Anal. Chem. 1987, 59,2252-2256
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equipment generally available to analytical laboratories, simple chlorination and dechlorination reactions can produce a number of compounds, normally expensive or commercially unavailable, for reference material in GC/MS.
ACKNOWLEDGMENT We thank Lance C. Nicolaysen for technical assistance with mass spectral data.
Eds.; Butterworth: Stoneham, MA, 1984; Chapter 2.
(IO) Nestrick, T. J.; Lamparski, L. L.; Stehl, R. H. Anal. Chem. 1979, 57, 2283-2276.
(1 1) Bell, R. A. Chlorinated Dioxins and Dibenzofurans in the Total Environ-
(12) (13) (14)
LITERATURE CITED (1) Rappe. C.; Marklund, S.; Berqvist, P.-A,; Hanson, M. Chlorinated Dloxins and Dlbenzofurans in the Total Environment ; Choudhary, G., Keith,
L. H., Rappe, C., Eds.; Butterworth: Stoneham, MA, 1983; Chapter 7. (2) Poland, A.; Glover, E.; Kende, A. S. J. BlOl. Chem. 1978, 251, 4936-4940. (3) Williams, C. H.; Prescott, C. L.; Stewart, P. 8.; Choudhary, G. Chlorinated Dioxins and Dlbenzofurans in h e Total Envlronment I I ; Keith, L. H., Rappe, C., Choudhary, G., Eds.; Butterworth: Stoneham, MA, 1985; Chapter 32. (4) Smith, L. W.; Stalling, D. L.; Johnson, L. Anal. Chem. 1984, 56, 1830- 1834. (5) Patterson, D. G.; Holler, J. S.; Smith, S.J.; Liddie, J. A,; Sampson, E. J.; Needham, L. L. Chemosphere 1988, 15,2055-2060. (6) Ryan, J. J.; Schecter, A. Chlorinated Dloxlns and Dibenzofurans in the Total Environment 111; Rappe, C., Choudhary, G., Keith, L. H., Eds.; Lewis: Chelsea, MI, 1986; Chapter 1. (7) Schecter, A.; Tiernan, T. 0.; Taylor, M. L.; VanNess, G. F.; Garrett, J. H.; Wagel, D. J.; Gitlltz, G.; Bogdasarian, M. Chlorinated Dloxins and Dibenzofurans in the Total Environment JI; Keith, L. H., Rappe, C., Choudhary, G., Eds.; Butterworth: Stoneham, MA, 1985; Chapter 18. (8) Gray, A. P.; Cepa, D. P.; Solomon, I.J.; Aniline, 0. J. Org. Chem. 1978, 4 1 , 2435-2437. (9) Taylor, M. L.; Tiernan, T. 0.; Ramaiingam, B.; Wagei, D. J.; Garrett, J. H.; Salch, J. G.; Ferguson, G. L. Chlorinated Dioxins and Dlbenzofwans in the Total Environment I I ; Keith, L. H., Rappe, C., Choudhary, G.,
15) 16)
17) 18) (19) (20)
ment; Choudhary, G., Keith, L. H., Rappe, C., Eds.; Butterworth: Stoneham, MA, 1983; Chapter 4. Safe, S. H.; Safe, L. M. J. Agric. Food Chem. 1984, 32, 68-71. Bell, R. A.; Gara, A. Chlorinated Dloxins and Dibenzofurans in the Total Environment I I ; Keith, L. H., Rappe, C., Choudhary, G., Eds.; Butterworth: Stoneham, MA, 1983; Chapter 1. Gelbaum, L. T.; Patterson, D. G., Jr.; Groce, F. G. Chlorinated Dioxins and Dibenzofurans in Perspective; Rappe, C., Keith, L. H., Choudhary, G., Eds.; Lewis Publisher: Chelsea, MI, 1986; Chapter 31. Hutzinger, 0.; Safe, S.; Zitko, V. Int. J. Environ. Anal. Chem. 1972, 2 , 95-106. Barnhart, E. R.; Patterson, D. G., Jr.; MacBride, J. A. H.; Alexander, L. R.; Alley, C. A.; Turner, W. E. Chlorlnated Dioxins and Dibenzofurans in Perspective; Rappe, C.. Choudhary, G., Keith, L. H., Eds.; Lewis: Chelsea, MI, 1986; Chapter 33. Muiiin, M. D.; Pochini, C. M.; McCrindle, S.; Romkes, M.;Safe, S. H.; Safe, L. M. Environ. Sci. Techno/. 1984, 18, 468-476. Ashley, D. L.; Barnhart, E. R.; Patterson, D. G., Jr.; Hill, R. H. Appl. Specfros ., in press. Colmsjo, A.; Rannug, A.; Rannug, U. Mutat. Res. 1984, 735,21-29. Ashley. 0. L.: Barnhart, E. R.; Patterson, D. G., Jr.; Hill, R. H., submitted for publication in Anal. Chem.
RECEIVED for review August 26, 1986. Resubmitted March 30,1987. Accepted June 8,1987. This study was supported by funds from the Agency for Toxic Substances and Disease Registry, Public Health Service, US. Department of Health and Human Services, Atlanta, GA. Use of trade names is for identification only and does not constitute endorsement by the Public Health Service or the U S . Department of Health and Human Services.
Single Potential-Alteration Surface Infrared Spectroscopy: Examination of Adsorbed Species Involved in Irreversible Electrode Reactions Dennis S. Corrigan, Lam-Wing H. Leung, and Michael J. Weaver*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
The fea8lbWy of obtaining poten#&dltference infrared spectra for reactive electrochemical adsorbates with a Fourier transform infrared (FTIR) spectrometer using only a slngie potential alteration during the spectra data acqulsnion Is IIlustrated and applied to examining the eiectrooxidatlon of carbon monoxkle and formic acid via adsorbed CO at piatinum-acidic aqueous Interfaces. thb form of such SPAIR (single potential-alteration infrared) spectra for such lrreversiMe electrode reactions is compared wlth corresponding data obtalned under more conventional conditions that utilize multiple potentlal steps between the base and sample potentials during the spectral acquisltlon. Several advantages of the former approach are apparent in these data, including informath on the nature and extent of COz product formatfon and the electrochemical reactlvlty of the adsorbed CO. The capability of obtaining SPAIR spectra for such adsorbed species withln ca. 5 s or less is noted, and posslble electrochemical klnetlc appllcations are pointed out.
External reflectance infrared spectroscopy has recently emerged as a major technique for the in situ molecular
characterization of electrodesolution interfaces (1,2). Besides the use of polarization modulation, the necessary subtraction of spectral interferences arising from the bulk solution is achieved by means of potential-difference methods, taking advantage of the sensitivity of the electrochemical surface structure to the electrode potential. Such potential-difference spectra are usually obtained in two ways (1, 2). The first, termed “EMIRS” (electrochemically modulated infrared spectroscopy), involves relatively rapid (ca. 10 Hz) potential modulation between the chosen “base” and “sample” values while the infrared wavelength is scanned slowly with a grating spectrometer. The alternative approach, dubbed “SNIFTIRS” (subtractively normalized interfacial Fourier transform infrared spectroscopy), involves recording a series of interferometer scans sequentially a t the base and sample potentials, data sets then being substracted from each other to yield the potential-difference infrared (PDIR) spectra ( 1 , 2). While the former technique necessarily requires a large number of Dotential modulations in order to acauire a mettrum over a suitably wide frequency range, the latter method in principle only requires that a single potential alteration, from the base and sample potentials, be applied during the spectral data acquisition. Indeed, SNIFTIR spectra can often readily be obtained in this manner for electrogenerated so-
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lution-phase species (1-4). In practice, however, it is usual to obtain SNIFTIR spectra for adsorbed species by altering the potential periodically (say every ca. 20-30 s) between the base and sample values while a sufficient number of interferometer scans are acquired a t each potential to yield a satisfactory signal-to-noise ratio. This is because the deleterious effects of instrument drift (source, detector fluctuations) and other time effects can thereby be largely eliminated over the longer periods (often 5-15 min) that are often taken to record such spectra for interfacial species. The forms of such EMIR and SNIFTIR spectra are therefore often identical since the latter as well as the former commonly employ multiple potential alterations between the base and sample values (Le., potential modulation). This procedure should be acceptable for systems where the potential-induced alterations in the surface spectra are entirely reversible, such as adsorbates having potential-dependent adsorption-desorption equilibria and/or vibrational frequency shifts. Not surprisingly, such systems have received the most attention to date (1,2). Another major projected application of potential-difference infrared methods involves the examination of heterogeneous redox mechanisms. The sensitivity of the thermodynamics and kinetics of such processes to the electrode potential underscores the inherent suitability of this approach. Provided that the electrode reaction is at least chemically (if not electrochemically) reversible, potential modulation methods are again applicable. However, many reactions of interest involving adsorbed intermediates, especially multistep electrocatalytic processes, are entirely irreversible so that the application of potential-modulation procedures is not straightforward. An important class of electrode processes of this type involves the electrooxidation of organic fuels on transition-metal electrodes. A number of surface infrared studies of these systems have appeared (5-9). In the present communication we demonstrate the feasibility of examining the reactivity of adsorbed species, specifically CO, involved in such irreversible reactions by means of PDIR spectra with a Fourier transform instrument when only a single potential alteration is applied during the spectral data acquisition. A similar procedure has been employed by Pons et al. to follow the irreversible adsorption of Fe(CN)63on platinum ( 4 ) . We shall refer to this approach as SPAIRS (single potential alteration infrared spectroscopy) as distinct from the corresponding potential-modulation techniques (PMIRS). Illustrative examples are discussed for the electrooxidation of adsorbed carbon monoxide formed from solution CO, formic acid, and methanol as examined with SPAIRS, along with comparisons with corresponding PMIR spectra.
EXPERIMENTAL SECTION Most experimental details of the surface infrared measurements are given in ref 10 and 11. The infrared spectrometer used was a Bruker-IBM IR 98-4A Fourier transform instrument, with a helical globar light source and a liquid-nitrogen-cooled InSb detector (Infrared Associates). The spectral resolution was 8 cm-l. The incident light was p-polarized by means of a KRS-5 wire-grid polarizer (Harrick). The polycrystalline platinum and gold electrodeswere 9.5 mm diameter disks mounted on a glass plunger. The platinum surface was pretreated immediately prior to use by mechanical polishing, dipping in hot chromic acid, followed by rinsing and potential cycling at 50 mV s-l between -0.25 and 1.4 V vs. SCE in 0.1 M HC104. The gold electrode was coated with a thin (ca. one to three monlayer) deposit of platinum (vide infra) by means of electrodeposition from 1 X M H,PtC16 in 0.5 M H2S04at a potential of 0.15 V vs. SCE (12). An electrochemical thin-layer configuration was formed by pressing the electrode up against the CaF, optical window. The acquisition of the interferometer scans was synchronized with the potential alterations by means of trigger pulses from the spectrometer to the potentiostat (Hi-Tek Model DT 2101) as outlined in ref 11.
2253
; : i 2400 1/, cm-'
I
I
1850
2400
v ,crr-'
1850
Figure 1. Potential-difference infrared spectra obtained in the 1850-2400 cm-' region for adsorption and electrooxidation of carbon monoxide at a platinum-coated gold electrode (see text) in 0.1 M H,SO, containing saturated (Le., ca. 1 mM) CO. Base (reference)potential is -0.2 V vs. SCE; Sample potentials are as indicated. SPAIR (single potential alteration infrared) spectra in A were obtained by recording 100 interferometer scans successively at the base and sample po-
tentials and subtracting the former from the latter. PMIR (potential modulation infrared) spectra in B were obtained similarly, but by acquiring 1024 interferometer scans at each of both potentials, the potential being altered back and forth between the two values every 32 scans. Formic acid (Fisher), methanol (Burdick & Jackson), and sulfuric acid and perchloric acid (G. F. Smith) were analytical reagent grade, and carbon monoxide was 99.8% purity (Matheson). Electrode potentials are quoted vs. the saturated calomel electrode (SCE), and all measurements were made at room temperature, 23 A 1 "C.
RESULTS AND DISCUSSION The catalytic electrooxidation of adsorbed carbon monoxide to C 0 2 on transition metals provides an especially convenient model system with which to compare surface PDIR spectra obtained by single and multiple potential alterations (SPAIRS and PMIRS) since the reaction is entirely irreversible and adsorbed CO yields an especially intense C-0 vibrational band, vco. An illustrative comparison of this type is shown in Figure 1,for a platinum-coated gold electrode in CO-saturated 0.1 M H2S04. (This and related systems, involving transition-metal overlayers on gold, are of particular interest to us since they display intense surface-enhanced Raman as well as infrared spectra (12). Figure 1A shows a series of SPAIR spectra obtained in the 1850-2400 cm-' region by using a single potential alteration from the base potential, -0.2 V, to the series of sample potentials indicated. This involved the acquisition of 100 interferometer scans consecutively at the appropriate pair of base and sample potentials, the former being subtracted from the latter to yield the spectra shown. The total of 200 scans so acquired consumed about 125 s. Comparable spectra could also be obtained by employing as little as 20 scans at each potential. As is conventional (1,2), the spectra are displayed as relative reflectance values, ARIR, plotted against the infrared frequency. Figure 1B shows a corresponding set of PDIR spectra obtained by using multiple potential alterations. This involved obtaining a total of 1024 interferometer scans at both the base and sample potentials,
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the potential being altered between these two values after every 32 scans (i.e., about every 20 9). In both cases, the electrode was pulled back from the CaFz window between the acquisition of each set of spectral scans in order to replenish the thin-layer cavity. The major bipolar uc0 band seen in both the SPAIR and PMIR spectral sets, a t about 2060-2080 cm-l, arises from CO bound to top-fold platinum sites ("terminal" configuration), whereas the smaller feature around 1880-1900 cm-' can be identified with a bridged form of this adsorbate (12-14). The bipolar form of these bands arises from the increasing uco frequency as the potential becomes more positive (14). (The negative- and positive-going components arise from infrared absorption a t the sample and base potentials, respectively.) For sample potentials negative of about 0.4 V, these bands have a very similar form in the single- and multistep spectra (Figure 1). This is consistent with the expected stability of the adsorbed CO in this potential region. However, marked differences in the SPAIR and PMIR spectra occur a t more positive sample potentials, where CO electrooxidation is known to occur from the observed onset of faradaic current. In the SPAIR spectra, the intensity of the negative-going component decreases progressively toward more positive potentials and disappears entirely by 0.8 V (Figure 1A). This is indicative of electrooxidative removal of the adsorbed CO, since the negative-going band reflects the state of the absorbate at the sample potential. The corresponding broadening and increase in intensity of the positive-going band around 2030 cm-' under these conditions is also consistent with this interpretation since the partial interference from the negative-going band is thereby removed. In contrast, the uco bands are entirely lost in the PMIR spectra under these conditions (Figure 1B). This is because the irreversible electrooxidation of adsorbed CO results in the partial and then complete disappearance of adsorbed CO a t the base as well as the sample potentials following one or more potential modulations. The other interesting feature of the single-step spectra in Figure 1A is the appearance of a negative-going band a t 2345 cm-' which becomes more intense at increasingly positive potentials. From its frequency, this band can be identified with COz, which is the bulk-phase product of CO electrooxidation. Indeed, the intensity of this band correlates approximately with the faradaic anodic charge passed a t the sample potential during the spectral data acquisition. Note that this band is entirely absent in the PMIR spectra (Figure 1B). This is because the irreversible electrooxidation of the entire reservoir of CO in the thin layer leads to the presence of COz a t the base as well as the sample potential under modulation conditions, so that the COz band is cancelled entirely in the PMIR spectra. It is also interesting to note that the COz band in the S P A R spectra emerges even at potentials, 0.25-0.45 V, prior to the electrooxidation of the adsorbed CO as signaled by the disappearance of the negative-going uco band (Figure 1A). This apparent disparity can be resolved by noting that solution CO is electrooxidized a t significantly smaller overpotentials on the gold substrate than on the platinum overlayer (12),so that this COz is presumably produced by reaction of CO on residual uncovered gold sites. Figure 2 shows a similar set of SPAIR spectra obtained for CO adsorbed on a conventional platinum electrode in 0.1 M HC104,employing a base potential of -0.15 V. In this example the solution CO was removed by nitrogen purging for 10 min prior to the acquisition of each spectrum, so that only irreversibly adsorbed CO remains. The removal of the adsorbed CO at 0.4 V, as indicated by the sharp intensity drop of the negative-going vco hand, is accompanied by the appearance
v 2065
*E-Y
2075-[
0.1
0.4
Figure 2. SPAIR spectra obtained for irreversibly adsorbed CO at platinum-aqueous 0.1 M HCIO,. Obtained by acquiring 20 interferometer scans successively at base potential, -0.15 V vs. SCE, and
then at sample potentials indicated. CO was preadsorbed from a CO-saturated 0.1 M HCIO4 solution, and then purged with nitrogen for 10 min to remove the dissolved CO. of the negative-going solution COz band at 2345 cm-' (Figure 2). This clearly indicates that this potential-dependent intensity decrease is caused by irreversible electrooxidation, rather than desorption, of the carbon monoxide. Note that these spectra involved the acquisition of only 20 interferometer scans a t the base and sample potentials; this consumed only about 15 s. The corresponding PMIR spectra (not shown) were essentially identical in the potential region prior to CO electrooxidation (C0.4V), yet, as before, no spectral features were obtained a t more positive sample potentials where irreversible CO electrooxidation occurs. A number of comparisons between corresponding SPAIR and PMIR spectra were also made for the electrooxidation of formic acid and methanol on platinum. On the basis of surface infrared spectroscopy, both of these reactants have been found to dissociate to yield adsorbed CO on platinum (5-9). Representative sets of such SPAIR and PMIR spectra for a series of sample potentials using a base potential of -0.2 V, with 0.25 M formic acid in 0.1 M HC104,are given in Figure 3, parts A and B, respectively. Comparable results were also obtained for methanol oxidation. The solid trace in Figure 4 is a typical anodic-cathodic cyclic voltammogram for this system. The dashed trace in this figure refers to the 0.1 M HC104 blank. Similar voltammograms were obtained whether or not the electrode was pushed up against the CaFz window. The frequency range in the vicinity of the bipolar uco band
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987 B
ov
035
20,56
1
IT2
I100
2343
2500
Y
Icm-’)
2255
IS50
Figure 3. Corresponding SPAIR and PMIR spectra (A and 8, respectively) in the 1850-2500 cm-‘ region for the electrooxidation of formic acid at platinum. Solution was 0.25 M HCOOH in 0.1 M HCIO,. SPAIR spectra (A) involved the acquisltlon of 10 interferometer scans at the base, -0.2 V, and then the sample potentials indicated. A 6-s delay was implemented between the potential step and the spectral data collection. Correspondlng PMIR spectra (B) consist of 512 scans at each potential, the potential being stepped after every 32 scans. Signals in the vco region, 2000-2125 cm-’, are magnified 10-fold relative to the rest of each spectrum.
in Figure 3 (around 2000-2125 cm-l) is shown on a 1OX expanded ARIR scale for clarity, since this feature would otherwise be dwarfed by the COzband at more positive potentials. Each of the SPAIR spectra in Figure 3A are comprised of only 10 interferometer scans obtained successively at the base and sample potentials. A delay of about 6 s was implemented following the potential step and prior to the spectral data acquisition to allow for the rise time of the potentiostat. Comparable spectra were also obtained by requiring up to 100 scans at each potential. However, the greater signal-to-noise ratio that might be expected by employing more scans is offset by spectral drift that is probably caused by the accumulation of COz bubbles in the thin-layer cavity. The PMIR spectra in Figure 3B consisted of 512 scans at each potential, this being altered after every 32 scans. In both cases, the electrode was retracted from the optical window and underwent two-tothree anodic-cathodic potential cycles between each spectra to regenerate a reproducible surface state. As before, for sample potentials less negative than the onset of faradaic anodic current, ca. 0.45 V (Figure 4), the SPAIR and PMIR spectra in Figure 3 are very similar, both exhibiting the usual bipolar band due to terminally bound CO. However, there are significant differences in these spectra at more positive sample potentials, where oxidation of formic acid occurs. The PMIR spectra (Figure 3B) exhibit vco bands of decreasing amplitude at progressively more positive potentials, leading to a complete disappearance of the negative-going band by around 0.8 V, although the positive-going band survives even at more positive potentials. In the former respect, these spectra are similar to the behavior observed for formic acid on platinum using EMIRS (5, 6). The form of the PMIR spectra can be accounted for by asserting that although electrooxidation of the adsorbed CO occurs each time the potential is stepped to sample values beyond 0.45 V, partial readsorption takes place upon each return to the base potential
60 0 200 E , rnV vs S C E
-200
Flgure 4. Cyclic voltammograms for electrooxidation of formic acid at platinum (soli trace). Solution conditions are given in Figure 3, 0.25 M HCOOH 0.1 M HCIO,. Sweep rate was 0.05 V s-’. Anodic current is plotted downward. Electrcde area is 0.71 cm2. Dashed trace was obtained for 0.1 M HCIO, alone, with current scale magnified 10-fold.
+
since a large reservoir of unreacted formic acid will remain in the thin-layer cavity. This therefore accounts for the partial retention of the positive-going band under these conditions. In contrast to the behavior of the foregoing systems, a small positiue-going COz band is also obtained in the PMIR spectra (Figure 3B). The surprising appearance of this feature can be understood from the extensive anodic current (and hence COz formation) that is observed around 0 to 0.5 V upon scanning back to more negative potentials (Figure 4). This could give rise to a small but consistently greater amount of COzpresent at the base potential than at the sample potential during the preceding modulation cycle, thus accounting for the small positive-going COz band that is observed. In the corresponding SPAIR spectra (Figure 3A), the negative-going component of the bipolar vco band survives to significantly more positive potentials than in the PMIR case (Figure 3B). This is expected in view of the smaller total time that is spent at the sample potentials in comparison with the potential-modulation case. The advantage of the SPAIR experiment is again apparent since the conditions under which the spectra in Fig. 3A were obtained allow them to be compared directly with the voltammetric response in Figure 4. Interestingly, the potential range, ca. 0.5-0.7 V, where oxidation of initially adsorbed CO commences as deduced from the dimension of the negative-going vco band intensity (Figure 3A) corresponds closely to the region where voltammetric oxidation of formic acid occurs (Figure 4). The occurrence of formic acid oxidation is also readily seen from the appearance of an intense negative-going COz band under these conditions in the SPAIR spectra (Figure 3A). This suggests that the adsorbed CO might act as a reactive intermediate for this process (but see ref 15). Similar spectra to those in Figure 3A were also obtained during positive-going potential sweeps. By use of suitably slow (1-5 mV s-l) voltammetric sweep rates, a sufficient number of interferometer scans could be acquired to enable the individual spectra to be obtained over potential increments as small as 10 mV or so. Each of these spectra were then referenced to the spectrum acquired at the initial (i.e., base) potential. Such a potential-dependent spectral sequence has the obvious virtue of enabling a direct comparison to be made with the voltammetric current-potential curve that is obtained
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simultaneously. Detailed applications of this procedure to the provision of mechanistic information for irreversible electrooxidation reactions will be described elsewhere. It is commonly accepted that the electrooxidation of formic acid on platinum can occur via “dual pathways” involving both weakly and strongly adsorbed intermediates (16). The identity of the latter species has been the subject of much debate; although the presence of adsorbed CO is indicated from surface infrared spectroscopy ( 3 ) ,electrochemical (16) and coupled mass spectrometric (“DEMS”)measurements suggest that the strongly adsorbed intermediate is COH (17, 18). While the present results certainly do not eliminate the latter possibility, they do nonetheless indicate that the initially adsorbed CO is sufficiently reactive to have potential as a possible catalytically active intermediate under voltammetric conditions (15).
CONCLUDING REMARKS Overall, the SPAIR spectra provide unusually direct information on the chemical nature, as well as extent, of the electrochemical reaction induced by a single potential step (or sweep) and how the reactivity of the adsorbed CO intermediate compares with that for the overall conversion of reactants to products. Although the spectra obtained under potential-modulation conditions can provide some information along these lines, such conventional PMIR are clearly of limited value for examining irreversible electrode processes. In addition, the rapid potential modulation that is necessarily employed in PMIRS when using a grating spectrometer (i.e., EMIRS) can have severe deleterious effects on the surface condition and, hence, on the chemical and physical state of the adsorbed species. Admittedly, the likely applicability of SPAIRS is limited, at least using present commercial FTIR spectrometers, to the examination of interfacial species with especially strong infrared absorptions. Nevertheless, a real virtue is that the spectra can be obtained under electrochemically well-defined potential-step conditions on time scales down to a few seconds. This offers the additional prospect of employing time-dependent SPAIRS to extract electrochemical rate data as well as mechanistic information for adsorbed species for com-
parison with kinetic data obtained by conventional electrochemical means. The virtue of such spectral measurements is that they could provide a direct connection between the vibrational properties and electrochemical reactivity of adsorbed species. A variety of experiments in our laboratory along these lines, focusing particular attention on the electrooxidation kinetics of CO and small organic fuels (IS),are under way or are planned for the near future.
ACKNOWLEDGMENT Some experimental assistance was provided by Feng Bao. Registry No. CO, 630-08-0;HC02H, 64-18-6;Pt, 7440-06-4; Au, 7440-57-5;COP, 124-38-9. LITERATURE CITED (1) Bewick, A.; Pons. S. In Advances in InfraredandRaman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heyden: New York, 1985; Vol. 12, Chapter 1. (2) Foiey, J. K.; Korzenlewski, C.; Daschbach, J. L.; Pons, S. In Electroanalytical Chemistry-A Serles of Advances; Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Voi. 14, p 309. (3) Foley, J. K.; Pons, S.;Smith, J. J. Langmulr 1985, 7 , 697. (4) Pons, S.;Datta, M.; McAleer, J. F.; Hinrnan, A . S. J . Elecfroanal. Chem. 1984, 760, 369. (5) Beden, 8.; Bewick, A . ; Larny, C. J . Electroanal. Chem. 1983, 748, 747. (6) Beden, B.; Bewick, A.; Larny, C. J . Necfroanal. Chem. 1983, 150, 505. (7) Kunirnatsu, K. J . Electroanal. Chem. 1982, 740, 205. ( 8 ) Kunirnatsu, K. J . Elecfroanal. Chem. 1983, 745, 219. (9) Kunimatsu, K. J . Electroanal. Chem. 1988, 273,149. 10) Corrigan, D. S.;Weaver, M. J. J . Phys. Chem. 1986, 90,5300. 11) Corrigan, D. S.;Milner. D. F.; Weaver, M. J. Rev. Sci. Instrum. 1985, 56, 1965. 12) Leung, L-W. H.;Weaver, M. J. J . Am. Chem. Soc., in press. 13) Sheppard, N.; and Nguyen, T. T. I n Advances in Infrared and Raman Sllecfroscow; Clark, R. J. H., Hester, R. E., Eds., Heyden: London, 1978; Voi. 5; p 67. 14) Beden, E.; Bewick, A.; Kunirnatsu, K.; Lamy, C. J . Electroanal. Chem. 1982, 142, 345. 15) Corrigan. D. S.;Weaver, M. J., submitted for publication in J . Nectroanal . Chem 16) Capon, A.; Parsons, R. J . Necfroanal. Chem. 1973, 4 5 , 205. 17) Wolter, 0.; Willsau, J.; Heitbaum, J. J . Nectrochem. Soc. 1985, 732, 1635. (18) Wiiisau, J.; Wolter, 0.; Heitbaurn, J., J . Necfroanal. Chem. 1985, 785,163.
.
RECEIVED for review March 6, 1987. Accepted May 28, 1987. This work is supported by the National Science Foundation.
Determination of Sulfite Using a Sulfite Oxidase Enzyme Electrode Vicki J. Smith Phillips Petroleum Company, Biotechnology Division, 21 1 CPL, Phillips Research Center, Bartlesville. Oklahoma 74004 A sulfite-senslng electrode was constructed by physically trapplng sulfite oxidase (EC 1.8.3.1) enzyme at the tip of a dissolved-oxygen electrode. The decrease In oxygen that occurs as sulfite is enzyrnatlcally oxidized to sulfate was measured amperometrlcaiiy. A linear response to three commonly used sulfltlng agents, sodium sulfite, sodlum blsulfite, and sodlum metablsulffte, was obtained. Precision of the measurements expressed as relative standard devlation was 1.0% at 100 ppm sodlum sulflte. Comparative determinations of the sulfite content of dried aprlcots, potato flakes, ros6 wfne, wine vinegar, and lemon julce uslng a colorimetric procedure and the sulflte electrode showed a 0.98 llnear correlation coefflcient. The electrode was stored refrigerated for at least 84 days without loss of response.
Interest in analytical methods capable of determining sulfiting agents has heightened due to recent Food and Drug Administration (FDA) rulings revoking the generally recognized as safe status of sulfiting agents for use on fruits and vegetables sold raw. FDA has also established 10 ppm as the threshold for declaration of sulfites in the labeling of foods, nonalcoholic beverages, and wine products ( I ) . This is the result of adverse and potentially life-threatening reactions in sulfite-sensitive individuals after eating foods treated with these agents (2-4). The mechanism of action and fate of sulfiting agents upon addition to food have been described (5-7) but are not completely understood. Among the analytical methods used for sulfite analysis are
0003-2700/87/0359-2256$01.50/0 42 1987 American Chemical Society
