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Anal. Chem. 1985, 57, 150-155
Pretreatment and Validation Procedure for Glassy Carbon Voltammetric Indicator Electrodes D. C. Thornton,’ K. T. Corby? V. A. Spendel: and Joseph Jordan Department of Chemistry, T h e Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802 Albert Robbat, Jr.,* and D. J. Rutstrom
Department of Chemistry, T u f t s University, Medford, Massachusetts 02155 Maurice Gross and G. Ritzler
Laboratoire d’Electrochimie et de Chimie Physique d u Corps Solide, Universite Louis Pasteur, Strasbourg, France
Metallographic pretreatment procedures are described which yleid surfaces whose eiectrochemlcaiiy active areas are Identical wlth their geometrlc areas. Glassy carbon indlcator electrodes (GCE) were used successfully in a stationary mode for cycllc voltammetry, In Levich’s rotated mode for hydrodynamlc voltammetry, and In both modes for dlfferentlai pulse voltammetry. Excellently reproduclble anodic and cathodlc current-voltage curves were obtained in aqueous and nonaqueous solvents with a varlety of inorganlc, organic, and biologlcal electroreactlve moieties. Validation crlterla for the GCE Include not only specified half-wave potentials but also electrochemicalrate constants In a Judlclously selected model system. Uslng as such 0.005 M ferrlcyanlde In 1 M aqueous KCI, the recommended speclflcation Is k o ‘ = ( 7 f 1) X IO-* cm s-’ at El,* = 0.226 V vs. SCE which compares wlth k o ’ = (8 f 2) X IO-‘ cm s-l reported at platlnum Indlcator electrodes, docurnentlng that thls represents a genuinely meaningful assignment.
The classical dropping mercury electrode (DME) and the hanging drop mercury electrode have unmatched advantages of reproducibility, because of their renewable surface. Furthermore, their working cathodic range of potentials (e.g., -1.8 V vs. SCE in neutral and basic aqueous solutions) compares favorably with solid voltammetric indicator electrodes due to the exceptionally high hydrogen overvoltage prevailing a t mercury cathodes. On the other hand, the anodic working range of the DME is severely limited by the dissolution of mercury occurring between zero and +0.6 V (depending on pH, supporting electrolyte, etc.). A synopsis of potential ranges of various electrode materials in aqueous and nonaqueous solutions is available in a recent monograph ( I ) . More than 15 years ago the use of so-called “glassy’! or “vitreous” carbon indicator electrodes was pioneered independently by investigators in several countries ( 2 , 3 ) .It became immediately evident that glassy carbon had the widest potential range of any known electrode material (3)and significant advantages over other forms of carbon used in electroanalytical chemistry. Thus, Adams’ ingenious carbon paste electrode ( 4 ) has an electrochemically active area which differs appreciably from its geometric area; pyrolytic graphite exhibits electrochemical kinetics which depend on the nature of the exposed crystalPresent address: Department of Chemistry,Drexel University, Philadelphia, PA 19104. 2Present address: E. I. du Pont de Nemours & Co., Wilmington, DE 19898. Present address: Procter and Gamble, Cincinnati, OH 45202.
lographic plane (5). Other graphite electrodes are permeable to liquids and have, customarily, been impregnated by wax for electrochemical use (6). On the other hand, glassy carbon is totally impermeable (nonporous). Its crystal structure is well characterized and consists of domains of trigonal carbon linked by tetragonal carbon atoms (7,8). Nevertheless, the literature of the 1970s is replete with reports ( S 1 2 )suggesting that electrooxidation-reduction kinetics (as well as associated wave analysis slopes, half-wave potentials, peak potentials, etc.) on glassy carbon depended greatly on prepolarization conditions, as well as on surface oxidation-reduction and adsorption phenomena. Indeed, these handicaps are common to all solid electrodes and account for the better reproducibility of DME. There is no prior reason to expect lesser reproducibility a t glassy carbon, provided an electrochemically “equireactive” surface is used in all experiments (13). Metallographic polishing procedures, developed on the basis of judicious considerations of the relevant physics, are described in this paper. They yield invariably reproducible electroreactive surfaces as has been documented by the determination of “true” (rather than “apparent”) electrochemical rate constants. These assignments are the same as the values obtained on other electrode materials (e.g., platinum) as indeed predicted by the Marcus theory (14). Thus, the procedures in this paper may represent a contribution toward converting solid electrode voltammetry from an empirical art into a more rigorous science. The scope of this report is represented by results obtained under a wide variety of experimental conditions, including (a) use of the glassy carbon electrode in the stationary mode for cyclic voltammetry and controlled potential chronoamperometry, (b) its use in Levich’s rotating mode for hydrodynamic voltammetry, (c) its use in both modes for normal and differential pulse voltammetry, and (d) aqueous and nonaqueous solvent-electrolyte systems further illustrating the exceptional quality of the current-voltage curves and the general applicability of the glassy carbon electrode. EXPERIMENTAL SECTION Equipment. A conventional three-electrode system was used in all electrochemical measurements. The electrolysis cell was jacketed and maintained at 25.0 f 0.1 O C by circulation of water from a constant temperature bath. The glassy carbon (grade GC 10) for the indicator electrode was obtained from Tokai Electrode Manufacturing Co. (Tokyo, Japan). The counterelectrode was a platinum wire. Three reference electrodes were used, viz., a conventional saturated calomel electrode (SCE) for experiments in aqueous solutions, a modified SCE, saturated in NaCl (in lieu of KCl), in a mixed DMF-water solvent containing perchloric acid as supporting electrolyte, and a (0.1 M) Ag+/Ag reference half-cell for experiments in the nonaqueous solvent acetonitrile. The relevant potentials (at 25 “C), referred t o the normal hydrogen
0003-2700/85/0357-0150$01.50/0@ 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
electrode, were conventional SCE, +0.242 V, modified SCE, +0.236 V, Ag+ (0.1 M)/Ag in acetonitrile, +0.581 V. Voltammetric measurements were obtained with EG + G PARC Models 173, 174, 175, and 179 electrochemical systems. The IR compensator was adjusted via the positive feedback control located on the Model 179 digital coulometer to the point just before “ringing” in the cyclic voltammograms and activation of the overload signal light was observed. Current-voltage curves were obtained with a Nicolet Explorer IIIA digital oscilloscope and plotted on a Houston Instruments Model 2000 X-Y recorder. A Pine Instruments Co. (Grove City, PA) analytical rotator and a Hall-effect driven rotating electrode assembly (15)were used in the hydrodynamic voltammetry experiments. Metallographic polishing equipment and abrasives (including a polisher/grinder unit, silicon carbide papers, polishing cloth, diamond abrasives, and alumina polish) were purchased from Buehler Ltd., Chicago, IL. Reagents. Spectral-grade acetonitrile obtained from Burdick and Jackson Laboratories (Muskegon, MI) was dried by refluxing over calcium hydride for 15 h and collected over $-A molecular sieves. N,N-Dimethylformamide (DMF), reagent grade, was purchased from the Fisher Scientific Co. (Fair Lawn, NJ) and, after storage over molecular sieves (Linde, 4 &, also purchased from Fisher Scientific),was deaerated with nitrogen and distilled in vacuo under a very slow stream of nitrogen. The “middle cut” was stored refrigerated under nitrogen and wa5,used within 72 hours after distillation. Water used in making solutions was triply distilled. Concentrated (70%)perchloric acid (analytical reagent grade obtained from Mallinckrodt Inc., St. Louis, MO) and ammonium hydroxide (ACS Reagent, Fisher Scientific) were used without further purification. Sodium perchlorate, tetra-n-butylammonium perchlorate (TBAP), potassium chloride, ferricyanide, and ferrocyanide were recrystallized before use, while ferrocene was sublimed prior to use. Dibenzothiophene and hematoporphyrin IX dihydrochloride (ICN Group, Cleveland, OH) were used without further purification. Metallographic Features and Preparation of Glassy Carbon Electrodes. Previous preparation methods described in the literature have included electrochemical pretreatments (cycling of electrode potential), chemical pretreatments (rinsing with nitric acid and distilled water), electroplating of mercury followed by its removal, or just simply polishing with an alumina suspension between experiments. Each of these methods could yield current-voltage curves which were reproducible per se, but results differed depending on the pretreatment. The goal of the present study was to prepare an electrode surface which yielded rigorously reproducible current-voltage curves, as well as meaningful electrochemical thermodynamic and kinetic assignments. We found that a so-called ”metallographic” polishing technique was best suited for this purpose. Glassy carbon can be purchased from several suppliers using different carbonaceous source materials and heat treatments. It is not our intention to compare the diverse commercially available glassy carbon materials but to focus on the physical phenomena occurring on the surface of the electrode during polishing and their effect on electrochemical results. The design of the electrodes used in this study was fashioned after a concept first described by Johnson (16)and is illustrated in Figure 1. Silver-filled thermosetting epoxy provided electrical contact between the glassy carbon and a brass rod as is apparent from Figure la. The brass section of the electrode can be precision machined to fit any rotator and is not germane to this paper. The glassy carbon was obtained by the manufacturer as a 3 mm x 50 mm rod. From this, the glassy carbon electrode (GCE) was assembled as follows. The glassy carbon rod was cut with a glass grinding wheel precisely normal to the cylinder’s long axis. The cut end was ground smooth, and the opposite circular face was given a very narrow beveled edge. A brass rod and Teflon sheath were machined to the forms pictured in Figure l a to ensure a tight seal. The electrode was assembled as follows: A cut piece of glassy carbon was inserted (beveled edge first) into a Teflon sheath heated at 250 OC for 1 h. Silver-filled, electrically conducting epoxy (Ohmex-Ag, Transene Co. Inc., Rawley, MA) was then applied to the carbon face. The brass rod was force-fitted into the Teflon until the beveled edge protruded out the far end of the sheath to a point just beyond the beveling. Beveling of the leading edge
151
iion
Carbon (a)
-I
Ic3rnrn
(b)
Flgure 1. Design and construction features of the glassy carbon electrode (GCE).
of the carbon prevented scarring of the Teflon by the carbon piece as it was pushed through, which would have impaired the tight Teflon-to,-carbon seal required. The above operations were perfbrmed as quickly as possible to prevent the sheath from cooling so much thatit would not slip over the brass rod. The assembled electrode was then placed in a 180 “C oven for 16 h to allow the epoxy (tle electrical contact) to cure. Once assembled, the electrode was polished by the two-step procedure outlined below. The first step was manual abrasion (grinding). A series of silicon carbide papers of decreasing roughness (down to 30 pm) was used (Carbimet grit papers supplied by Buehler). Continued abrasion with the 30-pmbilicon carbide paper had the effect of reducing the coarseness of the surface to the point where “cloth polishing” (see below) could be effectively applied. Fresh abrasive papers were used at each stage of the grinding process with a light covering of distilled water to lubricate the carbon/paper interface. The correct amount of water was important: excessive flooding had to be avoided to obviate sliding in lieu of cutting. On the other hand, insufficient water resulted in frictional heat which could damage the surface. The electrode was rotated 60” between each successive abrasion. Abrasion was continued until the scratches made by the rougher abrasive were removed by the next finer abrasive paper, etc. Care was taken not to damage the Teflon surrounding the glassy carbon and to ensure coplanarity. The integrity of the carbon-Teflon seal was verified by inspection with a low power reflectance microscope (60X). Abrasion with silicon carbide effectively leveled the carbon and Teflon surfaces but left behind irregularities (scratches) on the glassy carbon surfaces which are apparent in Figure 2. That figure depicts a scanning electron micrograph (350X) obtained after grinding with 30-pm silicon carbide paper. Chronoamperometric measurements revealed that-at this stage-the effective electrode area was nearly double the geometric surface area of 7.07 x cm-2. The second step of the metallographic procedure involved “fine polishing” with diamond pastes Suspended in light oil bases. The pastes (containing suspended diamond particles 30,6, and 1pm in diameter) were applied successively on cotton-backed, nylonnapped cloth fastened to a metallographic polishing wheel rotating at 1500 rpm. The diamond particles (suspended in the oil) moved freely under the load applied by the electrode which was positioned perpendicularly above the wheel: this removed the grinding damage (Figure 2) inflicted to the electrode surface by the fixed silicon carbide particles of the abrasive paper (used in the previous abrasion step).
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ANALYTICAL CHEMISTRY. VOL. 57. NO. 1. JANUARY 1985
mL-$3'LT I
i
ngUn 2. Scanning electron micrograph (350X) aner abraslMl with 3&pm silicon carbide paper. Monolayer of gdd vapw deposited on
Floun 3. Scanning electron micrograph (350X) obtained after fine polishing wHh 0.05-wndlameter alumina. Surface scratched to facilltate focusing.
surface.
The 'he" polishing WBB amomplished by holding the electrode f m l y 80 that a minimum of friction wan produced for 30 s while moving the electrode counterclockwise to the rotation of the p o l i wheel. This technique provided for a continuously new leading edge a~ the electrode move8 around the wheel. After esch "trip" around the wheel, oil and debris which acnnnulated on the surface of the electrode were removed by applying microsolution (detergent) and rinsing with a stream of distilled water. Finally, the electrode was polished with an aqueous alumina slurry containing A120sparticles 0.05 wn in diameter. This produced a glasslike mirror finish, completing the "fine polishing". On this surface cyclic voltammograms of 0.001M ferricyanide in 1M KCI yielded peak separations of 0.065 V at potential scan rates of 1 VIS (correspondingelectrochemical rate parameters are diacuased in the next section). Thereafter a monolayer of gold w a ~vapor deposited on the c a r h n surface and a metallographic instrument was used to scratch the surface to focus the seanning electron microscope. The corresponding micrograph (350X) is shown in Figure 3 the uniform smoothness of the electrode stands out in comparison with Figure 2. Chronoamperometric determination of the electrochemical area on the 'smooth" electrode surface yielded a value of (7.05 0.02) X 10" cm2 which is in agreement with the calculated geometric mea.
*
RESULTS AND DISCUSSION Working Range and General Capabilities. The potential region in which an electrode can operate is limited by the relevant residual currents which depend on a variety of factors including proneness of the electrode material to electrooxidation-reduction, effective decomposition potentials of solvent, and/or supporting electrolyte (these being frequently dependent on prevailing overvoltages, e.g., hydrogen and oxygen evolution overvoltage in aqueous solutions). Figure 4 documents the working range of our glassy carbon electrode in aqueous and nonaqueous media. It is apparent from the figure that the GCE has a wide working range, extending over spans of 1-2.5 V in aqueous solutions (depending on pH) and as much 88 6 V in aceto nitrile. This compares favorably with other solid indicator electrodes. The actual electroanalytical capabilities of the W E as a working electrode for h y d r o d p m i c and differential pulse voltammetry are illustrated in Figures 5-7. Figures 5
3.0
1.8 0.9 0 -0.9 -1.8 -3.0 Potential, volt vs S C E Fbum 4. Potential range of tha GCE in aqueous and nonaqueous meda. ReMual cwr6nta obtabksd h rotammode. ISOO rpn. Cuves shifted arbitrarily along me vertical axis.
and 6 show hydrodynamic voltammograas (ohtained with the GCE used in Levich's rotated mode) corresponding to the electroreduction of the hematoporphyrin dication, denoted PoH,'* in eq 1-3 below (the superscript zero designates a formal 'oxidation" number), in three successive steps as follows (17,18):
[PoH2+]
+ e- + H+ = [P(-I)H,]*+.
[P1H5]+-+ e- = [P1'H,]+
+
+
(1) (2)
PnH,+ 2e- 2H* = [P-'"H,]+ (3) The hematoporphyrin system is particularly difficult to inveatigate, because of the surface activity of porphyrin moieties
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
153
Table I. Typical Validation Data on the Electrochemially Effective Area of the GCE (Criterion I) area (cm2) x 100
2 250 -
-0.5
-0.3
-0.7 -0.9
-1.1
-1.3
Potential, volt vs modified SCE
Flgure 5. Hydrodynamic voltammograms of 0.505 X M hema6 M H,O; supporting toporphyrin dictation at GCE: solvent, DMF
+
electrolyte, 1.5 M HCIO,.
a 13.
61-
-
- 0.4
-0.6 -0.8 1.0 Potential, volt vs modified SCE Flgure 8. Comparison of differential pulse and conventional hydrodynamic voltammograms at the GCE rotated at 1600 rpm. Same solution as In Figure 5: potential pulse amplltude for left-side ordinate scale, A€ = 0.005 V. Potential of Rotated Disk, vs Ag + ( O . I M I / A g reference electrode 2.0 1.8 1.6 1.4 1.2 c 1 . 0
._
a . --.. -
t
- c
Peak B
\ I
n
m
Flgure 7. Differential pulse voltammogram of 1.1 X M dlbenzothiophene at the GCE rotated at 400 rpm: potential pulse amplitude, A€ = 0.025 V; solvent, acetonitrlle; supporting electrolyte, 0.1 M lithium perchlorate; electrooxldatlon products, (peak A) a dimeric 9 4 3 4 benzothiopheny1)dibenzothiophenium cation, (peak B) an analogous tetramer, and (peak C) an analogous octamer.
which are strongly adsorbed on electrodes. The well-defined limiting currents in Figure 5 are unmatched in porphyrin electrochemistry. Wave I corresponds to the sum of the two one-electron transfer steps in reactions 1and 2, which are not resolved in Figure 5. However, close scrutiny of wave I reveals that it actually consists of two closely spaced, overlapping reduction waves. They can be successfully resolved by differential pulse voltammetry a t the rotated GCE as seen in Figure 6, which clearly shows evidence of three, rather than two, discrete electroreduction processes. Wave I1 (in both
geometric 7.07 ”electrochemical”, after abrasion with S i c of 3 0 - ~ m 14 roughness electrochemical, after polishing with A1203of 7.05 i 0.02 0.05-pm diameter 7.1 electrochemical, after running 10 cyclic voltammograms of ferricyanide 7.5 electrochemical, after running 50 cyclic voltammograms of ferricyanide
Figures 5 and 6) corresponds to the two-electron transfer, reaction 3. Figure 7 is a differential pulse voltammogram of dibenzothiophene (DBT), illustrating the capabilities of the GCE as an indicator anode. DBT is electrooxidizable via successive two-electron abstraction reactions, yielding, in turn, a dimeric 9-(3-dibenzothiophenyl)dibenzothiopheniumcation (peak A), an analogous tetramer (peak B), and octamer (peak C) (15,19).Corresponding well-defined peaks are apparent in Figure 7 . Reproducibility of Electrochemical Thermodynamics and Kinetics at the Glassy Carbon Electrode (GCE). The construction and the metallographic pretreatment of the GCE have been described earlier in this paper. In this section we propose testing procedures for ascertaining the satisfactory (and reproducible) functioning of the polished electrode. It is proposed to use the following “validation criteria”: (I) an electrochemically effective area which is equal within 1% to the geometric area, (11) correct half-wave potentials in Levich‘s rotated mode, (111) correct standard electrochemical rate constants by cyclic voltammetry in the stationary mode, (IV) that the assignments for a given redox couple in a given supporting electrolyte and under identical experimental conditions (e.g., rate of rotation of the electrode in hydrodynamic voltammetry, potential scan rates in cyclic voltammetry, supporting electrolyte, etc.) be the same as on appropriately pretraated platinum and gold indicator electrodes. Table I illustrates the behavior of the GCE with respect to criterion I. It shows that the electrochemically effective area of the electrode after completion of the recommended pretreatment was, indeed, identical with the geometric area, as measured chronoamperometrically via the electrode reaction Fe(CN)t-
+ e- = Fe(CN)64-
(4)
in 1 M aqueous potassium chloride supporting electrolyte a t 25 “C, where the diffusion coefficient of ferricyanide was cm-2 s-l (20). assigned a value of D = 0.763 X Table I illustrates that prolonged “use” such as the running of 50 cyclic voltammograms of ferricyanide (and at times sooner when other electroreactive species were involved) increased the apparent effective electrochemical area of the electrode. Repolishing restored the effective electrochemical area to the value of the geometric area. In our experience it was generally sufficient to repeat only the last stage of the polishing procedure (with 0.05-wm alumina) in order to restore the area of the electrode to its pristine value (i.e., the geometric area). This can readily be accomplished in a couple of minutes whenever required. We found it sometimes necessary to repeat the polishing of the surface a t the beginning of the workday. We have not conclusively established the reason for the increase in electrochemically effective electrode area in the course of voltammetric experiments. We speculate that changes in the surface area were engendered by surface oxidation-reduction processes which “corrode” the electrode and, thus, enhance its roughness.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Polished With 0 . 0 5 ~ Alumina T
5.~1 ,,2 =+0.226V
Table 11. “Validation” of the GCE as to Electrochemical Thermodynamics and Kinetic Reproducibility criteria 11, 111, and IV recommended reproducibility criteria A in the rotated disk mode, the half-wave potential of 5 X M K,Fe(CN), in aqueous KCl at 900 rmp should be +0.226 V vs. SCE at 25 “C, reproducible within f0.005 V in tests extending over a period of 6 months, B in the stationary disk mode, the formal electrochemical rate constant of the ferri-ferrocyanide couple in 1 M KC1 should yield ko’ = (7 f 1) cm/s effective at E” ’ = 0.026 V vs. SCE
corresponding peak separations bv cvclic voltammetrv listed below scan rate, VIS peak separation, V
Potential, volt v s
SCE
Figure 8. Effect of polishlng on the shape of hydrodynamic voltammograms at the GCE rotated at 900 rpm: 0.001 M potasslum ferricyanide in 1 M aqueous KCI.
I/ \ h
Polished With 6p Diamond Abrasive
0.084 0.096 0.105
900 rpm of 0.005 M ferricyanide in 0.5 M aqueous KzS04 yielded a consistent half-wave potential of 0.226 f 0.005 V. Identical assignments were obtained a t platinum and gold rotated-disk electrodes. Figure 9 (cyclic voltammetry) suggests a “criterion of acceptability” for a voltammetric indicator electrode. As is apparent from the figure, the measured electrochemical rate constants clearly increased as the polishing progressed. After completing the recommended polishing procedure (with 0.05-bm alumina), we obtained invariably a value of
Polished With 3 0 p Diamond Abrasive
1
5 10 20
ko’ = (7 f 1) x
cm/s
(5)
for reaction 4,under the experimental conditions specified earlier (1M aqueous potassium chloride supporting electrolyte, 25 “C).This assignment is effective at the formal potential of
E”’= 0.226 V vs. SCE
I
0.50
I
I
I
0.30
Potential,
0.10 volt vs S C E
Flgure 9, Effect of polishing on the peak separation of cycllc voltammograms at the GCE: 0.001 M K,Fe(CN)6 In 1 M aqueous KCI; potential scan rate, v = 0.02 v s-’.
The significance of criteria I1 and TI1 is apparent from Figure 8 (half-wave potentials by hydrodynamic voltammetry) and Figure 9 (peak separations in cyclic voltammetry). Hydrodynamic voltammetry of ferricyanide and ferrocyanide at the GCE yielded well-defined waves for rotation rates between 400 and 10OOO rpm. A wave analysis of the plot (at 900 rpm) of E VS. log (i/il - i) for ferricyanide produced a slope of 0.064 f 0.004 V and half-wave potential of 0.226 f 0.004 V for the cathodic wave of ferricyanide and a slope of 0.063 f 0.004 V and a half-wave potential of 0.225 f 0.004 V for the anodic wave of ferricyanide. These results were the same in 1 M KC1 and 0.5 M KzS04 aqueous supporting electrolytes. As far as long-range reproducibility (6 months) is concerned, hydrodynamic voltammetry at rotation rates of
Equation 5 is, within experimental error, the same as the corresponding assignment at a platinum indicator electrode, viz., k”’ = (8 i 2) X lo-’ cm s-l (21). Further, cyclic voltammetric measurements of the ferrocene/ferrocenium couple in 0.1 M TBAP-DMF when compensated for IR loss resulted in a rate constant assignment a t the GCE of (7 f 2) X cm s-l which is in agreement with results reported by Kadish (22) at a platinum electrode. On the basis of these findings, we recommend the validation criteria listed in Table I1 for the GCE. The agreement between the rate constants at platinum and at glassy carbon suggests that these may approximate the “true” electrochemical rate constants since the potentials of zero charge are likely to be very different a t platinum and glassy carbon; the invariance of the rate constant suggests that the double layer corrections were negligible on both materials (rather than fortuitously the same). If so, the finding would be consistent with the prediction of the Marcus theory (14) that standard electrochemical rate constants ought not to depend appreciably on the electrode material (because the nature of the electrode material only affects the rate constant via a second-order term which contains the work function of the relevant metal). The empirical observation that apparent (measured) electrochemical rate constants actually depend a great deal on the electrode material (and even on the pretreatment of a given electrode material), which is common knowledge among practicing electrochemists and is probably accounted for by variable, uncontrolled, and irreproducible double-layer effects. The excellent consistency of the results reported in this paper suggests that the recommended pro-
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Anal. Chem. 1985, 57, 155-158
cedures can minimize these. Under such conditions, the GCE can be "validated" both as to electrochemical thermodynamic and kinetic reproducibilities.
(10) Blaedel, W.; Schieffer, G. W. J. Electroanal. Chem. 1977, 80, 259. (11) Bjelica, J.; Parsons, R.; Reeves, R. M. Croat. Chem. Acta 1980, 53, 211. (12) Bjellca, J.; Parsons, R.; Reeves, R. M. f r o c . Symp. Electrode Processes, 3rd 1980, 80-63, 190-212. (13) Jordan, J.; Carr, P. W. Plating (East Orange, N.J.) 1968, 589. (14) Marcus, R. A. J . Chem. fhys. 1965, 43, 679. (15) Robbat, A., Ph.D. Thesis, The Pennsylvania State University, University Park, 1980. (16) Johnson, D. C. Ph.D. Thesis, University of Minnesota, Minneapolis, 1967. (17) Spendel, V. A. Ph.D. Thesis, The Pennsylvania State University, University Park, lQ80. (18) Spendel, V. A.; Jordan, J. "Extended Abstracts"; Electrochemical Soclety: Princeton, NJ, 1982;p 523. (19) Jordan, J.; Ankabrandt, S. J.; Robbat, A,; Stutts, J. D. ACS Symp. Ser. 1961, No. 169, 427. (20) Adams, R. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969;p 219,Table 8.2. (21) Tamamushi, R. "Kinetic Parameters of Electrode Reactions"; Butterworths: London, 1975. (22) Kadish, K. M.; Ding, J. Q.; Malinski, T. Anal. Chem. 1984, 56, 1741.
ACKNOWLEDGMENT We thank Edward Wetzel and the Department of Energy, Pittsburgh Energy Technology Center (Analytical Branch), for the use of the scanning electron microscope. Registry No. P(O)H:+, 20670-60-4; DBT, 132-65-0;Fe(CN)6S-, 13408-62-3;Fe(CN)6", 13408-63-4;A1203,1344-28-1;C, 7440-44-0; ferrocene, 102-54-5;ferrocenium, 12125-80-3;diamond, 7782-40-3; silicon carbide, 409-21-2. LITERATURE CITED (1) Bard, A. J.; Faulkner, L. R. "Electrochemical Methods"; Wiley: New York, 1980. (2) Yoshimarl, T.; Arakawa, M.; Takeuchl, T. Talanta 1965, 12, 147. (3) Zittel, H. E.; Miller, J. F. Anal. Chem. 1965, 37, 200. (4) Adams, R. N. Anal. Chem. 1958, 30, 1576. (5) Laitinen, H. A.; Rhodes, D. R. J. Electrochem. SOC. 1962, 109, 413. (6) Elving, P. J.; Conrad, A. L.; Gaylor, V. F. Anal. Chem. 1953, 25, 1078. (7) Feely, F. R.; Kowalczyk, S. P.; Ley, L.; Cuvell, R.; Pollark, R. A,; Shirley, D. A. fhys. Rev. B : Solid State 1974, 9 ,5264. (8) Noda, T.; Inagaki, M.; Yamada, S. J . Non-Cryst. Solids 1969, I, 285. (9) Laser, D.; Arlel, M. H. Electroanal. Chem. Interfacial Electrochem. 1974, 52, 291.
RECEIVED for review June 13,1983. Resubmitted and accepted October 1, 1984. This work was supported by Grant AC2277ET10482 from the U.S. Department of Energy and was presented in part at the Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1982.
Square-Wave Anodic Stripping Analysis in the Presence of Dissolved Oxygen 4
Marek Wojciechowski, Winston Go, and Janet Osteryoung*
Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214
Substantial reduction in analysis time has been achieved in anodic stripping analysis by eliminating the necessity for oxygen removal. This was made possible by using square-wave voltammetry as a stripping technique in a low-pH electrolyte (0.1 M HCi04). Eiectroreduction of oxygen In acidic medium does not affect either the precision or the sensitivity of the method. Linear calibration plots were obtained for a model system at submicromolar concentration levels (0.1-1.3 pM Cd(I1)) using short deposition times (5-240 8 ) at -0.7 V vs. SCE.
Anodic stripping voltammetry is an important tool for the determination of trace amounts of metals. As for most analytical methods, the amount of metal detectable is limited by the background signal. In voltammetry, the background signal is composed of the faradaic current of impurities, the capacitative current, and electrical noise of the instruments. In anodic stripping voltammetry, the response is enhanced by electrolytically preconcentrating the metal of interest into or onto an electrode; this can reduce detection limits to picomolar levels. This technique has been successfully applied in the determination of a number of metals in different environments (1-3). Most procedures for anodic stripping voltammetry call for removal of dissolved oxygen from the sample. Oxygen is present in the millimolar range in nondeaerated samples, and its electrochemistry dominates the background current in the
potential range 50.2 V vs. SCE. In addition to this, dissolved oxygen and its reduction product, hydrogen peroxide, can strip metals chemically, including mercury, from the electrode according to the following reactions;
+ no, + 2H+ = 2M"+ + nH202 M + nHz02+ 2nH+ = 2Mn+ + 2 n H 2 0 M
(2)
Dissolved oxygen is most commonly removed by purging the sample with pure nitrogen, helium, or argon gas. This process is slow and in modern voltammetry generally occupies the majority of the analysis time. The use of nitrogen-activated glass nebulizers (4)or a rotated cell assembly (5) reportedly reduces the purging period to 20 and 75 s, respectively. A preferable approach, in the interest of simplicity, would be to eliminate altogether the purging process. The possibility of avoiding the removal of oxygen has been demonstrated by using current-sampled dc, pulse, and ac polarography (6), complexometric titration with fixed potential square-wave polarographic detection of the end point (7,8), differential pulse anodic stripping voltammetry (9),and subtractive anodic stripping voltammetry (10). In each case, the presence of oxygen raises the detection limit of the technique as a result of the high uncertainties in the background current due to fluctuations in the concentration of dissolved oxygen. Furthermore, it is not obvious that chemical interference by oxygen is absent. Distortions in the stripping peaks for cadmium, lead, and bismuth a t a concentration of 0.5 pM in solutions with varying concentrations of dissolved
0003-2700/85/0357-0155$01.50/00 1984 American Chemical Society
