890
Anal. Chem. 1984, 56,890-894
Zn2+,Pb2+,and Cd2+in concentrations ranging from 5 x to 2 X M with excellent analytical results (thus, e.g., the coefficient of variation for ten successive determinations of Cd2+at the M level was *0.8%). Hence, chronopotentiometry with a linear sweep of current at the DME appears to be a method with great analytical possibilities and has the additional advantage over classical chronopotentiometry at stationary electrodes with constant current of a clean and reproducible surface electrode. Finally, it is also interesting to show that eq 17 hints at the advantage of using the function I ( t ) = Iot7l6for DME chronopotentiometry because in this case the relationship between 7 and C is linear.
APPENDIX In eq 8 the u&A) functions are obtained by using the dimensionless parameter method previously described (2,5-7). Thus, we have CO,O(sA)
=
cA*
(AI)
PI = -
04
=-
3 ( 6 -~ 7) CA*
27
6w(3w
(-44)
+ 4) CA*
11P6w/7P(6w+l)/7
and the #'s are the functions defined by Kouteckjr (11). In turn, the biJ(sg) are obtained in an analogous way. Finally, from eq Al-A4 and taking into account that fii(0) = 1 ( l l ) , we find eq 10. Registry No. Cd, 7440-43-9; Pb, 7439-92-1; Zn, 7440-66-6; mercury, 7439-97-6.
LITERATURE CITED (1) Bard, Allen J.; Faulkner, Larry R. "Electrochemical Methods", Wlley:
New York, 1980; Chapter 7 and references therein. (2) Galvez, J. J . Electroanal. Chem. 1982, 732, 15. (3) Oldham, Keith B. Anal. Chem. 1969, 4 7 , 936. (4) Chow, Ll Hang; Ewing, Galen W. Anal. Chem. 1979, 57,322. (5) Galvez, J.; Molina, A. J . Electroanal. Chem. 1983, 746, 221. (6) Galvez, J.; Saura, R. J . Elecfroanal. Chem. 1983, 746, 233. (7) Galvez, J.; Fuente, T.; Mollna, A,; Saura, R. J . Nectroanal. Chem. 1983, 746, 243. (8) Heyrovsky, J.; Kuta, J. "Prlnclples of Polarography";Academic Press: New York, 1966; p 92. (9) Galvez, J.; Mollna, A,; Fuente, T. J . Nectroanal. Chem. 1980, 707, 217. (IO) Heyrovsky, J.; Kuta, J. "Prlnclples of Polarography";Academic Press: New York, 1966; p 542. (11) Kouteckf, J. Czech. J . Phys. 1953, 2 , 50.
where
RECEIVED for review September 29,1983. Accepted December 30, 1983. We thank the Comision Asesora de Investigacidn Cientifica y Tbcnica for supporting this study (Projects 321/81 and 854/81).
and
Spatial Resolution of Electrode Heterogeneity Using Iontophoresis Royce C. Engstrom Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069 The physlologlcal technique of iontophoresls has been adapted to the study of mlcroscoplc heterogeneity on solid electrode surfaces. Mlcroplpets havlng tlp dlameters of less than 1 pm were filled wlth a solution of potassium ferrocyanide and positioned over solid worklng electrode surfaces with a mlcroposltlonlng device. Electrophoretlc ejectlon of ferrocyanlde Ions was made onto a localized area of an electrode poised at 0.5 V vs. SCE. Electrochemical reaction of ferrocyanlde resulted In a translent Faradalc current, the amplltude of whlch was taken as a measure of the mlcroscoplcally local electrode actlvlty. The characterlstlcs of the technlque were studled wlth platinum mlcroelectrodes and epoxy-Impregnated retlculated vitreous carbon electrodes. Actlve reglons as small as 10 pm could be detected and two-dlmensional mapping of lndlvldual actlve regions was accompllshed wlth a spatial resolution of 10 pm.
I t is recognized that solid electrodes can present heterogeneous surfaces with respect to electrochemical activity. For
example, heterogeneity on an atomic level might arise from crystal defects or the presence of foreign adatoms. On a microscopic level or larger, heterogeneity might arise from adsorption processes, the presence of surface oxides, surface topography, microcrystallite orientation, or even deliberate electrode design. The first reports on the subject of electrode heterogeneity (1,2)recognized that deviations from expected electrochemical behavior could result from heterogeneity, and the extent of deviation depended on the sizes of active and inactive regions compared to the dimensions of the diffusion layer. Theoretical treatments of electrode heterogeneity have appeared (3-8) and attempt to explain the effects of heterogeneity on rotated-disk voltammetry, chronopotentiometry, chronoamperometry, linear potential sweep voltammetry, and cyclic voltammetry. In some cases, the theories have been tested through the use of model heterogeneous electrodes (4, 6, 7, 9-12). The theoretical treatments have been used to obtain estimates of the dimensions of heterogeneities at solid electrodes. Measuring the rotation-rate dependence of limiting current, Landsberg and Thiele estimated that their carbon paste
0 1984 American Chemical Society 0003-2700/84/0356-0890$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
electrodes had active regions with equivalent diameters of 11-38 pm (3). In another case, an anodized graphite electrode was reported to contain active regions with average radii of 4.1 pm, in inactive regions with radii of 8 pm (9). Adsorption of various alcohols onto platinum reportedly resulted in heterogeneous electrodes with active regions of 51-105 pm (13). Soviet researchers, relying on the models proposed by Landsberg and co-workers (3), have reported adsorption-related heterogeneities with dimensions in the range of 5-100 pm (14-19). Weisshaar and Tallman have used the theory of Gueshi et al. (6) to characterize composite electrodes made from graphite and KeL-F (20). Using chronoamperometry, they estimated the average equivalent radii of active sites to be in the range of 2-15 pm (depending on electrode composition) surrounded by inactive regions with equivalent radii of 40-96 pm. The models mentioned above necessarily make assumptions concerning the geometry and uniformity of active and inactive regions. As a result, estimates of dimensions that rely on those models yield average dimensions, without any information regarding true site geometry. Knowledge of true active site geometry, size, and distribution should help in understanding the role of electrode heterogeneity. T o obtain this knowledge, a technique is needed that has the capability of “mapping” an electrode surface with respect to its electrochemical activity. Spatial resolution of electrochemical activity has been applied to the study of corrosion processes (21-23). Using scanning microelectrode techniques, Issacs et al. studied the formation and propagation of pits resulting from corrosion of metal surfaces. In the work we are reporting here, a physiological technique called iontophoresis has been adapted to the characterization of electrode heterogeneity with spatial resolution in the micrometer range. Iontophoresis was introduced in 1953 by Nastuk as a physiological tool used to probe the sensitivity of biological membranes to pharmacological agents (24). The technique permits the application of an ionic chemical reagent to a microscopic area of a surface and has therefore been used to provide a correlation between physiological activity and microanatomical structure (25,26). In iontophoresis, a micropipet having a tip diameter of less than 1p m is fabricated and filled with a concentrated solution of an electrolyte. A nonpolarizable electrode, typically a chloridized silver wire, is inserted into the large diameter end of the micropipet. Another, external, electrode is placed in the solution into which the micropipet tip is dipped. Ordinarily, the solution would contain a submerged biological specimen. The tip of the micropipet is positioned within a few micrometers of the sample surface with the aid of micropositioning devices and a microscope. Passage of electrical current between the two nonpolarizable electrodes results in the migration of ions out of the micropipet tip. The amount of material “ejected” depends on the ejection current amplitude and duration, as well as the transference number of the ion of interest. The response of the sample surface to the action of the ejected material is measured (for example, as a change in the membrane potential of the biological sample). The sample surface activity can be “mapped” by moving the micropipet incrementally between ejections. The resolution of iontophoresis has been shown to be in the low micrometer range (27,28). In the adaptation of iontophoresis to the study of electrode heterogeneity, micropipets were filled with a solution of potassium ferrocyanide. A micropipet was positioned so that its tip was within a few micrometers of the electrode under study. A current pulse through the micropipet caused the ejection of electroactive ferrocyanide ions onto a microscopic area of the working electrode under study. The working electrode was held a t a potential capable of oxidizing ferro-
I ‘
I
Difference ArnDlifier
891
I
I
I l l
I
Micropositioner
R2
Flgure 1. Dlagram of Iontophoresis instrumentation.
cyanide, so a transient Faradaic current resulted as the ferrocyanide reached the electrode surface. A Faradaic response of relatively large amplitude resulted if the micropipet was positioned over an active region, whereas a smaller response or no response resulted if the micropipet was positioned over an inactive region. The position of the micropipet was moved incrementally over the surface of the working electrode, repeating the ejection and recording the Faradaic transient at each position. In this way, it was possible to map an electrode surface with respect t o its activity toward the oxidation of ferrocyanide.
EXPERIMENTAL SECTION Apparatus. A block diagram of the instrumentation system is shown in Figure 1. A brief description of the various components will be given here, A complete schematic diagram will be furnished on request. Micropipets were prepared by pulling capillary tubing (“Kwikfil”,WP Instruments, Inc., New Haven, CT) on a commercially available micropipet pulling apparatus (Model 700C, David Kopf Instru., Tujunga, CA). The micropipet tips were filled by immersing the large diameter end in a solution of 0.55 M potassium ferrocyanide. An inner filament in the capillaries caused their tips to fill within a few minutes, after which the f i i g of the micropipet body was completed with a syringe. A short length of platinum wire was inserted into the large diameter end of the micropipet to serve as the internal electrode. Another short length of platinum wire mounted in the Plexiglas cell compartment served as the counterelectrode in the iontophoresis circuit. Current pulses were made to flow through the micropipet (M) and its counterelectrode (C) with a current pulse generator (Model 161 MicroiontophoresisProgrammer, WP Instruments, Inc., New Haven, CT). The generator was capable of applying “ejection” currents of up to 1000 nA. When not ejecting, the generator applied a constant “braking current” to minimize diffusion of ferrocyanide ions from the micropipet. The duration of the ejection current pulse was set by the triggering circuitry, which was built in the laboratory around a monostable multivibrator integrated circuit (SN74121). The ejection duration could be varied continuously from 10 ms to 500 ms. The ejection current pulses were monitored by measuring the potential difference across a 100-kB resistor (Rl) in series with the micropipet and counterelectrode. An instrumentation amplifier circuit, made with two AD523 voltage followers and an AD510 difference amplifier (Analog Devices, Norwood, MA), sensed the potential difference across the resistor, which was recorded on one channel of a dual-channel digital storage oscilloscope (Model 054100, Gould, Inc., Cleveland, OH). Positioning of the micropipet was controlled by a three-dimensional translational stage (Model MR 50, Klinger Scientific Corp., Jamaica, NY). The manufacturer’s specifications rated the accuracy of movement at under 2 wm in each direction. Micropipet positioning was performed with the aid of a microscope operated at a magnification of 40X (Galen Model, Bausch and Lomb, Rochester, NY). The micropositioner, the cell container, and the microscope were all mounted on a marble slab sitting on
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ANALYTICAL CHEMISTRY, VOL. 50,NO. 6, MAY 1984
inner tubes to provide vibration isolation. These components were also housed in a Faraday cage which was opened to position the micropipet and closed during the ejection and recording of the Faradaic response. The working electrode (W) to be studied was housed in a Plexiglas cylinder with a diameter of 7 mm. This cylinder was fitted into the Plexiglas cell container, so that the working electrode surface was flush with the bottom of the solution cavity of the cell container. Supporting electrolyte solution was placed in the cell container to a depth of about 3 mm. The tip of a saturated calomel reference electrode was immersed in the solution, and a submerged platinum wire served as auxiliary electrode (A) in the conventional three-electrode circuit. Applied potential was controlled by a laboratory-built potentiostat with the potentiostating operational amplifier chosen for its low noise characteristics and high slew rate (AD504, Analog Devices, Norwood, MA). Current through the working electrode was measured with a high-speed current amplifier (Model 427, Keithley Instruments, Inc., Cleveland, OH). The output of the current amplifier was recorded on the second channel of the storage oscilloscope and then transferred to a strip chart recorder. The variable resistor (R2) between the working electrode and the current amplifier served to minimize capacitive coupling of the relatively large ejection current through the working electrode. The working electrode for experiments not involving spatial resolution was the cross section of a platinum wire having a diameter of 1mm. The wire was epoxied into a Plexiglas housing and machined to a flat surface. The final polishing step was 0.05-pm alumina, giving the surface a microscopically smooth finish. For demonstration of spatial resolution, arrays of platinum microelectrodes were prepared by embedding platinum wires having diameters of 100 pm or 25 pm (A. D. Mackay, Inc., Darien, CT) in epoxy (Epotek 320, Epoxy Technology, Inc., Billerica, MA). The surfaces of these array electrodes were machined flat and polished to a smooth finish. The array electrodes thus consisted of active regions of platinum with diameter of 100 pm or 25 pm, surrounded by inactive epoxy. A heterogeneous carbon electrode was prepared by mounting a small sliver of reticulated vitreous carbon (RVC) having 100 pores/in. (Fluorocarbon Co., Anaheim, CA) into a Plexiglas housing. When the RVC sliver was epoxied into the housing, care was taken to impregnate the end of the RVC sliver with epoxy. Upon curing of the epoxy, the end of the sliver was polished to a finish that was microscopicallyflat. The finished electrode consisted of exposed, irregularly shaped areas of carbon surrounded by a matrix of epoxy. The geometric area of the electrode was approximately 0.04 cm2,but the total active area of the electrode was only a few percent of the geometric area. Reagents. The salts used to prepare the 0.10 M potassium nitrate supporting electrolyte and the 0.55 M potassium ferrocyanide micropipet solution were of reagent grade and used without further purification. Solutions were prepared with water that had been distilled and further purified on a cartridge system (Nanopure, Barnstead Co., Boston, MA). Procedure. While micropipets were being filled with potassium ferrocyanide solution, the electrochemical cell was filled with supporting electrolyte and a potential of 0.5 V vs. SCE was applied. When the cell achieved a steady-state current, a micropipet was mounted on the arm of the three-dimension translational stage. The micropipet was moved into position with the aid of the microscope. During vertical positioning, occasional ejection current pulses were given and the Faradaic response was recorded. The distance between the micropipet and the working electrode surface was gradually decreased until the Faradaic response was judged to be of sufficient amplitude. Each data point in the mapping of an electrode surface consisted of adjusting the micropipet horizontal position, triggering the ejection current pulse, recording the Faradaic response on the oscilloscope and transferring the data to the recorder. Each data point required approximately 15 s for completion. The peak amplitudes were measured manually from the strip chart recording. RESULTS AND DISCUSSION Response t o E j e c t e d M a t e r i a l . Figure 2 shows the cur-
rent-time traces of an iontophoretic ejection of ferrocyanide and the resulting Faradaic response obtained a t a platinum working electrode at 0.5 V vs. SCE. The ejection current
LnA 500 mS
Figure 2. (A) Ejection current pulse and (B) Faradaic response for iontophoresis of ferrocyanide ions Onto platinum working electrode at 0.5 V vs. SCE.
I
f Applied Potential, V
Figure 3. Potential dependence of Faradaic response: (A) “iontophoresls” voltammogram showing amplitude of Faradaic response to ejection of 5 X equivalents of charge; (9)steady-state voltammogram of 1.O mM potassium ferrocyanide in 0.10 M potassium nitrate.
amplitude was 500 nA and the duration was 34 me, resulting in the ejection of 1.8 X equivalents of charge. Each mole of ferrocyanide carries 4 equivalents of charge in the ejection process and the transfer coefficient for ferrocyanide was calculated to be 0.71 (29). Therefore, it was estimated that 3.2 X mol of ferrocyanide was ejected. Integration of the Faradaic response of Figure 2 yielded 3.6 X equivalents. Since the Faradaic reaction of ferrocyanide involves mol of ferrocyanide was a one-electron transfer, 3.6 X detected in this case. The moles detected account for 11 % of the moles ejected. The fairly low percentage is not surprising, since once ejected the ferrocyanide ions are free to diffuse in all directions, not just toward the working electrode. Figure 2 also gives an indication of the noise level in the system, which was generally 0.1-0.5 nA. The artifact immediately preceding the Faradaic response results from coupling of the ejection current to the working electrode. Its severity differs somewhat from one micropipet to the next, but the example shown here is typical. It was important to verify that the responses such as that in Figure 2 were actually due to the Faradaic reaction of ferrocyanide at the working electrode. The response amplitude was measured as a function of the potential applied to the platinum working electrode. The results are shown in Figure 3, along with a bulk solution voltammogram of 1 mM potassium ferrocyanide taken in the same cell. the “iontophoresis” voltammogram is very similar to the steadystate voltammogram of ferrocyanide in bulk solution, indicating that the response to iontophoresis is Faradaic. As a result of this experiment, the working electrode potential was set at 0.50 V vs. SCE for all remaining experiments.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
893
3or
W YI C
a W
ir
- .
. .
- ' "$68
3'44 3'36 328 ?~60 ;52 Micropositioner Settlng , urn
A Distance
Flgure 4. Dependence of Faradaic response amplitude on dlstance between micropipet tip and electrode surface. Ejection current was 500 nA for 160 ms.
Reproducibility. The precision of the iontophoresis experiment was evaluated a t three levels: the ejection current pulse, the Faradaic response with a stationary micropipet, and the Faradaic response when the micropipet position was varied. For 30 replicate ejections with a nominal current pulse amplitude of 500 nA and duration of 30 ms, the relative standard deviation of the ejection pulse amplitude was 1 % and that of the duration was 4%. With the micropipet in a stationary position over a platinum working electrode, multiple ejections of ferrocyanide were made with six different micropipets. Between 25 and 30 Faradaic responses were recorded with each pipet. The relative standard deviations of response amplitude for the six pipets were 1 2 , 8 , 4 , 4 , 7 , and 10%. Finally, a pipet was moved in 10-pm increments over a smooth platinum electrode, covering an area 100 pm by 50 pm. A total of 50 data points were taken, and the relative standard deviation of the mean Faradaic response was 7% Ejection Current Amplitude and Duration. The amount of ejected electroactive material can be controlled by adjusting the ejection current amplitude and duration. Faradaic response amplitude vs. ejection current amplitude was linear over the ejection amplitude range of 0 to 1000 nA. The slope of the line was 0.0139 nA response/nA of ejection current (standard deviation of 0.002), or 8.0 X 10l2 nA/ equivalent when the pulse duration of 150 ms is taken into account. Based on a signal-to-noise ratio of 2, the detection limit resulted when the ejection current amplitude was about 100 nA. The amount ejected at the detection limit was 1.5 X equivalents of charge, and the amount of ferrocyanide detected was just a few femptomoles. The effect of ejection pulse duration was determined by applying ejection currents of 500 nA with varying durations. A plot of Faradaic response amplitude vs. ejection pulse duration had a slope of 0.128 nA/ms (standard deviaton of 0.008), or 2.3 X 1013nA/equivalent. In this case, the minimum detectable response resulted from a 10-ms ejection duration, corresponding to 5.3 x equivalents of charge. The difference in the slopes of these two experiments expressed in nanoamperes per equivalent is not surprising considering the sensitivity of the response to the distance between micropipet and electrode surface, as shown in the next section. I
Distance between Micropipet Tip and Electrode Surface. It was found that the distance between the micropipet tip and the working electrode surface (the vertical position of the micropipet) had a profound influence on the amplitude of the Faradaic response. Because of the micropipet tip size, it was impossible to determine accurately the vertical position by microscopic observation. Therefore, the amplitude of the Faradaic response was measured as a function of the vertical micropositioner setting and is plotted in Figure 4. The absolute values of the vertical positions shown in the figure bear no relationship to the micropipet-electrode distance. The shape of the curve shows that as the micropipet tip was moved closer to the electrode (toward the left on the x axis) the Faradaic response amplitude increased up to a
LL
,...'
Distance
... B
Flgure 5. One-dlmensional iontophoresis of (A) three platinum electrodes of 100 p m diameter spaced approximately 350 p m apart (ejection current was 30 nA for 27 ms); (B) two platinum microelectrodes of 100 p m diameter separated by 80 p m (ejection current was 300 nA for 30 ms).
point. Movement of the micropipet closer than a setting of 362 pm resulted in no further increase, indicating that the micropipet tip was probably resting on the electrode surface at that point. One could then assign an absolute vertical micropipet position of zero to the micropositioner setting of 362 pm. Movement of the micropipet away from the electrode (to the right on the x axis) by even 2 pm resulted in a measurable decrease in response amplitude. Clearly, to maximize response amplitude, the tip should be placed within just a few micrometers of the electrode surface. It is estimated that with practice, the micropipet tip could routinely be placed within 2-3 pm of the surface. Great care must be taken in the vertical positioning; more than a few micropipet tips have been broken upon contacting the electrode surface! Spatial Resolution of Electrode Activity. The resolving power of iontophoresis was first studied by mapping arrays of platinum microelectrodes prepared as described in the Experimental Section. One-dimensional mapping of electrode surface activity is illustrated in Figure 5, where Faradaic response amplitudes are shown vs. micropipet position for single passes over adjacent platinum microelectrodes. Figure 5A resulted from iontophoresis of three microelectrodes having nominal diameters of 100 wm and separated by approximately 250 pm. The three microelectrodes were detected iontophoretically and showed up as peaks in the Faradaic response amplitudes. The widths of the peaks at half-maximum height are 103,94, and 107 pm, which is in good agreement with the nominal diameter of the platinum electrodes. The smaller height of the third peak probably indicates a change in the distance between the electrode surface and micropipet tip over the 1 mm distance that Figure 5A represents. Figure 5B is the one-dimensional iontophoresis of two platinum electrodes of 100 pm diameter, spaced only 80 pm apart. Base line resolution of the two electrodes was obtained. The boundaries of the platinum electrodes as determined microscopically compared well with the boundaries measured iontophoretically. Microscopically, the four boundaries were observed at micropositioner settings of 145, 260, 340, and 440 pm. The positions of half-maximum response read from Figure 5B (at the arrows) are 148, 245, 347, and 435 pm. Two-dimensional mapping of a single platinum microelectrode is shown in Figure 6. In this case, the microelectrode had a nominal diameter of 25 pm, and the micropipet was moved in 10-pm increments. The vertical axis of Figure 6 represents the Faradaic response amplitude, and each intersection on the plot corresponds to a single data point. From
894
0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
IO
20
30
40
50
60
x , m Flgure 6. Two-dimensional iontophoresis of a single platinum electrode with dlameter of 25 pm. Ejection current was 320 nA for 22 ms. Each intersection represents a single data point, placed at 10-pm intervals in both directtons.
A
Distance
Flgure 8. Two-dimensional iontophoresis of a slngle active region of epoxy-impregnated reticulated vitreous carbon electrode, shown in "digitized" form (seetext). Each square represents a single data point, placed at 10-pm intervals in both directions.
and active site distribution plots such as Figure 7 should prove valuable in extending the applicability of theories involving electrode heterogeneity. To our knowledge, these plots represent the first spatial chracteristics of electrode activity a t the micrometer level.
Note Added in Proof. Heterogeneous electrodes based on reticulated vitreous carbon such as those used here were recently described by Slesynski, Osteryoung, and Carter (30). Registry No. Platinum, 7440-06-4; carbon, 7440-44-0; potassium ferrocyanide, 13943-58-3. LITERATURE CITED
B
Dlstance
Figure 7. One-dimensional iontophoresis of two areas of an epoxyimpregnated reticulated vitreous carbon electrode. Ejection current was 600 nA for 32 ms.
the shape of Figure 6, it is apparent that electrochemical activity is localized to an area on the electrode surface having dimensions of just a few tens of micrometers. Iontophoresis can be used to determine both the number of active regions on an electrode surface and the shape of an individual active region. The epoxy-impregnated reticulated vitreous carbon (epoxy RVC) electrode, described in the Experimental Section, possessed a surface having irregularly shaped active regions with a wide variety of dimensions. Figure 7 shows one-dimensional mappings of two different regions of the surface taken with spatial increments of 10 pm. The same micropipet was used in both cases; however, it was apparently positioned closer to the electrode surface for trace B, resulting in larger amplitude responses and an improved signal-to-noise ratio. Nevertheless, the positions and dimensions of active regions can be readily determined from such one-dimensional plots. It is important to note that active regions as small as 10 pm can be detected. A two-dimensional map of a single, irregularly shaped active region on the epoxy-RVC electrode is shown in Figure 8. A contour map such as that of Figure 6 was first obtained, and the map was redrawn in a digitized fashion. All data points generating a response greater than five times the noise level of the system (chosen arbitrarily) were plotted as a tall rectangle whose center lies a t the appropriate X-Y coordinate. Responses less than five times the noise level were plotted as a short rectangle. The plot clearly defines the shape and boundaries of the active region with a resolution of 10 pm. Detailed two-dimensional maps such as the one in Figure 8
Vetter, K. J. 2.Phys. Chem. (Wlesbaden) 1952, 799,300. Llopis, J.; Fernandez-Biarge, J.; Fernandez, M. P. Electrochim. Acta 1959, 1 , 130. Landsberg, R.; Thiele, R. Elecfrochim. Acta 1988, 7 7 , 1243. Scheller, F.; Muller, S.; Landsberg, R.; Spltzer, H. J. J. Electroanal. Chem. 1988. 79. 187. Levart, E.; Schuhmann, D.; Contamin, 0.; Etman, M. J. Electroanal. Chem. 1976. 70. 117. Gueshi, T.; Tokuda, K.; Matsuda, H. J. Electroanai. Chem. 1978,89, 747. Gueshi, T.; Tokuda, K.; Matsuda, H. J. Nectroanal. Chem. 1979, 707, 29. Reller, H.;Klrowa-Eisner, E.; Glleadi, E. J. Electroanal. Chem. 1982, 138, 65.
Scheller, F.; Landsberg, R.; Wolf, H. Electrochim. Acta 1970, 75, 525. Llndemann, J.; Landsberg, R. J. Electroanal. Chem. 1971,2 9 , 261. Lindemann, J.; Landsberg, R. J. Electroanal. Chem. 1971, 30, 79. Scheller, F.; Landsberg, R., Muller, S. J. Electroanai. Chem. I Q W 20, 375. Lindemann, J.; Landsberg, R. J. Nectroanal. Chem. 1971, 3 7 , 107. Povarov, Y.; Eroskina, L.; Lukovstev, P. Nektrokhlmiya 1988, 4, 464. Povarov, Y.; Trukhan, A.; Lukovstev, P. Elektrokhlmiya 1970, 6, 602. Trukhan, A.; Povarov, Y.; Lukovstev, P. Nekfrokhimiya 1970,6, 425. Barbasheva, I.; Povarov. Y.; Lukovstev, P. €lektrokhlmiya 1970. 6, 175. Povarov, Y.; Lukovstev, P. Nektrokhlmiya 1971, 7, 1715. Povarov, Y.; Lukovstev, P. Nectrochim. Acta 1973, 78,13. Weisshaar, D. E.; Tallman, D. E. Anal. Chem. 1983, 55, 1146. Isaacs, H. S.; Kissel, G. J. Electrochem. SOC. 1972, 779,1628. Isaacs, H. S.; Kendlg, M. W. Corrosion 1960, 3 6 , 269. Isaacs, H. S. Locallzed Corros. 1980, 36, 269. Nastuk, W. Fed. Proc., Fed. Am. SOC. Exp. Biol. 1953, 12, 102. Nastuk, W. L., Ed. "Physical Techniques in Blologlcal Research"; Academic Press: New York, 1964; Vol. 5. Dennls, M.; Harris, A,; Kuffler, S. Proc. R . SOC.London, Ser. B 1971, 777, 500. Peper, K.; McMahan, U. Proc. R. SOC.London, Ser. B 1072, 787, 431. Kuffler, S.; Yoshlkaml, D. J. Physlol. 1975,2 4 4 , 703. Bockrls, J. O'M.; Reddy, A. K. N. "Modern Electrochemistry"; Plenum Press: New York, 1970; Vol. 1, p 400. (30) Sleszynski, N.; Osteryoung, J.; Carter, M. Anal. Chem. 1984, 56. 130.
RECEIVED for review December 15,1983. Accepted February 1,1984. This work was supported in part by a Northwest Area Foundation Grant of the Research Corporation. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
