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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Although special conditions are known to exist in different regions of the discharge (9),further studies would be required to test this explanation of these negative signals. Finally, the sometimes bidirectional signals exhibited by some metastable atom lines will be considered, Figure 3D. An increase in impedance (corresponding to our negative signals) in dc mode lamps irradiated with a CW laser has been attributed by Kakeski, Keller, and Englemen, Jr. ( 4 ) to the loss of atoms from long-lived metastable states caused by laser induced excitation to higher energy levels and subsequent radiative deactivation. Since the metastable atom population is a good reservoir for potential ions, the impedance of the lamp is increased by the loss of metastable atoms. Since all of the neon lines in Table I that showed a negative excursion (whether preceded by a positive peak or not) are due to transitions that originate from metastable states, it seems that our results support this as a possible mechanism in the dc mode. In addition, our observations show that the negative component of these bidirectional signals does not start until the laser pulse is almost over and they reach a maximum value several microseconds later. This delay may be due to the fact that regions of relatively high concentrations of charge carriers, such as the Faraday dark space (9), can temporarily continue to supply the lost charge carriers to other regions where only low concentrations are required. The impedance of the lamp increases by a small but not immediately observable amount, until the regions of high concentration begin to become significantly depleted. The impedance then increases more rapidly and the lamp voltage noticeably increases. The maximum value is reached when the discharge (under the influence of the increased voltage gradients) begins to return to its original conditions prior to the laser pulse. Metastable atoms are relatively more important in maintaining the discharge at low lamp currents than at high lamp currents ( 4 ) . Therefore, a laser induced decrease in the metastable atom population at lower lamp currents causes a larger relative increase in lamp impedance and a correspondingly larger negative signal, as observed. At higher lamp currents, a positive peak is observed prior to the negative peak for these metastable atom lines. The
positive peak begins shortly after the laser pulse begins, but reaches its maximum value 0.5-1 p s after the laser pulse is over. Apparently laser-induced ionization gradually becomes more important as the lamp current is increased, causing a short lived (1-2 ps) decrease in impedance (positive signal) before the influence of the deactivated metastable atoms becomes predominant. The fact that the influence of the ionized metastable atoms lasts for such a short time compared to the influence of the deactivated metastable atoms may be due to the ionization and deactivating processes occurring predominantly in different regions of the discharge, where ionizing collision rates differ. Although these pulsed experiments introduce additional complexities to the interpretation of the results, the transient observations provide additional and useful information that is not apparent in steady-state observations. In any case, these data show that the laser-induced change in impedance can be readily observed in hollow cathode lamps operated in a pulsed mode, and that the pulsed mode offers a greater signal-to-noise ratio, especially for lines of the cathode material. The narrower lines produced in the pulsed mode are an advantage when the lamp is to be used for wavelength calibration of pulsed tunable lasers.
LITERATURE CITED (1) Green, R. B.; Keller, R . A,; Luther, G. G.; Schemck, P. K.; Travis, J. C. Appl. Phys. Lett. 1976, 29, 727-729. (2) Green, R . B.; Keller, R. A,; Schenck, P. K.; Travis, J. C.; Luther, G. G. J . Am. Chem. SOC. 1976, 98,6517-6516. (3) Bridges, W. G. J. Opt. SOC. Am. 1978, 68, 352-360. (4) Zalewski, E. F.; Keller, R . A.; Engleman, R., Jr. J. Chem. Phys. 1979, 70, 1015-1026. (5) Piepmeier, E. H. "Atomic Absorpton Spectroscopy with Laser Primary Sources, Chapter 3 in "Analytical Laser Spectroscopy", Omenetto, N.. Ed.; John Wiley & Sons: New York, 1979. (6) Mitchell, A. C. G.; Zemansky, M. W. "Resonance Radiation and Excited Atoms"; Cambridge University Press: London, 1961. (7) Piepmeier, E. H.; de Galan, L. Spectrochim. Acta, Part B 1975, 30, 263-279. (6) De Jong, G. J.; Piepmeier, E. H. Spectrochim. Acta. Part B 1974, 29, 159-177. (9) Skvin, P. J.; Harrison, W. W. Appl. Spectrosc. Rev. 1975, 10, 201-255.
RECEIVED for review May 29, 1979. Accepted June 20, 1979. The authors thank the National Science Foundation for their support and award of Grant MPS-7305031.
Anodic Stripping Voltammetry at a Reticulated Mercury Vitreous Carbon Electrode W. J. Blaedel" and Joseph Wang Department of Chemisfty, University of Wisconsin-
Madison, Madison, Wisconsin 53706
Anodic stripping voltammetry has been modified with a porous mercury-coated electrode of reticulated vitreous carbon in a flow-through configuration. Exploratory experiments have shown the dependence of the stripping peak current upon the deposition time, scan rate, flow rate, electrode length, and concentration. The deposition step involves deposition of a substantial fraction of the metals. For convenient quantitation M, a of concentration ranging from 3 X lo-' M to 3 X 0.5- to 5-min deposition is sufficient. High precision of results is maintained on the same film. Electrodes are easily fabricated, and are inexpensive enough to be disposable.
The need for a rapid technique for measuring trace metals in a variety of matrices, has led recently to the adoption of 0003-2700/79/0351-1724501 OO/O
a variety of flow systems (1-9) to anodic stripping voltammetry (ASV). Flow cells for ASV have been mentioned in connection with on-site analysis of natural waters (2, 6, 8 ) , monitoring of chromatographic effluents (3),and with automated analysis, based on the AutoAnalyzer principle (9). Various configurations have been designed for these purposes. These include cells in which the solution flows through an open tubular electrode (1-3, 5 , 7), or onto a fixed ( 4 ) or rotating disk (6) electrode. The characteristic common to all of these configurations is that they utilize only a very small fraction of the available metals for their response. Large surface area electrodes, that would increase the deposition yield, have not been employed successfully, even though sensitive ASV modes, recently adapted to flowing streams ( 4 , 5 ) , would discriminate ef1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
fectively against the high background currents that accompany the high peak currents. ASV employing high ratio of electrode surface to solution volume has been described, using a batch thin-layer cell (10). The use of a gold minigrid mercury-coated electrode for ASV was limited by the tendency of the gold to form intermetallic compounds (11). Of the substrates used when plating the mercury films, carbon appears to best fulfill the requirements of chemical and electrochemical inertness throughout the potential range employed. A graphite cloth was unsuited for analytical measurements because of its nonreproducible behavior (12). Reticulated vitreous carbon (RVC) serves as a substrate for a thin mercury film electrode for ASV in a flow-through cell. RVC is a relatively new porous carbon material, possessing high surface area together with many hydrodynamic and electrochemical advantages that make it well suited as an electrode material in flow-through cells (13). Electrolysis yields may range up to 10070,depending on the residence time of an element of solution in the electrode (14,15). RVC has been exploited recently for various analytical applications, such as an optically transparent electrode (16),a flow-through detector for flow injection analysis (In,and as a pH indicator electrode (18).
RVC is unique among the electrodes that have been employed in ASV flow cells due to its special structure. High deposition yields and high sensitivity are associated with its large surface area. The electrodes are very inexpensive and easily fabricated. No polishing, impregnation, or glueing are required. The properties and behavior of an ASV flow cell, utilizing deposition a t a RVC mercury-coated electrode, are explored in this study.
THEORY For mercury film electrodes the stripping peak current is given by (19):
i,
= 1.1157 X lo6 n2 ACJu
(1)
In Equation 1, C, is the concentration of the metal in the mercury, A and 1 are the surface area and the thickness of the mercury film, respectively, and u is the potential scan rate. By Faraday's law, iLtdep
c, = nFAl In Equation 2, iL is the limiting current for the deposition of the metal in question, and tdepis the deposition time. The limiting current is constant under the conditions of continuous replenishment of solution which prevents concentration depletion during the deposition step. Sioda (20) has given an expression for the limiting current on porous electrodes that has been confirmed experimentally (15) for the RVC electrode: iL = r#CbVf[l - exp(-j~u'-~Vf~-'L)I (3) In Equation 3, Cb is the concentration of the metal ion in the bulk solution, Vf is the volume flow rate, j is a proportionality constant, s and a are the specific surface and the cross-sectional areas of the porous electrode, respectively, L is the electrode length, and a is a constant, that characterizes the type of flow and the shape of the electrode. Combining Equations 1-3 (to eliminate C, and iL) leads to the following expression for the peak current for a mercury-coated porous electrode: ip = 1.1157 x lo6 n2 utdepCbVf[l- exp(-js a'-"Vf"-'L)]
(4) EXPERIMENTAL Apparatus. The RVC flow cell and the flowing system have been described in detail previously (15). Unless otherwise stated, all data were obtained with 1 disk of 100 pores per inch (ppi).
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The disk was 0.218 inch (5.5 mm) in diameter and 0.096 inch (2.4 mm) in length. The disk had a calculated volume of 0.058 mL (97% void) and a surface area of 3.8 cm2. All experiments were performed with a Sargent model FS polarograph. All potentials in this paper are given with respect to the Ag-AgC1 reference electrode in 0.1 M KCl (+0.282 V vs. the normal hydrogen electrode). Reagents. All solutions were prepared from analytical grade chemicals and deionized water (Continental Water System (charcoal bed filter, mixed bed deionizer, 0.2-pm Gelman filter)). Standard metal solutions,0.001 M, were prepared from analytical grade nitrate salts. The solutions were stored in polyethylene bottles. Portions of these solutions were diluted as required for standard addition. The mercury plating solution was 0.5 mM in HgClz and 0.1 M in KC1. All samples were prepared in a supporting electrolyte that contained 0.04 M acetic acid and 0.04 M ammonium acetate (pH 4.8). Mercury Film Deposition. The mercury film was deposited at the beginning of each day, and yielded reproducible results during the course of a day. Before plating the mercury, any small air bubbles inadvertently trapped in the RVC were removed by pumping deionized water up and down rapidly with a plastic syringe located at the solution outlet. The mercury film was deposited by holding the potential of the RVC at -0.8 V and passing the deaerated plating solution through at 3.32 mL/min for 15 min. From the limiting current of 510 FA, the average mercury film thickness was estimated as 0.092 pm, and the amount of mercury at the RVC as 6.24 X mol Hg/cm2. For electrodes consisting of more than one disk, uniform mercury films could not be obtained by deposition from a flowing solution. Instead the film was deposited from quiescent solution in two stages, each of which consisted of pumping the mercury plating solution into the RVC, stopping the flow, and plating for 2.5 min. Similar peak and residual currents were observed for these different film plating procedures. After plating the mercury film, the plating solution beaker was replaced by the sample solution beaker and nitrogen was sparged through the sample solution for 20 min. During this period the RVC was held at f0.15 for the first 10 min, followed by 8 min at 4 . 9 V and 2 min at +0.15 V. Such a conditioning process was found to be necessary to reduce traces of oxygen that may remain in the electrode, to strip trace metals which may have co-plated with the mercury, and to reduce any Hg2C12that may have formed previously. The porous structure of the RVC precluded wiping its surface to remove the mercury film at the end of a series of experiments. To ensure complete removal of the film, the electrode must be held at +LO V for at least 30 min. This anodization cannot be performed when a chloride medium such as seawater is analyzed, because chlorine is generated, which causes degeneration of the carbon surface (21). Chloride media should therefore be exchanged before any anodization is performed. Analysis Procedure. After plating and conditioning the mercury film, the sample solution was allowed to flow through the cell. A plating potential, selected according to the cations to be determined, was applied at the electrode for a selected time determined by their concentration level. At the end of this plating period, the metals were stripped from the mercury by applying a linear anodic potential scan, while the sample solution still flowed through the electrode. The scan was stopped at +0.1 V, and this potential was maintained for 30 s, after which the mercury film was ready for the next determination.
RESULTS AND DISCUSSION Analytical Application. Figure 1illustrates typical anodic stripping voltammograms for some common cations, present a t the 5 ppb (kg/L) level, obtained at the mercury-coated RVC electrode by employing relatively short (2 min) deposition periods. The peaks are well defined and separated even though a relatively fast scan rate (2 V/min) was employed. The peak potentials are observed a t -0.01 V (CUI,-0.47 V (Pb), -0.66 V (Cd), and -1.07 V (Zn). To prevent complication caused by Zn-Cu intermetallic formation, zinc was deposited in the absence of other metal ions. High sensitivity is obtained for the determination of zinc, in contrast to the insensitivity
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
a
b
C
d
I -1.2
I
- 0.8
1
- 0.4
u -
I
0.8 -0.4 APPLIED POTEKTIAL,VOLTS
0
APPLIED POTENTIAL, VOLTS
Figure 1. Characteristic anodic stripping voltammograms. (a) 90 nM zinc. (b) 50 nM cadmium, 25 nM lead, and 90 nM copper in pH 4.8 acetate buffer. Two-minute depositions at -1.4 V (a)and -0.95 V (b), 5.8 mL/min, 2 V/min. The dotted lines represent the blank solutions
reported previously for some carbon-based thin mercury film electrodes (22,23). Poor sensitivity has also been obtained a t the bare RVC under the same experimental conditions. Quantitative evaluation is based on the linear correlation between peak currents and concentration, expected from Equation 4. Two separate experiments were performed to confirm this linearity. Eight-point standard additions from 30 to 240 nM added lead a t 5.8 mL/min, and five-point standard additions from 100 to 500 nM added copper a t 3.32 mL/min were linear for both metals (2-min depositions, 2 V/min). A least-squares analysis of the standard addition data yielded slopes of 93.2 f 1.7 nA/nM (90% confidence limits) for lead and 60.8 f 0.8 nA/nM for copper. The sensitivity (Le.,peak current per concentration unit, for a given deposition time, expressed as iP/Cbtdep)is remarkably high, being 49 and 30 nA/nM min for the lead and copper, respectively, for a scan rate of about 2 V/min. These values are much higher than the corresponding values (0.6 to 3.3 nA/nM min) that have been observed under similar conditions for other ASV flow cells suggested t o date (1,4-6). The enhanced sensitivity is due to the much higher yields of metal reduced into the amalgam during the deposition step. The high peak currents are accompanied by high background currents, since the charging current is directly proportional to the electrode area (24). However, despite the high background currents, the detectability obtained compares favorably with that of other conventional ASV flow cells (I, 2). This is demonstrated in Figure 2 by the peak currents obtained after successive standard additions of cadmium and lead to the working solution, each addition effecting a 5 nM increase in their concentrations (corresponding to 0.6 ppb Cd and 1 ppb Pb). Five-minute depositions were employed. For convenient quantitation of concentrations ranging from 3 to 300 nM, 5- to 0.5-minute depositions are sufficient, at relatively low flow rates of about 5 mL/min. T o fully realize the improved sensitivity of the RVC, work is in progress toward the adaptation of the collection mode (ASVWC), that discriminates against the charging current, and that should permit determinations a t the sub-ppb level without prolonging the deposition period. The Effect of Experimental Parameters upon the Peak Current. Table I shows the dependence of the peak current upon the deposition time. It is apparent that the peak current is directly proportional to deposition time, as expected from Equation 4.
Figure 2. Anodic stripping voltammograms obtained after increasing the cadmium and the lead concentrations in 5-nM steps (a-d). pH 4.8 acetate buffer. Five-minutedeposkions at -0.95 V, 4.9 mL/min, 2 V/min
0
I
I
I
I
I
I
I
1
I
Table I. Dependence of Peak Current on Deposition Time4 tdep,
min
ip,
@Ab 2.7
1 2 3 5
14.1
7
19.2
10
26.1
5.6 8.6
wAimin 2.70 2.80 2.86 2.82 2.74 2.61
ip/tdep,
a 100 nM lead in pH 4.8 acetate buffer, 2.26 mL/min, 2 Vimin. Corrected for zero deposition time, estimated as the peak obtained by a potential scan from the deposition potential to the peak potential.
Figure 3 shows the dependence of the peak current on the volume flow rate. As was shown in previous studies (14, 151, an increase in sensitivity is obtained with increasing the flow rate. As the flow rate increases, the peak current rises rapidly at first and then more slowly (the conversion yield declines from 49% to 10% as the flow rate increases over the range studied (15)). When replotted on a log-log scale, the data of Figure 3 gives a straight line with a slope of 0.31. This value is in good agreement with the value of 0.32 that was reported for laminar flow regimes a t flow rates greater than 2 mL/min (15). A deviation of the experimental point from the straight line for the lowest flow rate is observed and indicates perhaps
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
-0.7 -0.5 VOLTS
I
I1
VOLTS -0.7 I 1 10 15 DAYS OF USE
1727
.-0.5
I o
201.0
1.5
SCAN RATE, 2 a V/MIN
01 0
2.5
Flgure 4. Dependence of peak current on scan rate. 700 nM lead in pH 4.8 acetate buffer. 1.5minute depositions at -0.8 V, 3.32 mL/min
="I
4
q20
5W E 3 u
Y
4
I
)\
00
I
I
-08 u -0.4
I
I
2 3 NUMBEROF DISKS, N NUMBER
VOLTS
4
Figure 5. Dependence of peak current and shape on number of disks. 100 nM lead in pH 4.8 acetate buffer. 4.9 mllmin, 2 V/min. (Peak potentials are at -0.47 V in all the cases)
a change in the nature of the flow regime, as was observed in a previous study (15). The dependence of the peak current on the potential scan rate is given in Figure 4. Because the total deposition time depends on scan rate, the peak currents were corrected by a factor tdep/(tdep tp), where t , is the time taken to scan from the deposition potential to the peak potential (22). The curvature toward the X axis in Figure 4 means that the increase in the corrected peak current is less than the direct proportionality predicted by Equation 4. When replotted on a log-log scale, the data from Figure 4 give a straight line with a slope of 0.68 instead of the value of 1predicted by Equation 4. Such a deviation was observed with other thin film electrode techniques (5,22),and was attributed to the limited reversibility of the electrode reaction at higher scan rates. At the RVC electrode, which is characterized by high currents, the curvature may also be caused in part by distortion of the peak current due to IR drop effects. Figure 5 presents the dependence of the peak current upon the number of disks, N, where N is proportional to the length of the electrode. Peak current ratios were 2.1:1.84:1.49:1.0 for N values of 4,3, 2, and 1, respectively, showing that the peak current was not simply proportional to the length of the electrode. This is in accord with Equation 4, and due to the substantial fraction of metal removed as the solution passes through each disk. (For the flow rate used in this study, the conversion yield increases from 13% to 40% as N increases from 1to 4 (15)) The charging current is directly proportional to the number of disks forming the working electrode and thus, overall, the detectability is not improved with increasing the
+
1
5
2
20
Figure 6. Time-dependence of electrode sensitivity over a 3-week period. Sensitivity is for cadmium in pH 4.8 acetate buffer. ASV conditions: Two-minute depositions i t -0.95 V, 5.8 mL/min, 2 V/min. 100 nM cadmium
electrode length, as demonstrated by the voltammograms shown in Figure 5. However, the use of a long electrode might be made advantageous, if combined with sensitive or differential ASV techniques which provide high discrimination against the charging current (e.g., ref. 4 and 5). The peak half-width increases from 59 mV at 1 disk to 65 mV at 2 disks, 70 mV a t 3 disks, and 71 mV at 4 disks. The theoretical minimum half-width for a two-electron transfer stripping peak a t a thin mercury film is 37.7 niV (19). The increased half-width at the RVC mercury-coated electrode may be related to a nonuniform amalgam concentration along the electrode and to the high IR drop effects. Similar broadening was also observed a t the mercury-coated open tubular electrode ( I ) . Precision and Stability. A series of six determinations was carried out in the same mercury film, each at 30-min intervals on 100 nM Cd(I1) in pH 4.8 acetate buffer (conditions: 2-min deposition time; -0.95 V; 5.8 mL/min; 2 V/min). The relative standard deviation was only 1.3%. On different electrodes, with different mercury films, and on different days, the relative standard deviation was around 5%. The precision obtained a t the RVC electrode (which cannot be polished) compares favorably with the precision reported for various pre-polished carbon-based mercury film electrodes employed in ASV flow cells, using the same concentration level (1,5, 6). The long-term stability of the electrode was studied by measuring the peak current of cadmium, under the same experimental conditions and at different mercury films, over a 3-week time period. The results are shown in Figure 6. The electrode functioned in a normal fashion for the first 10 days of use, with a slowly decreasing sensitivity. Thereafter, electrode failure occurred, even though the sensitivity became quite constant. Electrode failure was indicated by a sharp increase of the background current over most of the operating potential range. This high background current partly obscured the stripping peak current (see Figure 6). Another RVC disk, that was not systemically studied, functioned in the normal fashion for the first 17 days of use, after which similar failure was observed. Electrode failure is a well known problem with all forms of carbon substrates used in ASV, and their useful lifetimes range from a few days to several weeks (22,23,25). Failure of the RVC electrode is permanent and does not appear to be reparable with simple chemical or electrochemical treatments. It does not seem worthwhile at this time to elucidate the mechanism of failure. RVC is very inexpensive (about $0.15 per electrode), and the cell construction permits easy and fast replacement of electrodes.
LITERATURE CITED (1) Seitz, W R : Jones, R.; Klatt, L. N ; Mason, W D. Anal. Cbem. 1973, 45, 840.
(2) (3) (4) (5) (6) (7) (8) (9) (10' (11) (12) (1 3) (14) (15) (16)
Lieberman, S. H.; Zirino, A. Anal. Chem. 1974, 46, 20. Andrews, R. W.; Johnson, D. C. Anal. Chem. 1978, 48, 1056. Wang, J.; Ariel, M, J . Nectroanal. Chem. 1977, 85, 289. Schieffer, G. W.; Blaedel, W. J. Anal. Chem. 1977, 49, 49. Wang, J., Ariel, M. Anal. Chim. Acta 1978, 99, 89. Schieffer, G. W.; Blaedei, W. J. Anal. Chem. 1978, 50, 99. Zirino, A.; Lieberman, S. H., Clavell, C. Environ. Sci. Techno/. 1978, 12,73. Wang, J.; Ariel, M. Anal. Chim. Acta 1978, 701,1. DeAngelii, T. P.; Bond, R. E.; Brooks, E. E.; Heineman, W. R. Anal. Chem. 1977, 49, 1792. Meyer, M. L.; DeAngeiis, T. P.; Heineman, W. R. Anal. Chem. 1977, 49, 602. Yaniv, D.; Ariel, M. J . Electroanal. Chem. 1977, 79, 159. "ReticulatedVitreous Carbon (RVC)", 1978 Chemotronics International Ann Arbor, Mich. 48104. Strohi, A. N.; Curran, D. J. Anal. Chem. 1979, 51,353. Blaedel, W. J.; Wang, J. Anal. Chem. 1979, 51,799. Norvell, V. E.; Mamantov, G. Anal. Chem. 1977, 49, 1470
(17) Strohl, A. N., Curran, D. J. Presented at the 177th National Meeting of the American Chemical Society, Honolulu, Hawaii, 1979. (18) Strohl, A. N.; Curran, D. J. Anal. Chim. Acta, in press. (19) devries, W. T.; van Dalen, E. J . Electroanal. Chem. 1987, 14, 315. (20) Sioda, R. E. Nectrochim. Acta 1970, 15,783. (21) McLaren, K. G.,Batiey, G. E. J . Electroanal. Chem. 1977, 79, 169. (22) Florence, T. M. J. Electroanal. Chem. 1979, 27, 273. (23) Clem, R. G.;Litton, G.; Ornelas, L. D. Anal. Chem. 1973, 4 5 , 1306. (24) Ellis, W. D. J . Chem. Educ. 1973, 50, A131. (25) Clem, R. G. Anal. Chem. 1975, 47, 1778.
RECEIVED for review April 16, 1979. Accepted June 19,1979. This work was funded in part by the University Sea Grant Program under a grant from the Office of Sea Grant, National Oceanic and Atmospheric Administration, U S . Department of Commerce, and by the State of Wisconsin.
Elimination of Intermetallic Compound Interferences in Twin-Electrode Thin-Layer Anodic Stripping Voltammetry Daryl A. Roston, Elwood E. Brooks,' and William R. Heineman" Depaement of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
Anodic stripping voltammetry (ASV) performed in a twlnelectrode thin-layer cell eliminates Cu-Zn and Cu-Cd Intermetallic interferences that are often encountered In ASV. Cu is first exhaustively deposited on one electrode, and then the second electrode is used to complete the Zn and/or Cd determination. Intermetallic interferences are circumvented since the Interfering constituents are deposited on different working electrodes. Effects of increasing Cu concentiatlon on the Cd and Zn stripping peaks are shown. Linear calibration curves for Cd and Zn (13 to 130 ppm) are obtained from standard solutions in which Cu2+ concentratlons vary from 16 to 76 ppm. The feasibility of determining Pb, Cd, Zn, and Cu simultaneously by anodic stripping voltammetry with no Intermetallic interferences is enhanced with the twin-electrode thin-layer cell.
Thin-layer electrochemical cells have been used for studying a variety of electrochemical phenomena: adsorption at electrode surfaces; spectroelectrochemical measurements of biological, inorganic, and organic systems; and kinetics of chemical reactions coupled to electrode processes (1-5). In addition to these applications, the analytical utility of the thin-layer cell has been demonstrated. An early application of the thin-layer cell for analytical measurements was the determination of Cu2+,PbZt, CdZt, and Zn2+by thin-layer coulometry (6). The capability of performing differential pulse anodic stripping voltammetry in a thin-layer cell was recently demonstrated by the determination of Pb2+,Cd2+,CU", and Tl' down to 30 ng/mL (7, 8). Thin-layer differential pulse voltammetry was also shown to be applicable to electroactive organic compounds by the determination of the drug diazepam down to 100 ng/mL. Such techniques combine the smallvolume capability inherent in thin-layer electrochemistry with the low detection limit of differential pulse voltammetry. Present address: Department of Chemistry, H o w a r d University, Washington, D.C. 20059. 0003-2700/79/0351-1728$01.OO/O
An inherent difficulty with stripping voltammetry is the potential interaction between materials which have been preconcentrated into or onto the electrode. One such interaction is the formation of intermetallic compounds by metals deposited into mercury in anodic stripping voltammetry (ASV). Numerous intermetallic compounds have been reported (S12). The formation of such compounds can cause erroneous analytical results since metals in the compounc, ;an oxidize a t different potentials than the individual metals oxidize. This problem has been noted a t the two commonly used electrodes for ASV the hanging mercury drop electrode (HMDE) and the mercury film electrode (MFE) (13). Effects of intermetallic compound formation have also been observed in thin-layer ASV. The Cu-Zn interference in A S ? has been frequently noted ( S l l ,13-17). Under certain conditions, the deposition of Cu and Zn in a mercury film electrode (MFE) or a hanging mercury drop electrode (HMDE) results in formation of a Cu-Zn intermetallic compound, CuZn, where x = 1 , 2 or 3 (13). The intermetallic compound is oxidized from mercury at the same potential as Cu, causing enhancement of the Cu stripping peak and depression of the Zn stripping response. As much as 16% of the Cu and Zn present can be nonelectroactive as a result of Cu-Zn interactions. The Cu-Cd interference has not been as widely reported, and the nature of the interference is less clearly defined. Some authors have not detected any interfering effects between Cu and Cd (14); others have reported substantial interference (18 and references therein). Ostapczuk and Kublik have shown that there are no detectable differences in the Cd stripping responses from a saturated Cu amalgam HMDE and a pure Hg HMDE (18). However, when the solubility of Cu in the HMDE is exceeded, a depression in the Cd stripping wave results. They propose that exceeding the solubility limit of Cu results in other metals plating onto nondissolved Cu atoms on the surface of the mercury, forming binary intermetallic compounds. In a pratical sense, interferences among Cu-Zn and Cu-Cd are particularly important since these metals are frequently determined by ASV. The interferences described above pose 0 1979 American Chemical Society
