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Anal. Chem. 1984, 56, 136-141
Characterization of Electrochemically Pretreated Glassy Carbon Electrodes Royce C. Engstrom* and Vernon A. Strasser
Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069
Electrochemlcal pretreatment of glassy carbon electrodes was performed by applylng 1.75 V vs. SCE for 5 mln followed by -1.0 V for 1 mln. The effects of pretreatment on the functlonal, physlcal, and chemical characterlstlcs of the electrodes were studled. The pretreated electrodes showed enhanced electrochemlcal actlvlty, hlgher background currents, Increased wettabllity, and no change In electrochemlcal surface area or topography as determlned by scanning electron microscopy. X-ray photoelectron spectroscopy showed that pretreatment produces a surface more highly oxygenated than that of a freshly polished electrode. Electrochemlcal pretreatment cleans the surface of contamlnants Introduced In the polishing stage of electrode preparatlon and changes the chemlcal nature of the glassy carbon surface Itself. The chemlcal changes Influence the reactlon of ascorbic acid but do not Influence reactlon of the ferrlcyanlde-ferrocyanide system.
Several groups have noted that the performance of carbon electrodes can be improved by electrochemical pretreatment (1-12). In all cases, the pretreatment included the application of relatively large, positive potentials to the working electrode, and frequently, negative potentials were applied as well. The exact amplitude and duration of the applied potentials varied considerably, with many groups recommending the use of potential cycling or AC wave forms (4-6, 10). In a previous paper (I), we reported on the electrochemical pretreatment of glassy carbon electrodes. It was found that electrodes that had been freshly polished on emery paper and alumina showed relatively poor activity toward the oxidations of several electroactive species. The application of a potential greater than approximately 1.5 V vs. SCE for 5 min, followed by the application of approximately -1.0 V for a brief time resulted in voltammetric waves that were more sharply defined than their counterparts obtained at freshly polished electrodes. It was observed that the anodization step of the pretreatment produced a layer on the electrode that inhibited the oxidation of ferrocyanide, but that the cathodization step reduced the layer, leaving a surface with increased activity. The reason electrochemically pretreated carbon electrodes show increased activity over freshly polished electrodes is unknown. It has been suggested that pretreatment introduces or alters the nature of functional groups on the electrode surface (8) and that such groups might serve as mediators of electrons between the electrode and the electroactive species (13). Quinone functionalities appear to be likely candidates as mediators, since there is substantial evidence suggesting the presence of such groups on oxidized carbon surfaces (13-20). Alternatively, it had been proposed that on carbon paste electrodes, anodization causes the removal of organic pasting liquid from the carbon surface, rendering the electrode more hydrophilic and therefore more accessible to solution species (3). Another possible explanation is that electrochemical pretreatment might simply serve to clean the electrode of contaminants introduced in the polishing stage of electrode preparation.
The purpose of the work reported here was to further characterize the effects of electrochemical pretreatment on glassy carbon electrodes. The electrodes were studied with respect to changes in their functional, physical, and chemical characteristics resulting from pretreatment. The techniques used in this study were voltammetry a t the rotating-disk electrode, cyclic voltammetry, contact-angle measurements, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The electrochemical pretreatment under study consisted of applying to a freshly polished electrode 1.75 V vs. SCE for 5 min followed by -1.0 V for 1 min. The thesis of Myers represents a characterization of glassy carbon sufaces and the effects of chemical treatment on the surface (21). Several of the techniques we have used in the present study were used by Myers.
EXPERIMENTAL SECTION Apparatus. Electrochemical experiments were performed on a laboratory-built rotating-disk electrode apparatus, connected to either a PAR Model 174A polarographic analyzer (EG&G Princeton Applied Research, Princeton, NJ) or a Model CV-1B cyclic voltammetry instrument (BioanalyticalSystems, Inc., West Lafayette, IN). The model 174A was interfaced to a HewlettPackard Model 9825A calculator (Palo Alto, CA) via an AMS, Inc., Model 704 interface (Lake Elmo, MN), which was programmed to perform background substraction (22). Voltammetric data were displayed on a Hewlett-Packard Model 9872A digital plotter. Working electrodes were short lengths of 3 mm diameter rods (Grade 30s, Tokai Mfg., Tokyo) epoxied into cast acrylic housing or press-fitted into Teflon housing. The reference electrode was a saturated calomel electrode (SCE) and all potentials reported herein refer to that reference. A platinum wire served as auxiliary. Scanning electron microscopy was carried out on an IS1 Alpha-9 (Santa Clara, CA) housed in the University of South Dakota School of Medicine. Electrodes studied by SEM were mounted onto the top of an aluminum specimen holder with a copper-based conducting epoxy (Epoxy Technology, Inc., Billerica, MA). The arrangement permitted ready use of the electrode in an electrochemical cell and in the electron microscope. X-ray photoelectron spectroscopy was carried out at the NSF Regional Instrumentation Facility for Surface Analysis at the University of Minnesota. The instrument was a Physical Electronics Industry Model 555 (Eden Prairie, MN) equipped with a Mg K a X-ray source (1254 eV). The resolution of the instrument was approximately 0.05 eV in the high-resolution mode. A PDP-11/04 computer (Digital Equipment Corp., Marlborough, MA) was used in conjunction with the XPS instrument. In the high-resolution mode, signal averaging was performed until suitable signal-to-noise characteristics were obtained. All highresolution spectra shown here were corrected for inelastic backscattering and were obtained with charge compensation in effect. Reagents. Supporting electrolyte was 0.10 F potassium nitrate unless stated otherwise, prepared from a reagent grade salt and water purified by distillation and purification on a commercial cartridge system (Barnstead Nanopure, Boston, MA). Potassium ferricyanide,potassium ferrocyanide, ferrous ammonium sulfate, hydroquinone, catechol, and ascorbic acid were reagent grade. Iron(II1) solutions were prepared from high-purity iron wire. Nicotinamide adenine dinucleotide (NAD) and dihydronicotinamide adenine dinucleotide (NADH) were obtained in the purest form available from Sigma Chemical Co. (St. Louis, MO). 1,4Dihydroxynaphthalene was obtained from Aldrich Chemical Co.
0 1964 American Chemical Society 0003-2700/64/0356-0136$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
I
Table I. Effect of Electrochemical Pretreatment on Half- Wave Potentials of Various Electrochemical Reactions at the Rotated-Disk Electrode
electroactive species 00
-05
-IO
137
-15
Applied Potential, volts Flgure 1. Effect of electrochemical pretreatment on the reduction of oxygen at (a) a freshly polished electrode and (b) a pretreated electrode: supporting electrolyte, air-saturated 0.10 F potassium nitrate; scan rate, 10 mVls; rotation rate, 500 rpm.
(Milwaukee, WI). 1,4-Benzoquinoneand 1,Cnaphthoquinonewere obtained from Eastman Kodak (Rochester, NY). Solutions of electroactive species were prepared fresh daily at a concentration of 1.0 mM. Procedure. Electrodes were hand-polished for 1 min on each of the following media: 4/0 emery paper (3M, St. Paul, MN), 0.3 pm alumina and 0.05 pm alumina (Sargent-Welch, Skokie, IL). All polishing media were kept wet with distilled-deionized water during polishing. The electrodes were rinsed copiously with water between steps and at the end of the polishing. Experiments were usually performed on freshly polished electrodes and then again after the electrode was pretreated as specified in the introduction. A t all times, care was taken to ensure that the electrode surface was never touched with a tissue or allowed to dry during an electrochemical experiment. Voltammetry at the rotating-disk electrode was performed at a scan rate of 10 mV/s and at a rotation rate of 500 rpm. Background processes were studied by use of cyclic voltammetry at a scan rate of 100 mV/s. Contact angle measurements were made by using a sessile drop method (23). The electrode was removed from the electrochemical cell, rinsed with water, and clamped in an inverted position to air-dry. A 2.0-pL drop of water or 0.10 F potassium nitrate was placed on the electrode surface with a chromatography syringe. The drop and the electrode were photographed from the side by using a close-up lens, and contact angles were measured from enlargements of the developed photographs. Measurement of oxygen evolution during anodization was performed with a Yellow Springs Model 53 dissolved oxygen meter (Yello Springs, OH). An electrochemicalcell was arranged so that the sensing element of the oxygen meter was situated less than one millimeter away from the surface of the glassy carbon electrode. Currents through the glassy carbon electrode and the oxygen probe were recorded simultaneously.
RESULTS AND DISCUSSION Effect of Pretreatment on Electrochemical Reactions. As has been shown ( I ) , electrochemical pretreatment causes the half-wave potential of several anodic reactions to shift to less anodic values than those observed a t freshly polished electrodes. The effect of pretreatment on a variety of electrochemical oxidations and reductions is shown in Table I, where half-wave potentials are listed for reactions studied at freshly polished and pretreated rotated-disk electrodes. Upon pretreatment, the half-wave potential for most species shifted to less extreme values; however, the magnitude of the shift depended to a great degree on the electroactive species. For example, the reduction of oxygen showed a particularly dramatic dependence on pretreatment as illustrated in Figure 1. At the freshly polished electrode (trace A), one well-defined wave was observed with a half-wave potential of -0.68 V. A second wave, beginning at -1.2 V, was largely obscured by background decomposition. After pretreatment (trace B), two waves were discernible with half-wave potentials of -0.31 V and -0.52 V. The limiting current of the second wave at the pretreated electrode was 1.95 times the limiting current of the
E,,, at E,,, at prepolished treated electrode, electrode, v vs. v vs. SCE SCE
AEuz
Oxidations hydroquinone 0.332 1,4-dihydroxynaphthalene 0.038 catechol 0.391 ascorbic acid 0.165 Fe(I1) in 0.1 M H,SO, 0.683 NADH 0.341 ferrocyanide 0.172 phosphatea 0.210 citrate 0.265 pyrophosphate 0.169
0.187
-0.145
-0.054 0.323 0.060 0.391 0.220 0.166 0.170 0.202 0.155
-0.092 -0.068 -0.105 -0.292 -0.121
Reductions -0.194 -0.407 0.016 -1.406 -0.68 -1.2 0.154 0.117 0.132 0.146
-0.197 -0.401 0.291 -1.353 -0.31 -0.52 0.155 0.158 0.152 0.155
-0.003 0.006 0.275 0.053 0.37
1,4-benzoquinone 1,4-naphthoquinone Fe(II1) in 0.1 M H,SO, NAD 0, (wave 1) (wave 2) ferricyanide phosphatea citrate pyrophosphate
-0.006
-0.040 -0.063 -0.014
0.7
0.001 0.041 0.020 0.009
a These buffering components were added at 0.01 F in addition to the 0.10 F KNO,. The pH was adjusted to
1.0.
single well-defined wave a t the freshly polished electrode, suggesting that the complete reduction of oxygen took place at the pretreated electrode, whereas only the partial reduction took place a t the freshly polished electrode. The species dependence of pretreatment effectiveness can be important even between both members of a redox couple. For example, the half-wave potential for the oxidation of hydroquinone shifted by 145 mV upon pretreatment, whereas the half-wave potential for the reduction of 1,Cbenzoquinone did not change significantly. A similar effect was seen for the naphthoquinone-l,4-dihydroxynaphthalene system; the anodic wave shifted by 92 mV and the cathodic wave shifted by only 6 mV. Another interesting comparison can be made between the iron(I1)-iron(II1) system and the ferricyanide-ferrocyanide system. The former showed a profound dependence upon pretreatment while the latter showed a much smaller dependence. Comparisons such as these suggest that pretreatment influences the specific chemical interactions associated with the electrochemical process, such as adsorption or changes in hydration of the electroactive species. The effectiveness of pretreatment also appears to be influenced by the supporting electrolyte composition, as shown by the data for the ferricyanide-ferrocyanide system in Table I. The buffering component (present at a total concentration of 0.010 F and adjusted to pH 7.00) influenced the change in half-wave potential associated with pretreatment. In fact, the effect of pretreatment was least when the buffering component was absent. Such observations have been noted previously, where anodization of pyrolytic carbon in citrate-acetate media resulted in activation, but anodization in acetate alone was not effective (11). An explanation for the observed supporting electrolyte composition effects is not apparent a t this time, but probably involves changes in the double-layer structure connected with the anions of the buffering component. Background Processes. Coincident with enhancement of electrochemical kinetics following pretreatment are changes
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
h a
Y b
Figure 2. Background processes at the glassy carbon electrode: (a) freshly polished electrode, (b) initial scan after anodization, (c) steady-state scan after anodization; scan rate, 100 mV/s; initial potential, 0.0 V; anodic limit, 1.50 V; cathodic limit, -1.05 V.
in the nature of the background current. Figure 2 shows cyclic voltammograms of the glassy carbon electrode taken in 0.10 F potassium nitrate without any electroactive species present. Trace A shows that the voltammogram obtained at a freshly polished electrode is relatively featureless except for the anodic and cathodic background decomposition currents. Trace B shows the first complete cycle obtained after anodization of the electrode a t 1.75 V for 5 min. (Note the change in sensitivity.) A signficantly lowered background current was observed upon initiating the scan in the anodic direction. When the potential reached negative values for the first time, a cathodic process of large amplitude resulted. Once the cathodic process was complete, the background current showed a substantial increase. Trace C shows the voltammograms that resulted after several additional cycles had been made. As compared to trace A, the background current was about twice that observed a t the freshly polished electrode. In addition, a surface-bound reversible redox process appeared at -0.2 V, and the decomposition potentials for both the anodic and cathodic limits shifted to less extreme values. Several aspects of this behavior have been reported previously. The existence of a surface bound redox couple has been observed on glassy carbon electrodes (2,18,19) and on other forms of carbon (16,17) and has usually been attributed to the presence of a surface quinone-hydroquinone system. The only notable difference between the behavior observed in this work and that of others is that the surface-bound couple appears to be reversible here, whereas it is usually reported to be quasi-reversible. The degree of reversibility would no doubt be a function of the source of the carbon and the composition of the supporting electrolyte. The increase in background current at the anodized electrode has been noted before also. Bjelica et al. reported that the differential capacity of glassy carbon increased by more than an order of magnitude upon anodic preoxidation, resulting in increased background currents (7). Similar effects were noted for glassy carbon electrodes that had been oxidized chemically (24). The large, cathodic peak obtained upon the first negative-going scan was reported earlier (1). It appears to reflect the reduction of a multilayer film formed during anodization, most likely an “oxide” of some sort. The film inhibits the normal background processes at the electrode, as well as inhibiting some electrochemical processes as well ( 1 ) . Current during Anodization. The current through a freshly polished electrode during anodization in 0.10 F potassium nitrate was monitored and is shown in Figure 3a. When the applied potential was switched to 1.75 V, a current spike resulted, due to charging of the double layer. Rather than showing a monotonic decay, the current began to increase
J
Figure 3. Current during anodization of a glassy carbon electrode: (a) anodization of freshly polished electrode, (b) a second anodization of the same electrode; applied potential, 1.75 V.
after abdut 30 s. A maximum was reached at about 1.4 min, followed by a gradual decrease to a steady value. Figure 3b is the current-time trace for anodization of the same electrode a short while later, without repolishing. During the second anodization, the maximum did not appear; the current simply decreased to the same steady-state value achieved in Figure 3a. These results indicated that the freshly polished electrode undergoes activation with respect to some electrode process taking place at the anodization protential of 1.75 V. Once the electrode had been activated, the anodic process could occur at its maximum rate immediately upon switching to the anodization potential. The process responsible for the majority of the current in Figure 3 is probably the oxidation of water to produce oxygen. An experiment was performed to confirm that oxygen evolution was taking place during anodization. A cell was assembled that permitted the placement of a dissolved oxygen probe directly opposite and a few tenths of a millimeter away from the glassy carbon surface. The currents through the glassy carbon electrode and the dissolved oxygen probe were monitored simultaneously. The output of the dissolved oxygen meter showed an increase that began immediately upon application of the anodization potential, showing conclusively that oxygen was actively evolved during anodization at 1.75 V. The possibility that electrode activation results from indirect oxidation of the electrode surface by oxygen produced during the anodization step was tested. Pure oxygen was bubbled over a freshly polished electrode in 0.10 F potassium nitrate for 15 min in the absence of an applied potential. No change in any voltammogram was observed as a result of bubbling oxygen, indicating that the presence of oxygen alone is not sufficient to activate the electrode. Surface Area and Topography of the Electrode. The increased background currents observed after pretreatment (Figure 2) and the current behavior during anodization (Figure 3) could be interpreted as an increase in the true surface area of the electrode upon pretreatment. Previous workers have noted that glassy carbon electrodes subjected to prolonged electrolysis at extreme positive potentials showed roughening of the surface (7). The same has been noted for pyrolytic carbon (11)and an increase in capacity of anodized pyrolytic
ANALYTICAL CHEMISTRY. VOL. 56, NO. 2. FEBRUARY 1984
A
B Flgure 4. Scanning electron micrographs of (A) a freshly polished electrode and (6)a pretreated electrode. Magnlncatlon is 15 OOOX. The ~ a in p Me Iwrizontal lines at the bottom of each micrograph represent 0.5 pm carbon was taken as evidence of an increase in true surface area (I 7). Our results showed that the electrode surface area does not increase as a result of the pretreatment process used here. Electrode areas were determined by measuring the rotation rate dependence of the limiting current for the reduction of ferricyanide. Plots of limiting current vs. the square root of rotation rate were prepared. The slopes of the plots obtained from a least-squares fit were 3.28 0.02 FA s-'12 and 3.12 i 0.02 WAs112before and after pretreatment, respectively. A second electrode yielded slopes of 2.73 f 0.01 pA s-112 and 2.72 i 0.01 WAs-1/2. The uncertainties shown are the standard deviations obtained from the leastsquares tit. In both cases, the slope, and thus the electrode area, did not increase as a result of electrochemical pretreatment. Perhaps the somewhat higher potential of 2.2 V used by Bjelica et al. accounts for their observation of surface roughening (7). The surfaces of the glassy carbon electrodes were examined by scanning electron microscopy (SEM). Examples of micrographs obtained a t freshly polished and pretreated electrodes at a magnification of 15000X are shown in Figure 4. Two important conclusions can he drawn from the micrographs. First, a small amount of alumina appears to remain on the surface after polishing. However, most of the surface was devoid of alumina particles; the micrograph shown in
*
+
139
Figure 4A was taken of an area particularly dense with alumina particles. Considering the very small fraction of the electrode surface actually covered by alumina, it does not seem likely that alumina particles play a significant role in the behavior of the systems studied here. A second conclusion drawn from the micrographs is that no significant change in the surface topography results from pretreatment. This is in agreement with the above observation that the electrode surface area showed no increase. Electrode Wettability. Contact-angle measurements can be used to measure the degree of interaction between a solid and a liquid (23). For example, contact-angle measurements were made recently on platinum electrodes that had been coated with poly(viny1ferrocene) films to show that the wettability of the film increases upon anodic oxidation (25). In our work, contamangle measurements were made on g h y carbon electrodes before and after electrochemical pretreatment, and the results show that the electrode became more hydrophilic upon pretreatment. When a 2-wL drop of 0.10 F potassium nitrate was placed on a freshly polished electrode, the average contact angle of four replicates was 72' with a standard deviation of 7". After pretreatment, the average value was 54' (standard deviation of 6"). With 2-sL drops of distilled-deionized water, the contact angles were 66O (14') and 49" (4') before and after pretreatment, respectively. The signifcant decrease in contact angle upon pretreatment clearly shows that interaction between the aqueous phase and the electrode surface is increased after pretreatment. It is known that decreases in contact angles can he due to surface roughness (23),hut the results from the previous section rule out that explanation. Thus, the changes in contact angles shown here can he attributed to changes in the chemical nature of the surface due to electrochemical pretreatment. Chemical Characterization of Electrode Surface. T h e glassy carbon electrode surface was subjected to analysis by XPS after polishing, after anodization a t 1.75 V for 5 min, and again after cathodization at -1.0 V for 1 min. Each analysis consisted of obtaining a low-resolution survey spectrum and high-resolution spectra of the C(1s) and the O(1s) regions. The survey spectrum of the freshly polished surface showed the presence of fluorine (F(1s)at 690 eV and F (Auger) a t 604 eV), oxygen (O(ls) at 532 eV), and carbon (C(1s)a t 284 eV). No peak in the vicinity of 73 eV was detectable, indicating the absence of aluminum on the surface. This observation agrees with that found by SEM, where only a small amount of alumina was noted. The only change in the survey spectrum arising during the course of the experiment (other than the changes in signal amplitudes to be discussed below) was the appearance of a barely detectable nitrogen signal (401eV) and a barely detectable silicon signal (157eV and 105 eV). The origin of these signals is unknown. The fluorine signal in the survey spectra resulted from the geometric arrangement of the sample. The 3 mm diameter glassy carbon electrode was surrounded by a Teflon housing to give a total diameter of 9.5 mm, which was the size necessary for the XPS instrument. The area sampled by the instrument was somewhat larger than the area of the glassy carbon: consequently signals arising from the Teflon housing were observed. Teflon was chosen as a shroud material because its C(1s) signal would be shifted to higher hinding energy from the signal of the glassy carhon, permitting distinction between C(1s) signals arising from the electrode and housing. The high-resolution spectra of the C(ls) region and the O(ls) region for the steps involved in the pretreatment procegs are shown in Figure 5. The C(ls) spectra showed a high binding-energy peak (291eV) attributable to carbon aaMciated with Teflon and a low binding-energy peak (264 eV) awxiated
140
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
538
526 298
278
Binding Energy, eV Flgure 5. High-resolution spectra of the O(1.s) and C(1s) regions: (a) freshly polished; (b) after anodizatlon; (c) after cathodization.
with the glassy carbon. The oxygen-to-carbon ratio of the electrode surface was calculated by fitting the Teflon-carbon peak to a normal distribution and subtracting its area from the C(1s) spectrum. The remaining area was determined by manual integration, as was the area under the O(1s) peak. After correcting for differences in sensitivities to oxygen and carbon (26), we calculated the O/C ratios. The ratios were 0.084, 0.22 and 0.22 for the freshly polished, anodized, and cathodized electrode, respectively. The increase in the O/C ratio shows that the surface became oxygenated during pretreatment, which is compatible with the notion that oxygencontaining functional groups can be created on the surface of glassy carbon electrodes through anodization. T h s observation that the ratio remained unchanged upon cathodization indicated that those groups may be reduced, but not removed, by cathodization. This observation is in agreement with the conclusions of Laser and Ariel (19) and is consistent with the appearance of a surface-bound redox couple discussed earlier. The O/C ratio of 0.22 observed here is similar to the value obtained by Kuwana and co-workers a t glassy carbon electrodes that had been subjected to oxygen plasma treatments (27,28),where oxidation of the electrode surface was presumed to occur. The O/C ratio we obtained at the freshly polished electrode was somewhat lower than their “polished” value of 0.14, however. The high-resolution spectra of the C(1s) region show that the amount of carbon of Teflon origin (291 eV) decreased throughout the pretreatment process. Apparently, Teflon was carried across the electrode surface during the polishing stage (the effects of carry-over are discussed below) and application of extreme potentials of either sign caused removal of the Teflon film. The small Teflon-associated signal remaining after cathodization probably arose from the Teflon housing itself. The major peak in the C(1s) spectrum (284.6 0.2 eV) changed in shape as a result of pretreatment. The width a t half-maximum height went from 1.9 eV at the polished electrode to 2.7 eV and 2.4 eV at the anodized and cathodized electrodes, respectively. This peak broadening has been interpreted to mean that the environment of surface carbon becomes more complicated upon pretreatment. A noticeable shoulder appeared in the anodized and cathodized spectra at
*
approximately 288 eV, indicating the presence of a distinct form of carbon having higher binding energy than the remainder of the carbon associated with the electrode. However, it is not safe to say that the shoulder arose from the pretreatment since there appears to be a higher binding-energy component (288-289 eV) at the freshly polished electrode as well, although less well-defined. The high-resolution O(1s) spectra showed an increase in amplitude over the course of the pretreatment as noted above by the O/C ratios. The peak shape showed no significant change throughout the experiment. The Polishing Process. An effort was made to determine the source of electrode deactivation that takes place during the polishing stage. The above XPS data indicated that housing carry-over occurred during polishing. Previous workers have indicated that housing carryover significantly decreased the reversibility of electrochemical reactions at glassy carbon electrodes and indicated that glass housing material was preferable over Kel-F in that regard (29). An experiment was performed to investigate the influence of housing carry-over on electrode performance. An electrode was prepared in which the glassy carbon could be pushed out so that its surface extended out from the Teflon housing during polishing and could be pushed back in so that its surface was flush with the housing surface during the electrochemical experiment. In this way, the electrode could be polished without the possibility of housing carry-over. At the freshly polished electrode, a voltammogram of 1.0 mM hydroquinone yielded a half-wave potential of 0.35 V, and after pretreatment, the value was 0.22 V. Ascorbic acid showed similar effects; the polished electrode yielded a half-wave potential of 0.28 V and pretreatment decreased the value to 0.072 V. In both cases, electrochemical pretreatment caused a decrease in the half-wave potential even when housing carry-over could not have taken place. Therefore, if carry-over does occur under ordinary polishing conditions, it is not the primary cause of deactivation of the electrode. Significant deactivation of the electrode does result from contaminants associated with the emery polishing paper and the polishing felt used in the alumina stages of polishing. This was determined by polishing electrodes with silicon carbide powder (500 mesh) on a glass plate followed by polishing with the two alumina slurries, also on glass plates. For the ferricyanide-ferrocyanide system, background-corrected voltammograms obtained at glass-polished electrodes were identical with those obtained at conventionally polished electrodes that had been pretreated. Furthermore, pretreatment did not alter the voltammograms obtained at the glass-polished electrodes. These results show that a primary function of electrochemical pretreatment is one of cleaning the glassy carbon of contaminants introduced in the conventional polishing process. Other electroactive species showed a dependence upon electrochemical pretreatment even at electrodes polished on glass plates. The voltammogram of ascorbic acid oxidation showed a decrease in half-wave potential of 60 mV when the glass-polished electrode was pretreated. The reduction of oxygen still showed behavior similar to that in Figure 1even when electrodes were polished on glass. Clearly, electrochemical pretreatment must also cause chemical changes in the electrode surface beyond those associated with simple cleaning of the electrode. Relatively simple electrochemical reactions, such as those of the ferricyanide-ferrocyanide system (30),would be expected to be less dependent on the surface chemistry of the electrode than more complicated reactions such as those of ascorbic acid and oxygen. Whether or not electrochemical pretreatment is indicated depends upon the conditions of polishing and the nature of the electrochemical system under study. For the ferricyanide-ferro-
Anal. Chem. 1984, 56, 141-747
cyanide system, a pretreated glassy carbon electrode offers no advantage over a clean electrode, and in fact the increased background current a t pretreated electrodes would be a disadvantage.
ACKNOWLEDGMENT Scanning electron micrographs were obtained with the help of Dan Neufeld of the University of South Dakota School of Medicine. The help of John Evans, Ed Bowdin, and Ganapathy Swami of the University of Minnesota is greatly appreciated. Registry No. Fe, 7439-89-6; 02, 7782-44-7; KNOB,7757-79-1; C, 7440-44-0; NADH, 58-68-4; NAD, 53-84-9; hydroquinone, 571-60-8; catechol, 120-80-9; 123-31-9;1,4-dihydroxynaphthalene, ascorbic acid, 50-81-7;ferrocyanide,13408-63-4;1,4-benzoquinone, 106-51-4; 1,4-naphthoquinone,130-15-4;ferricyanide, 13408-62-3. LITERATURE CITED Engstrom, R. C. Anal. Chem. 1982, 54, 2310. Gunasingham, H.; Fleet, E. Analyst (London) 1982, 107, 896. Rice, M. E.; Galus, Z.; Adams, R. N. J. Electroanal. Chem. 1983, 743, 89. Gonon, F. G.;Fombarlet. C. M.; Euda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386. Falat, L.; Cheng, H. Y. Anal. Chem. 1982, 5 4 , 2111. Van Rooijen, H. W.; Poppe, H. Anal. Chim. Acta 1981, 130, 23. Ejelica, L.; Parsons, R.; Reeves, R. M. Croat. Chem. Acta 1980, 53. 211. Elaedel, W. J.; Jenkins, R. A. Anal. Chem. 1974, 4 6 , 1952. Elaedel, W. J.; Jenkins, R. A. Anal. Chem. 1975, 4 7 , 1337. Elaedel, W. J.; Mabbott, G. A. Anal. Chem. 1978, 50, 933.
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Wlahtman. R. M.; Paik, E. C.; Eorman, S.; Dayton, M. A. Anal. Chem. 19?8, 5 0 , 1410. Eggll, R. Anal. Chlm. Acta 1978, 9 7 , 195. Evans, J. F.; Kuwana, T.; Henne, M. T.; Royer, C. P. J. Nectroanal. Chem. 1977, 8 0 , 409. Hallum, J. V.; Drushel, H. V. J. Phys. Chem. 1958, 6 2 , 110. Jones, I. F.; Kaye, R. C. J. Nectroanal. Chem. 1989, 2 0 , 213. Drushel, H. V.; Hallum, H. V. J. Phys. Chem. 1958, 62. 1502. Epstein, B. D.; Dalle-Molle, E.; Mattson, J. S. Carbon 1971, 9. 609. Blurton, K. F. Nectrochlm. Acta 1973, 78, 889. Laser, D.; Ariel, M. J. Electroanal. Chem. 1974, 52, 291. Evans, J. F.; Kuwana, T. Anal. Chem. 1977, 4 9 , 1632. Myers, Robert E. Ph.D. Thesis, Duke University, 1980. Strasser, V. A. Master's Thesis, Universlty of South Dakota, 1983. Adamson, A. W. "Physical Chemistry of Surfaces", 4th ed.; Wiiey: New York, 1982; p 341. Taylor, R. J.; Humffray, A. A. J. Electroanal. Chem. 1973, 42, 347. Willman, K. W.; Murray, R. W. Anal. Chem. 1983, 55, 1139. Riggs, W. M.; Parker, M. J. "Methods of Surface Analysis", Czanderna A. W., Ed.; Elsevier: Amsterdam, 1975; p 112. Evans, J. F.; Kuwana, T. Anal. Chem. 1979, 57, 358. Miller, C. W.; Karweik, D. H.; Kuwana, T. Anal. Chem. 1981, 53, 2319. Anderson, J. E.; Tailman, D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051. Hale, J. M. "Reactions of Molecules at Electrodes"; Hush, N. S., Ed.; Wiley-Interscience: New York, 1971; p 247.
RECEIVED for review August 1, 1983. Accepted October 24, 1983. This work was supported in part by a Northwest Area Foundation Grant of the Research Corporation and by the University of Minnesota NSF Regional Instrumentation Facility, Grant No. NSF CHE-7916206. This work was presented in part at the 1983 ACS Annual Meeting in Washington, DC.
Data Analysis for Concentration Measurements in the Nonlinear Response Region of Ion-Selective Electrodes Ravi Jain and Jerome S. Schultz* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109
Various methods for the determination of concentratlons In the nonllnear response region of Ion-selectlve electrodes are developed. Data analysls Is done with a computer program based on Marquardt's method for determlnlng the nonllnear parameters. For the multiple standard addltlon method, It Is shown that a high degree of correlatlon between the model and the experlmental data Is needed for accurate results. Nonlinear callbratlons do give good results, even when the correlatlon between the model and the data Is only falr. The regression procedures that minlmlze the least-squares error may not work well because they may demand a degree of correlatlon not possessed by the orlglnal data. Measurements on the chlorlde and the nitrate electrodes are Included to compare various methods. The methods discussed In this paper can be easlly Implemented and can substantlally Increase the worklng range of Ion-selective electrodes wlthout sacrlflce of accuracy.
During the past 2 decades developments in the area of ion-selective electrodes have made them viable alternatives for measuring various ions as well as the gases that are converted to ions in solution. Although electrodes can respond
to changes in concentrations until a statistical limit of detection is reached ( I ) , measurements are usually limited to the linear region of response. The lower limits of detection depend on different factors for different electrodes. For halide and similar solid-state electrodes, this limit is determined by the solubility of the membrane; for electrodes employing liquid ion exchangers, the limit is determined by the distribution of the exchanger between the membrane and the solution phase. Other factors, such as the impurities in the reagents and the interference from other ions in the solution, also affect the detection limit, as discussed by Midgley (1). Midgley (1) also points out that the detection limit for most electrodes extends to far lower concentrations than the linear or Nernstian response limit. The concentrations in the linear region of electrode response are normally measured with either a calibration curve or the method of known standard addition. Multiple additions of a standard solution can also be performed to improve the accuracy (see Mascini (2) for a review of these techniques). For most cases these techniques work reasonably well. Measurements in the nonlinear region of electrode response can be made to fully utilize the capabilities of the electrodes. Among the few published methods for doing this are the graphical calibration methods and the methods by Midgley (3), Parthasarathy et al. (4), and Frazer et al. (5). Because
0003-2700/84/0356-0141$01.50/00 1984 American Chemical Society
