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Anal. Chem. 1980, 52, 2392-2396 Korenman, I. M.; Gur'ev, I. A. J . Anal. Chem. USSR(€ngl. Trans/.) 1975, 30, 1601. Gur'ev, I. A,; Gur'eva, 2. M. Tr. Khim. Khim. Tekhnol. 1975, 98;Chsm. Abstr. 1976, 86, 1 3 3 1 3 6 ~ . Gur'ev, I. A. Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 1977, 20,504; Chem. Abstr. 1977. 8 7 , 145391j. Gur'ev. I. A,; Lizunova. G. M.; Korenman, I. M. Fiz.-Khim. Metody Anal. 1976, 1 , 78; Chem. Abstr. 1976, 88. 15573e. Gustavii, K.; Johansson, P. A.; Brandstrom. A. Acta Pharm. Suec. 1976, 13, 391. Johansson, P. A., Astra Pharmaceuticals, ABS-I51 85. Siidertalje, Sweden, presented at Euroanalysis I11 Conference, Dublin, Aug 1978. Ratajewicz, D.; Ratajewicz, 2. Chem. Analifyczne 1971, 16, 1299. Johansson. P. A. Acta Pharm. Suec. 1977, 14, 363. Gur'ev. I. A.; Lizunova, G. M. Fiz.-Khim. Metody Anal. 1977, 2 , 85; Chem. Abstr. 1979, 90, 1 9 7 1 6 8 ~ . Puon, S. Ph.D. Thesis, University of Alberta, 1979. Puon, S.; Cantwell, F. F. Anal. Chem. 1977, 49, 1256. "United States Pharmacopea"; Mack Printing Co.: Easton, PA, 1975; 19th Rev., p 335. Saunders. L.; Srivastava, R. S. J . Pharm. Pharmacol. 1951, 3 , 78. Schill, G. In "Ion Exchange and Solvent Extraction"; Marinsky, J. A., and Marcus, Y., Eds; Marcel Dekker: New York, 1974: Vol. 6, Chapter 1.
(17) Grinstead, R. R. Solvent Extr. Chem. Roc. Int. Conf.. 1966 1967, 426. (18) Hhfeldt, E. SolventExtr. Chem., Proceed. Int. Conf.. 19681069, 157. (19) Chuchani, G.; Hamandez, J. A.; Zabicky. J. Uture(London) 1985, 207, 1385. (20) Mohammed, H. Y.; Cantwell, F. F. Anal. Chem. 1980, 52, 553. (21) Dye, J. L.; Nicely, V. A. J . Chem. Educ. 1971, 48. 443. (22) Cantwell, F. F. University of Alberta, unpublished work, 1980. (23) Sandell, K. B. Naturwissenschaffen 1964, 57, 336. (24) Duffy, M. J. Endo Laboratories, Garden City, NY, private communlcatbn, 1975. (25) Kortum. G.: Vogel, W.; Andrussow, K. "Dissociation Constants of Organic Acids in Aqueous Solution"; Butterworth: London, 1961; p 450. (26) Kemula, W.; Buchowski, H. J . Phys. Chem. 1959, 63, 155. R. F. Chem. Rev. 1974, 74, 5. Cookson, (27) (28) Bates, R. G. "Determination of pH", 2nd ed.; Wiley: New York, 1973; Chapter 8.
RECEIVED for review May 21, 1980. Accepted August 5,1980. This work was supported by the Natural Sciences and Engineering Research Council of Canada and the University of Alberta.
Response of Microvoltammetric Electrodes to Homogeneous Catalytic and Slow Heterogeneous Charge-Transfer Reactions Mark A. Dayton, Andrew G. Ewing, and R. Mark Wightman' Department of Chemistry, Indiana University, Bloomington, Indiana 4 7405
Voltammetric electrodes whose area is defined by the radius of a carbon fiber (2.8 pm) have been used in the electrooxidation of dopamine, dihydroxyphenylacetlc acid, and ascorbic acid. The results are compared to those predicted by the equations for a hemispherical electrode of Identical radius. The equations are derived by evaluating the pertinent flux equations that were derived in spherical coordinates. Charge-transfer rates deduced from this analysis are found lo be the same at carbon fiber electrodes as at carbon paste for dopamine oxidation. The quaskeverdble behavior of dihydroxyphenylacetic acid at carbon paste is accentuated at the fiber employed in this report. Because of the small size of carbon fibers, enhanced current from the catalytic oxidation of ascorbic acid by oxidized dopamine is minimized. The result is also in accord with that predicted by the equations for a homogeneous catalytic reaction at a hemispherical electrode of similar dimensions.
In a recent paper we demonstrated that microvoltammetric electrodes fabricated from carbon fibers, with a surface area cm', exhibit a greatly different current response of 3 X than electrodes of conventional size (1). Current at an electrode of these dimensions is essentially time independent at times greater than 100 ms since the rate of diffusion exceeds the rate of depletion by electrolysis. Because of the small size of these electrodes, planar models for a mathematical description of the response provide an unsuitable prediction. In this paper we report the effects on the current a t a microvoltammetric electrode for the case of slow heterogeneous charge transfer and also for the case of a homogeneous catalytic reaction following electron transfer. Approximate solutions for both of these cases can be obtained by adapting results derived for spherical electrodes. Oxidations of the neurochemically important compounds dopamine (DA), dihydroxyphenylacetic acid (DOPAC), and ascorbic acid (AA) 0003-2700/80/0352-2392$0 1 .OO/O
at these electrodes are compared with conventional carbon paste electrode responses to demonstrate the application of these concepts. A major concern in any voltammetric experiment is the rate of heterogeneous charge transfer. Electroanalytical measurements are difficult when the rate of charge transfer at the standard potential (k,) is very slow since the voltammetric waves become drawn out and shifted on the voltage axis. It has been suggested that voltammograms obtained at electrodes of small size may be more sensitive to the value of k , than conventionally sized electrodes (2). Our experimental results, and the theory adapted from spherical electrodes, demonstrate that electrodes fabricated from carbon fibers are not sufficiently small to accentuate this effect. However, the theory facilitates recognition of differences in charge-transfer parameters between different forms of carbon. The unique voltammetric properties of microvoltammetric electrodes are of especial interest because of their application as chemical sensors of electroactive substances in mammalian brain. This research area is directed a t determining changes in the concentration of neurotransmitters such as DA following neuronal activity (3-12). A major interference in these measurements is AA, a compound present in millimolar concentrations in mammalian brain. Although AA is thermodynamically easier to oxidize than DA, the peak potential for AA oxidation a t carbon electrodes is displaced several hundred millivolts in a positive direction and is actually positive of the peak potential for DA (13, 14). Oxidation of DA a t carbon electrodes in the presence of AA results in a homogeneous catalytic oxidation of AA. The regenerated DA returns to the electrode resulting in an enhanced current.
DA Z~DOQ 4
DOQ
+ AA
-2H
k.
I
DA
+ DHA
(DOQ and DHA represent the oxidized forms of DA and AA, 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2393
Table I. Electrochemical Parameters at Carbon Paste and Carbon Fiber Electrodes differential pulse voltammetry carbon paste i, A?;,' E, mV PA 260 1.1 175 120 0.42 90 235 0.18 140 105 2.4 80 7
AA, 10-3 M DA, 10-4 M DOPAC, 10-4 M AA, M, and DA, 10-4 M a
Average current a t 120 ms, E ,
$ p:
550 110 480 110
260 =
E , = -0.2 V.
carbon fiber i, AE,I/z, mV PA 20 24 3 188 130
350 70
300 250
Background corrected.
chronoamperometry a carbon paste carbon fiberC i a t 0.3 i at 0.6 i at 0.3 i at 0.6 V,pA V,pA V,nA V,nA 6.9 1.6 1.2 15.9
16.9 1.6 2.0 19.1
0.51 0.20 0.076 0.69
0.89
0.23 0.23 1.12
Backstep corrected.
respectively.) The catalytic rate (k,) for this reaction is sufficiently fast that the faradaic current is equal to the sum of the diffusion controlled currents for DA and AA. Because of the small area of microelectrodes, only a small portion of the catalytically regenerated DA can return to the electrode and, thus, these electrodes are essentially blind to this interference. In this paper, this intuitive notion is supported by theory and experiment.
EXPERIMENTAL SECTION Chemicals. Dopamine-HC1 and dihydroxyphenylacetic acid (Sigma) were dissolved in a citratephosphate (9.2 mM, 181.6 mM, pH 7.4) buffer to prepare lo-* M stock solutions. Ascorbic acid (Mdinckrodt) stock solutions, made in the same buffer, were lo-' M. All stock solutions were prepared fresh for each experiment. The buffer was made with water distilled in glass from alkaline permanganate. All solutions were deoxygenated by bubbling with water-saturated nitrogen. Electrodes. The microelectrodes were composed of combination working-reference electrode assemblies. A schematic of the two-barrel electrode assembly is depicted in Figure 1. The electrodes were constructed by employing a modification of our technique for the construction of single-barrelcarbon fiber working electrodes previously described ( I ) . Carbon fibers (Type H28G, AERE Harwell, Oxfordshire, England) were aspirated into one barrel of a double-barrel microfilament capillary glass (GCF(2)-100-4,A-M Systems, Inc., Toledo, OH). After the pipet was pulled, but before the working electrode compartment was sealed with epoxy, the empty barrel was filled with water to ensure the tip of the reference chamber was not occluded by the epoxy. The water evaporated during the subsequent curing of the epoxy. An Ag wire coated with AgCl inserted into the open barrel comprised the reference electrode. Contact was made with the external solution by filling the reference barrel with normal saline49 70 NaCl (0.16 M). The macroelectrode consisted of carbon paste packed into a small well formed by force fitting Teflon tubing (1.5 mm i.d.) over a piece of copper wire (1.56 mm diameter). The paste was composed of carbon powder (UCP lM, Ultra Carbon, Bay City, MI) thoroughly mixed with mineral oil (25% mineral oil by weight). Apparatus. A polarographic analyzer (Princeton Applied Research Corp., Model 174A, Princeton, NJ) provided potentiostatic control and generated the waveforms for cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The polarograph was modified to allow use of faster scan rates in the pulse modes (I5,16). The time constants for the sample and holds involved in processing the current responses were decreased from 105 to 10.8 ms (R60 (220 kQ) was replaced with a 3.01-kQresistor and R232 was decreased from 21 to 2.16 kR). The scan rate for DPV was 20 mV s-l and that for CV was 100 mV s-l. Pulses of 25 mV were applied to the ramp every 0.5 s in DPV scans. Chronoamperometry was performed and evaluated with a locally constructed microcomputer interfaced with the electrochemical cell via the polarographic analyzer and a locally constructed amplifier. This system has been described in an earlier paper (1). Potential steps of 0.125 s duration were applied. Data collected at times shorter than 10 ms were not analyzed. The data have been corrected for nonfaradaic contributions to the observed current. For the carbon paste experiments, this involves subtraction of the current generated by a potential step in the
Figure 1. Schematic of a two-barrel microelectrode tip: Ag, AgCl coated Ag wire; C, carbon fiber; E, epoxy: G, glass capillary wall; Hg, mercury contact to the carbon fiber; S, saline solution.
supporting electrolyte/solvent solution. With microvoltammetric electrodes, the observation has been made that faradaic current on the back step of a double-potential step chronoamperometric experiment is negligible ( I ) . This is because species generated on the front step have diffused away from the vicinity of the electrode tip. Therefore, current from the back step has been used to correct for background contributions to the observed current at the carbon fiber microelectrodes. Due to the low current (PA) measured at microelectrodes, several methods have been used to enhance the signal-to-noise ratio. All experiments were conducted within a Faraday cage. Chronoamperometric data obtained at microelectrodes are presented as the result of an ensemble average of at least five runs. In addition, since the current response is essentially steady state at microelectrodes, the data presented for chronoamperometry are the time average of the current values measured during the last 10% of a run. As will be discussed more fully in a subsequent publication, by a judicious choice of step times 60 cycle noise from the back step can provide destructive interference with its counterpart in the front step to enhance the signal-to-noise ratio. The electrochemical cell consisted of a 30-mL bottle with a plastic top drilled to allow N2 purging and electrode immersion. The Ag/AgCl reference barrel of the microelectrode assembly provided the potential reference for both the carbon fiber and the carbon paste electrodes. The auxiliary electrode was a Pt wire. Temperature was maintained at 22 f 1 "C.
RESULTS Oxidation of DA, DOPAC, and AA. Of the three compounds examined, none undergoes totally reversible electron transfer at carbon paste as shown by CV. DA is more reversible than DOPAC, whereas AA is totally irreversible (Figure 2A-C). Comparison of CV a t microelectrodes and conventionally sized electrodes is difficult because of the steady-state behavior of the microelectrodes (Figure 2D-F). However, the results for DPV a t both types of electrodes are qualitatively equivalent ( I ) and are thus useful for comparison of the responses of the two electrodes (Table I). The peak potentials and width a t half-height obtained by DPV for the oxidation of DA a t carbon paste and carbon fiber electrodes are essentially the
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
chemical wave. The current (i) obtained a t a hemispherical electrode for the oxidation of species R is given by
where the current is in amperes, D is the diffusion coefficient of species R (cm2s-'), CRb is the bulk concentration of species R (mol ~ m - ~r )is, the electrode radius (cm), and the other terms have their usual electrochemical meaning. Equation 1 can approximately describe the chronoamperometric results and, more importantly, it predicts the predominantly steady-state behavior measured with carbon fiber electrodes (1).
Shain, Martin, and Ross have considered the equation for spherical electrodes where a potential step is made under conditions of charge-transfer control (20). For a hemispherical electrode, in the special case where the diffusion coefficients of the oxidized and reduced form are equal, the current can be shown to be
1 Figure 2. Comparison of cyclic voltammetry ( 100 rnV s-I scan rate) at carbon paste (A, B, C) and carbon fiber (D, E, F) electrodes: (A, D) DA, M; (B, E) DOPAC, M; (C, F) AA, M. Crosses in D and E were generated by using eq 3 with (1 - a ) = 0.5, k,A = 2.5 X cm s-', and k-,, =8X cm s-'.
same. However, for DOPAC and AA the carbon fiber response differs from that observed at carbon paste. At microelectrodes, the DOPAC DPV peak is displaced to more positive potentials and is much broader than at macroelectrodes. The peak for the oxidation of AA seen by DPV is also broader a t microelectrodes. The peak potential for AA at the microelectrode is variable, apparently depending on the surface state of the carbon. A surface wave that can be monitored by DPV in a solution containing only supporting electrolyte tends to appear in the vicinity of 0.0 V after repetitive voltage scans. The surface wave depends on the history of the carbon fiber electrode, becoming most evident after prolonged electrolysis a t potentials positive of about 0.4 V vs. Ag/AgCl, and is capable of modifying subsequent faradaic electrochemistry. When the surface wave is present, the oxidation of AA appears a t more negative potentials than in the absence of the wave. Similar shifts in potential are not observed for DOPAC or DA. Repeated short-time chronoamperometric experiments had little effect on the magnitude of the surface wave and on the position of the ascorbic acid peak. Voltammograms at Microelectrodes. An analytical solution for the model which best approximates carbon fiber microelectrodes, an isolated disk, has not been achieved because of the mathematical complexity ( I 7,18). Furthermore, because of the difficulty in construction of these electrodes and their small size, a precise definition of the geometry of the surface area of our electrodes cannot be achieved. Approximate predictions of the current can be obtained by considering the solution of the diffusion equations to a hemispherical electrode in an insulating plane. Solutions for this model are readily obtained by appropriately defining the boundary conditions and applying them to the flux equations derived in spherical coordinates. Diffusion to spherical electrodes has been considered for a wide number of controlled potential situations, including those where homogeneous chemical reactions affect the flux of electroactive species. For a complete review see ref 19. The simplest case to demonstrate this approach is a potential step to the diffusion controlled portion of an electro-
where Cob is the concentration of the oxidized form, k f i = k , exp[-(an,F/RT)(E -E0)], k b h = k, exp[(l - a)(n,F/RT)(EE?)], a is the electron-transfer coefficient, and n, is the number of electrons in the rate-determining step. For times longer than 100 ms at carbon fiber electrodes and in solutions containing only an oxidizable species, eq 2 reduces to
i=
-nF27rr2kbhC~b 1 f ( r / D ) ( b t , kbh)
(3)
Equation 3 is time independent and thus predicts the current-voltage curve for a hemispherical microelectrode at slow scan rates. As shown in Figure 2D eq 3 follows the current-voltage curve obtained at microelectrodes for DA. The values for n,[1 - a ] (0.5) and k, (2.5 x cm s-l) used for the calculation of the crosses were obtained from an analysis of the separation of peak potentials in the cyclic voltammogram obtained a t a carbon paste electrode (21). While it is not expected a priori that these parameters will be the same for DA at the two forms of carbon, Figure 2D demonstrates that eq 3 does predict the experimentally determined result. Similarly, the reduction of ferricyanide can be shown to have a similar heterogeneous charge-transfer rate at carbon paste and carbon fibers using our previously reported data (1). In contrast, the currentvoltage curve for DOPAC is not followed by eq 3 when the approximate parameters obtained from carbon paste electrodes are employed (k,= 8 X lo4 cm s-', n,[l - a] = 0.5) (Figure 2E). Catalysis of AA Oxidation. Cyclic voltammetry at a carbon paste electrode in a mixture of DA M) and AA M) demonstrates the effect of a classic EC homogeneous catalytic reaction. In the presence of ascorbic acid, the DA peak is larger than expected, the ascorbic acid oxidation wave is absent, and the wave for the reduction of DO$ is not apparent, Similarly, DPV of a mixture of these compounds gives one peak at the same potential as for DA alone (Table I). We have examined this reaction by using chronoamperometry at a carbon paste electrode. During an anodic step in a solution M) to 0.6 V vs. Ag/AgCl, sufof DA M) and AA
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2395
L
4-
current enhancement from the catalytic reaction a t a hemispherical electrode. For k,'t greater than 5 , this ratio is lk,' - kC"l2 +D1f2/r napp= - (6)
i
t -
3
I
, 0.05 t (si
0 10
Figure 3. Background corrected chronoamperometric results for the oxidation of DA (10-4 M) alone and DA (1O4 M) i- AA ( 10-3 M). ,Estep = 0.30 V, 0.125-s duration: (A) carbon paste; (B) carbon fiber.
ficiently positive to completely oxidize DA and AA at the electrode surface, the current from the mixture is the sum of the individual currents obtained in separate DA and AA solutions. In contrast, when the step is 0.3 V, where the DA surface concentration is completely oxidized but the concentration profile for AA is not completely polarized by the electrode, a current which is a function of the D A and AA concentrations is obtained. As can be noted, the current is not the sum of the chronoamperometric currents from steps in solutions of the individual components but reflects the homogeneous catalytic reaction (Table I). The rate constant, k,' ( k l = k c C u ) , for the DOQ, AA reaction can be calculated from chronoamperometric data given in Figure 3A by using eq 4, where nappis calculated from the ratio of the current at
time t for DA in the presence of AA to that of DA alone (22). This equation was derived assuming that the species being catalytically oxidized is not electroactive a t the applied potential, which is not the case for this system. T o partially offset this error in the determination of the rate constant, we digitally subtracted the AA contribution to the current from the current obtained in solutions containing mixtures of DA and AA. Solving for h, a t several different times in the experiment gives a rate constant of (3.2 f 0.3) X lo5M-' s?. This result is within the range predicted by Tse et al. ( 2 3 ) . Whereas electrochemistry a t the macroelectrode is dominated by the regeneration of DA, the sensitivity to the catalytic reaction at carbon fibers is significantly reduced. Two peaks can be discerned in D P V (Table I). Chronoamperometry a t a microelectrode at 0.3 V in the same DA, AA mixture results in a current which is only slightly larger than that of DA alone (Figure 3B). In this case, the oxidation current obtained in a AA solution at 0.3 V was not subtracted. Catalytic Reactions at Microelectrodes. Delmastro and Smith have derived the chronoamperometric result for the catalytic current arising for a reaction such as the oxidation of DA and AA (24). The derivation assumes a sufficiently high concentration of the material being catalytically oxidized is present so that its concentration is not depleted during the reaction. In addition, the diffusion coefficients of the oxidized and reduced forms are assumed to be equal. The solution of the flux equation for these conditions to a hemispherical electrode is given by eq 5 . Dividing eq 5 by eq 1 gives the
l / ( ~ t ) ' /+~D 1 / ' / r
When r >> D'f2,eq 6 gives the same result as eq 4 for k , / t > 5 . Equation 6 predicts that very little catalytic current should be observed at microelectrodes and approaches a limiting value (napp= 3.2 for a hemispherical electrode of 2.8 x lo4 cm radius and k,' = 3.20 X lo2 M-'s-' ), the result experimentally obtained.
DISCUSSION The qualitative agreement of our experimental data with the results predicted from solution of the flux equations for a hemispherical electrode demonstrate that this is a good model for electrodes fabricated from carbon fibers. Equation 3 enables a comparison of the degree of electrochemical reversibility a t microelectrodes with conventionally sized electrodes. Both ferricyanide and dopamine exhibit similar behavior at macro carbon paste and micro carbon fiber electrodes. DOPAC appears to be much more irreversible at the carbon fiber electrodes employed in this study. The best fit of eq 3 to the current voltage curve was obtained with n,[ 1 - C Y ] = 0.075, a value that is physically unrealistic. Since the model employed for these calculations is relatively simple, we will not attempt to interpret the physical significance of this result (for example, see ref 25 for a full description of hydroquinone oxidation a t a mercury electrode). (Simulation of the current-voltage curve for AA has not been attempted because of its extreme degree of irreversibility and complication with a rapid following chemical reaction (26).) It is interesting to note that eq 3 predicts exponential currentvoltage curves for electrodes of 100 times smaller radius than carbon fibers. The degree of electrochemical irreversibility obviously depends on the particular carbon material (29,the orientation of the graphitic crystallites (28),and the chemical functional groups on the surface (14). However, with both of the forms of carbon employed here, DOPAC is more irreversible than DA. Since the electrophore is identical for both of these compounds, it appears that the charge of the side chain plays an important role in the apparent charge-transfer rate. The data reported by Ponchon et al. ( 1 1 ) show a similar trendaromatic compounds which differ only in their alkyl side chain with a terminal carboxylate group rather than an amine are more difficult to oxidize a t carbon fibers, in agreement with our results. The extreme degree of irreversibility of DOPAC with the carbon fiber we have employed is actually advantageous since, at 0.3 V, measurement in solutions containing mixtures of DA and DOPAC will show a larger contribution from changes in the DA concentration. The dependence of the AA peak potential on the history of the carbon microelectrode correlates to the surface state of the fiber. The appearance of the surface wave of 0.0 V vs. Ag/AgCl appears to catalyze ascorbic acid oxidation. The variable response observed on repeated scans of AA emphasizes an important concept related to the use of solid electrodes. When it is not possible to constantly renew the surface of the electrode, it is important that the electrode is inactive with respect to solution electrolysis for the majority of the time (9,291. For this reason, short-time chronoamperometric ex-
2396
Anal. Chem. 1980, 52, 2396-2400
periments produce fewer changes in the electrochemical response than do scanning experiments such as semidifferential DPV (7), or CV (7). voltammetry (8), Determination of DA in the presence of ascorbic acid is clearly facilitated with microelectrodes, since contributions from catalytic chemical reaction are minimized. This result is predicted by eq 6 for hemispherical electrodes of identical radius. However, this result can also be rationalized by considering the rate of the catalytic reaction. Under the conditions employed here, the oxidized DA has a 2.1-ms half-life. Assuming that the diffusion coefficient for DOQ is 1X cm2sd, it will diffuse a little over 1.4 pm before being reduced to DA. This distance is much less than the radius of our carbon paste electrode (790 pm) but is only about half the radius of a carbon fiber electrode (2.8 pm). As has been shown, the minimization of catalytic reactions, t.he recognition of the irreversibility of DOPAC, and the shape of the current-voltage curves for well-behaved electrochemical systems can all be readily deduced from existing electrochemical theories. These relationships should lead to an optimization of the use of carbon fiber electrodes in specific applications. It is important to note that the observations reported here reflect the time scale of the measurements; faster measurements should result in conventional time-dependent currents a t microelectrodes.
LITERATURE CITED Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 5 0 , 946-950. Pletcher, D.; Fleishmann, M. University of Southampton, Southampton, England, personal communication, 1980. Adams, R. N. Anal. Chem. 1978, 48, 1126A-1137A. Wightman, R. M.; Strope, E.; Plotsky, p. M.; Adams, R. N. Nature(L0ndon) 1978, 262, 145-146. Kissinger, P. T.; Hart, J. B.; Adams, R. N. Brain Res. 1973, 55, 209-213. Marsden, C. A.; Conti, J.; Strope, E.; Curzon, G.; Adams, R. N. Brain Res. 1979, 171, 85-99. Cheng, Y.-Y.; Schenk, J.; Huff, R.; Adams, R. N. J. Electroanal. Chem. 1979, 100,23-31.
(8) Lane, R. F.; Hubbard, A. T.; Blaha, C. D. Bloelectrochem. Bioenerg. 1978, 5 , 504-525. (9) Lane, R. F.; Hubbard, A. T.; Blaha, C. D. J . Nectroanal. Chem. 1979, 95. 117-122. (10) Lane, R. F.; Hubbard, A. T.; Fukunaga, K.; Blanchard, R. J. Brain Res. 1978. 114. 346-352. (1 1) GOnOn, F.; Cespuglio, R.; Ponchon, J.-L.; Buda, M.; Jouvet, M.; Adams, R. N.; PujOl, J.-F. C.R. h b d . Seances, Acad. Sci., Ser. D 1978, 286, 1203-1206. (12) Ponchon, J.-L.; Cespuglio, R.; Gonon, F.; Jouvet, M.; Pujol, J.-F. Anal. Chem. 1979, 51, 1483-1486. (13) Loach, P. A. "Handbook of Biochemistry and Molecular Bblogy", 3rd ed.; Fasman, G. D., Ed.; CRC Press: Cleveland, 1976; Vol. 1, pp 122-130. (14) Evans, J. F.; Kuwana, T.; Henne, M. T.; Royer, G. P. J . Electroanal. Chem., 1977, 80, 409-416. (15) Abel, R. H.; Christie, J. H.; Jackson, L. L.; Osteryoung, J. G.; Osteryoung, R. A. Chem. Instrum. (N.Y.) 1978, 7 , 123-138. (16) Burrows, K. C.; Brindle, M. P.; Hughes, M. C. Anal. Chem. 1977, 49, 1459-1461. (17) Soos, 2. G.; Lingane. P. J. J . Phys. Chem. 1984, 68, 3821-3828. (18) Sarangapani. S.; DeLevle, R. J . Electroanal. Chem. 1979. 102, 165-174. (19) Galus, 2. "Fundamentals of Electrochemical Analysis"; Halsted Press: New York, 1976. (20) Shain, I.; Martin, K. J.; Ross, J. W. J. Phys. Chem. 1981, 65. 259-261. (21) Nicholson, R. S. Anal. Chem. 1985, 3 7 , 1351-1355. (22) Delahay, P. "New Instrumental Methods of Anaiysls"; Wiley-Interscience: New York, 1954 p 103. (23) Tse, D. C. S.; McCreery, R. L.; Adams. R. N. J . Med. Chem. 1978, 19, 37-40. (24) Delmastro, J. R.; Smith, D. E. J. Phys. Chem. 1987. 7 1 , 2138-2149. (25) Vetter, K. J. Z . Electrochem. 1952, 56, 797-806. (26) Perone, S. P.; Kretlow, W. J. Anal. Chem. 1988, 38, 1760-1763. (27) Lindaulst. J. J. Electroanal. Chem. 1974. 52. 37-56. i28j Wightman,RI M.; Paik, E. C.;Borman, S.;'Dason, M. A. Anal. Chem. 1978. 50, 1410-1414. (29) Lane, R. F.; Hubbard, A. T. Anal. Chem. 1978, 48, 1287-1293.
RECEIVED for review June 9, 1980. Accepted September 19, 1980. This research was supported by the National Science Foundation (Grant NO.BNS 77-28254). M.A.D. is a combined Medical-Ph.D. candidate, Indiana University. R.M.W. is the recipient Of a Research Career Award from the National Institutes of Health (Grant No. 1 KO4 NS 00356).
Determination of Sulfur Dioxide by Reaction with Electrogenerated Bromine in a Thin-Layer Cell Having a Gas-Porous Wall Stanley Bruckenstein," Kevin A. Tucker, and Paul R. Glfford Department of Chemistv, State University of New York at Buffalo, Buffalo, New York
An electrochemical sensor is described for the determination of ambient sulfur dioxide by reactlon with electrochemically generated bromine in a thin-layer cell. Sulfur dioxide in the gas phase diffuses through a porous, hydrophoblc membrane into a thin layer of solution in which bromlne is generated electrochemically. The quantity of bromine at the gas-solution interface is determined by reduction at a gold cathode sltuated on the solution side of the porous wall separating the gas phase from the solution phase. The dlffusion of sulfur dioxide through the porous membrane into the thin-layer cell decreases the amount of bromine reaching the gold cathode. This decrease produces a change In current that Is proportional to the concentration of sulfur dioxide in the gas phase. The devlce was evaluated for sensitivity and stabllity for the determination of parts-per-million levels of sulfur dioxide.
Electrochemical analyzers have been extensively used for
14214
monitoring ambient sulfur dioxide levels, and several commercial instruments using the "coulometric" principle are presently available. These instruments provide a sensitive method for the determination of sulfur dioxide and the technique has been defined in ref 1 as an equivalent method to the manual reference colorimetric procedure. The method was initially developed by Shaffer, Briglio, and Brockman for the detection of mustard gas (2) and was later extended to sulfur dioxide determinations. The principle behind these analyzers is based on the oxidation of sulfur dioxide to sulfate by electrogenerated halogen (iodine or bromine). A gas stream containing sulfur dioxide is bubbled through a stirred electrolyte containing halide ion (Br- or I-) in an electrochemical cell, and the halogen is generated by electrooxidation of the halide ion. The cell contains two pairs of electrodes. One pair consists of an indicator and reference electrode, and the emf between these two electrodes is determined potentiometrically; this emf is a measure of free halogen concentration in solution. The other
0003-2700/80/0352-2396$01,00/00 1980 American Chemical Society
