Glasgow, New Zealand, 1995; p 361.
Istva´ n Fa´ bia´ n* and Do´ ra Szu1 cs Lajos Kossuth University Department of Inorganic and Analytical Chemistry Debrecen 10, P.O.Box 21 Hungary, H-4010 ES971059H
