Anlaal. Chem. 1983, 55, 933-936
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Dual Coulometric-Voltammetric Cells for On-Line Stripping Analysis Joseph Wang” and Howard B. Dewald Department of Chemlstty, New Mexlco State Unlverslty, bas Cruces, New Mexico 88003
The application of dual coulometric-voltammetrlc flow cells to on-line strlpplng analysls Is Investigated. I f the “upstream” coulometrlc coll Is held at a lower potentlal than the deposltlon potentlal of the analytlcal cell, on-line strlpplng analysls Is Improved with respect to Interferences due to Intermetallic compounds, dissolved oxygen, and overlapplng peaks. Intermetallic Interferences are ellmlnated slnce the lnterferlng constltuents aire deposlted on dlfferent working electrodes. Dissolved oxygen Is electrochemlcally removed at the “upstream” eloctrods, ellmlnatlng the need for a tlme-consumlng deaeratlon step. The resolutlon between two overlapping peaks Is Improved by removal ob the Ion wlth the more posltlve peak potential at the “upstream” electrode. Due to Its slmpllclty and stability, the system seems well-sulted for contlnuous menltorlng of heavy metals In aqueous medla.
Interest in flow systems in connection with electroanalysis has increased considerably during recent years. The need for a rapid and sensitive means of measuring trace metals in a variety of matrices has led to the development of flow systems for anodic stripping voltammetry (ASV). Such systems have been mentioned in connection with on-line analysis of seawater (1,2), with automated analysis of discrete samples ( 3 , 4 ) ,and with monitoring of chromatographic effluents (5). Most work reported to date has been concerned with improving the sensitivity of the response by using advanced stripping modes (6, 7)or effective cell configurations (1,2,8). While considerable research effort has gone into these aspects, little attention has been paid to some of the common problems associated with practical ASV flow analysis. Among these are the formation of intermetallic compounds, unsatisfactory resolution, or the presence of dissolved oxygen. In this paper, we describe dual coulometric-voltammetric flow cells aimed at circumventing the problems described above. A dual coulometric-amperometric cell was utilized by Schieffer (9) to increase the selectivity in liquid chromatography/electrochemistry of organic molecules. Roston et al. employed a twin-electrode thin-layer cell for eliminating intermetallic compounds in batch ASV analysis (10). A porous silver electrode was employed to remove oxygen prior to polarographic flow-through detection ( I I ) . However, a dual (coulometric-voltamretric) cell approach for on-line ASV analysis has not been reported. The present approach utilizes an “upstream” coulometric cell with a mercury-coated reticulated vitreow carbon working electrode and a “downstream” wall-jet detector with a mercury-coated glassy carbon disk working electrode. The results reported here demonstrate that by proper adjuaitment of the deposition potentials applied a t these working electrodes, problems such as intermetallic compounds, unsatisfactory resolution, or oxygen background currents can be eliminated. The strategy is to selectively and exhaustively electrolyze the interfering constituent at the “upstream” cell prior to the ASV detection at the “downstream” (cell. EXPERIMENTAL SECTION Apparatus. The coulometric cell design is shown schematically in Figure 1. The body consisted of a Plexiglas cylinder (3.18 em
diameter, 7.30 cm long). A solution flow channel (0.55 cnn diameter) was drilled through the Plexiglas body. The working electrode was composed of a reticulated vitreous carbon plug (IRVC type, 2x343; 100 ppi; 1.2 cm long, 0.55 cm diameter, purchased from Fluorocarbon Co. (the production rights were transferred recently to ERG, Inc., Oakland, CA)). The RVC plug was held at the lower end of the solution channel by a snug fit. Electrical contact to the RVC was made by pressure t o one end of a short glassy carbon rod (2.5 mm diameter) that was introduced to the cell through a hole in its wall. The Ag/AgC1(3 M NaC1) reference and the platinum wire (0.33mm diameter) counterelectrodes were introduced to the flow channel through two holes in the cell wall. The “downstream”wall-jet voltammetric detector was described in detail previously (12). The working electrode was a glassy carbon disk (2.5 mm diameter) onto which a solution stream was directed from an inlet nozzle (0.34 mm diameter). The glassy carbon disk was polished by a 1-wm alumina slurry, until a mirrorlike surface was obtained. All potential values were determined with respect to a Ag/AgCl (3 M NaC1) reference electrode. The cell was connected to the coulometric cell via 2.7 cm long Teflon tubing. The sample solution was stored in a 400-mL Nalgene beaker fitted with a cover. Solution flow was maintained with a F’ML Lab pump (Model RPP, Fluid Metering, Inc., Oyster Bay, TQY), located between the sample reservoir and the coulometric cell. The connecting tubing (0.5 mm i.d.) was made of Teflon. The coulometric cell was connected to a Princeton Applied Research Model 364 polarographic analyzer. Stripping voltammograms (at the “downstream”cell) were recorded with a SargenbWelch Model 4001 polarograph. Reagents. Deionized water was used to prepare all solutions. Metal ion stock solutions, M, prepared by dissolving the metal or its nitrate salt in nitric or hydrochloric acids and diluting as required,were t3tored in polyethylene containers. Portions of these solutions were diluted as required for standard addition. A 5 X M mercuric ion solution was used for the in situ mercury film plating. Most samples were prepared in 0.1 M potassium nitrate supporting electrolyte,with some in 0.1 M hydrochloric acid. Rio Grande river water samples were taken from surface water near the river shore at Las Cruces, NM. Analytical grade K,Fe(CN),, KH2P04,and KzHP04were employed in the evaluation of the electrolytic yield. Procedure. The (Florence type) mercury films were deposited at the beginning of each day. The supporting electrolyte and 10 mL of the mercuric ion solution (total volume, 200 mL) were introduced into the sample reservoir. The glassy-carbon “downstreamnworking electrode was conditioned first by holding its potential at -0.7 V and passing the plating solution through at 0.3 mL/min for 20 min. After this period the potential of the “downstream”working electrode was switched to 0.0 V, and plating of the mercury at the ”upstream” RVC electrode began by applying a potential of -0.7 V for 20 min while passing the soluition through at 0.3 mL/min. Then, the “upstream” potential was switched 0.0 V, and the electrodes were ready for use in the analytical run. Measurements with scrubbing of the interfering species were performed in the following way. The “scrubbing” potential was imposed on the RVC electrode according to the species to be removed, while a plating potential (selected according to the ions to be determined) was applied at the “downstream” wall-jet electrode. Following the deposition period, the solution flow ‘was stopped and after a 15-s rest period the metals were stripped from the “downstream”working electrode by applying an anodic potential scan to 0.0 V. The stripping voltammogram was recorded simultaneously. Keeping the electrode at 0.0 V for 60 s, wlhile
0003-2’700/83/0355-0933$01.50/0 0 1983 American Chemical Society
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97%) electrolyses were obtained at the six flow rates employed. The “downstream” wall-jet detector maintains high sensitivity even at the low flow rates required to achieve exhaustive electrolysis (because of its thin nozzle that increases the linear velocity of the fluid). Intermetallic Compounds. The formation of intermetallic compounds during ASV analysis can cause error in the trace metals quantitation. For example, the determination of zinc and cadmium in the presence of copper is a common analytical problem due to the formation of the Cu-Zn and Cu-Cd compounds (10, 16). Most of the recent work on intermetallics has been focused on the Cu-Zn compound, because of the high demand for the determination of copper and zinc. One approach used to circumvent this problem has been the preferential formation of another intermetallic compound, e.g., addition of gallium which forms a Cu-Ga compound instead of the Cu-Zn compound (17). The dual flow-cell assembly, employed in this work, permits physical separation of the interfering ions, as compared with the batch twin-electrode thin-layer voltammetric approach employed by Roston et al. (10). The strategy employed in the present work is to selectively and exhaustively deposit copper at the “upstream” coulometric cell; thus, the solution reaching the “downstream” voltammetric cell is free of copper, and zinc is measured without interference.
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Flgure 2. Comparison of stripping voltammograms recorded at the “downstream” detector utilizing potentials of 0.0 V (a) and -0.6 V (b) at the “upstream” cell: 5 X M copper and 1 X M zinc in 0.1 M KNOB; 3-mln depositions at -1.6 V; flow rate, 0.2 mLlmin; differential pulse amplitude, 50 mV; scan rate, 0.5 V/min.
The advantage in using the dual flow-cell assembly is demonstrated in Figure 2. Stripping voltammograms for a solution containing Zn2+ and Cu2+ were recorded at the “downstream” voltammetric cell. In curve a, both Cu and Zn were deposited into the “downstream” mercury film (no scrubbing of copper a t the “upstream” cell); a zinc peak (ca. -1.1 V) and a combined copper and copper-zinc peak (ca. -0.02 V) are observed. In curve b, the copper was exhaustively scrubbed at the “upstream” electrode (scrubbing potential of -0.60 V) during the deposition of zinc at the voltammetric cell. This resulted with a 120% enhancement of the zinc peak and “disappearance” of the copper peak. As expected, the suppression of the zinc stripping response is a function of the relative concentrations of the interfering metals in the solution (and therefore in the mercury). Two additional 5 X lo-’ M concentration increments of copper resulted with 64 and 77% reduction of the zinc peak shown in Figure 2a; in contrast, when the copper was scrubbed at the “upstream” cell, only negligible changes in the zinc peak (shown in Figure 2b) were observed. The “scrubbed” metal ion (Cu in the intermetallic experiment) can be determined by recording the stripping response of the coulometric electrode. However, for reasons of signal-to-background characteristics, we preferred to measure this ion in a subsequent deposition/stripping cycle at the “downstream” voltammetric cell (Figure 3a). In this case, the deposition potential of the voltammetric cell was lowered so that only the copper was plated and measured. Figure 3b is the response for the zinc in this mixture, obtained with the copper scrubbing. As the voltammogram shows, no peak is obtained at -0.1 V for Cu stripping indicating again the effectiveness of the scrubbing process. Dissolved Oxygen. Stripping analysis of trace metals is usually performed in the absence of oxygen. The reduction of oxygen gives rise to background current that obscures the stripping peaks. Oxygen also affects the response by chemical oxidation of the metals in the amalgam and by precipitation of metal hydroxides by hydroxyl ions (formed during the
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6,MAY 1983
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M copper and 1 X Flgure 3. Stripping voltammograms for 5 X IO-’ M zinc in 0.l M KNO,: deposition potentials, -0.8 V (a) and -1.6 V (b); 2-min depositions with flow rate of 0.2 mL/min; scrubbing potential (b) -0.6 V; amplitude and scan rate, as in Figure 2. The small peaks at -0.65 ‘V and -0.45 V are due to cadmium and lead ions present in the blirnk solution.
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Figure 4. Compririson of stripping voltammograms recorded at the “downstream” detector utilizing potentials of -0.50 V (a) and 0.0 V (b) at the “upstream” cell: 5 X IO4 M lead and cadmium in 0.1 M KNO,; 3-min depositions at -1.3 V; flow rate, 0.46 mL/min; linear scan of 1 V/min.
reduction of ox:ygen). The deaeration step, accomplished by bubbling an inert gas, is time-consuming and inconvenient (especially in continuous flow systems). Different continuous deaeration deviices have been suggested to facilitate this step in flowing streams (2, 18), but they are based on effective nitrogen purging. With the RVC scrubber, the oxygen is removed electrochemically, which is more suitable for field measurements and automation. An electrochemical scrubber based on a porous silver electrode was used to clean oxygen in on-line polarographic analysis (11). Figure 4 compares stripping voltammograms, obtained under identical conditions, utilizing the RVC scrubber (a) and in the presence of dissolved oxygen (b). The lead and cadmium peaks are almost completely obscured by the high oxygen reduction current. In contrast, by application of a scrubbing potential of -0.50 V a t the “upstream” cell, an oxygen-free solution enters the “downstreamn detector, and well-defined lead and cadmium peaks are observed. Note also
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-E,V Figure 5. Comparlson of strlpping voltammograms recorded at the “downstream” detector, utilizing oxygen cleanup by electrochemical scrubbing (a) arid by conventional nitrogen purge (b): 2.5 X loe7 M lead and 4.3 X lo-’ M cadmium in 0.1 M KNO,; 3-min depositions at -1.1 V; flow rate, 0.28 mL/min; linear scan of 1 V/min.
the disappearance of the copper peak when the scrubbing potential is applied. This is a minor drawback of the method since the determination of few metals with relatively positive peak potentials (Cu, Bi, Sb) is not feasible. Fortunately, most of the metal ions determined by ASV (e.g., Cd, Zn, T1, Pb, In, Sn) are not reduced in the potential used for the oxygen scrubbing. Figure 5 compares the efficiency of the electrochemical oxygen scrubbing with that obtained by conventional nitrogen purge. Similar background and peak currents are observed, and the only difference is the absence of the copper peak when the RVC scrubber is employed. Lower noise level is also noted in the scrubbing experiment. Except for the voltammogram in Figure 5b, all the voltammograms presented in this paper were recorded without a nitrogen purge. In many cases, by utilizing the electrochemical scrubbing, two problems are eliminated simultaneously; e.g., the potential -0.6 V used to remove the copper in the intermetallic study removes also the dissolved oxygen. In general, the interference of oxygen upon the differential pulse stripping voltammograms was found to be much less severe than its interference in the linear scan stripping studies. The detailed effect of oxygen upon the differential pulse stripping response is presently under investigation in our laboratory. Resolution. The most common type of interference in stripping analysis is when two stripping peaks have similar potentials, so that they coalesce or partially merge. For example, the simultaneous determinations of indium with cadmium, copper with bismuth, or tin with lead are common problems due to the similarity in their oxidation potentials. This problem can be minimized by changing the supporting electrolyte to alter the peak potentials or by decreasing the deposition pokntial to the point where only the metal ion with the more positive peak potential is plated; with the later approach, the ion with the more negative peak potential cannot be determined. The dual flaw-cell assembly can be used to detect the ion with the more negative peak potential in such mixtures. This is accomplished by the removal of the more easily reducible metal a t the “upstream” coulometric cell. Figure 6 illustrates the determination of a mixture of cadmium and indium. the two metals yield peak currents that partially merge (curve a). By proper adjustment of the potential of the “upstream” cell, the indium is plated a t the RVC electrode, and thus the cadmium is determined at the “downstream” detector without interference (curve c). The success of this approach depends on the separation between the peak potentials of the metals, and it is effective for Uplarger than 50 mV. When AI?,,is
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Flgure 6. Determination of 1 X lo-’ M indium and cadmium: 3-min depositions at -1.2 V; scrubber potential, 0.0 V (a), -1.0 V (b), and -1.06 V (c); flow rate, 0.2 mL/min; differential pulse conditlons, as In Figure 2. The small peaks at -0.45 V and -0.1 V (voltammogram (a)) are due to lead and copper ions present in the blank solution.
smaller than 50 mV, significant decrease of the peak current of interest is observed due to the difficulty in precisely controlling the RVC potential (that may be attributed to the RVC ohmic drop). For AE, of 50-100 mV, a small fraction of the ion with the more negative potential is also removed by the ”upstream” cell (compare Cd peaks in curves a and c), but this does not affect the quantitation process. This was indicated M indium ion from successive standard additions of 4 x to a test solution containing also 4 X M lead. By use of a potential of -1.02 V a t the RVC electrode, the lead was scrubbed while an indium peak current proportional to the solution concentration was obtained (not shown). Similar advantage was obtained in the determination of 2 X lo4 M copper in the presence of 4.7 X M bismuth in 0.1 M HC1 (conditions: scrubbing potential, -0.525 V; deposition at -0.8 V for 2 min; flow rate, 0.14 mL/min). Stability. The potential of the system for continuous monitoring is demonstrated by the precision obtained during an unbroken 3-h period of operation. A series of 12 determinations were performed (using a river water sample and the same mercury film), each at 15-min intervals; six of these runsare represented in Figure 7 . The mean lead peak current found was 0.83 pA with a range of 0.76-0.99 p A . The relative standard deviation over the complete series was 8%. In this study, the “upstream” cell was held continuously at -0.6 V, thus scrubbing the dissolved oxygen and traces of copper present in the river water. The gradual decrease in the stripping current is attributed to a variety of experimental parameters (e.g., adsorption of contaminants, mechanical stress) that cause gradual deterioration of mercury films. When working with relatively high concentration of the “scrubbed” ion, we used periodic cleaning of the RVC electrode, e.g., switching to 0.0 V for 2 min after every 30 min of operation, In this manner, the same RVC cylinder was employed for more than 45 days. We found the RVC scrubber to be effective in protecting the “downstream”detector against air bubbles that may occasionally be introduced into the flow system. Protection of the “downstream” detector against particulate matter and algae present in environmental samples is another advantage of the scrubber. In conclusion, the dual cell assembly is a useful device for improving the analytical capability of flowing stream ASV. Selective electrolysis of interfering constituents onto different electrodes eliminates or minimizes problems such as inter-
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-E,V Flgure 7. Behavior of the system under continuous use, river water (pH 1.9) spiked with 1 X lo-’ M lead and cadmium: depositions, 2 min at -1.2 V; scrubber potential, -0.6 V; flow rate, 0.2 mL/min; differential pulse conditions, as In Figure 2. The number on each curve represents the run number.
metallic effects, dissolved oxygen, and overlapping peaks. Applicability of the assembly to other electrochemical flow systems seems clear. Work is in progress to adapt the dual cell for purification of effluents from heavy metal contaminants, utilizing on-line stripping monitoring of the purified stream.
ACKNOWLEDGMENT The assistance of J. Tobin in the machining of the coulometric cell is highly appreciated. Registry No. HzO, 7732-18-5;Cu, 7440-50-8;Pb, 7439-92-1; Cd, 7440-43-9; Zn, 7440-66-6; In, 7440-74-6. LITERATURE CITED (I) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11)
Lieberman, S. H.; Zlrlno, A. Anal. Chem. 1974, 4 6 , 20. Wang, J.; Ariel, M. Anal. Chlm. Acta 1978, 99, 89. Wang, J.; Arlel, M. Anal. Chim. Acta 1978, 101, 1. Anderson, L; Jagner, D.; Josefson, M. Anal. Chem. 1982, 5 4 , 1371. Andrews, R. W.; Johnson, D. C. Anal. Chem. 1976, 4 8 , 1056. Wang, J.; Ariel, M. J . Nectroanal. Chem. 1977, 85, 289. Schieffer, G. W.; Blaedel, W. J. Anal. Chem. 1978, 5 0 , 99. Blaedel, W. J.; Wang, J. Anal. Chem. 1979, 51. 1724. Schleffer, G. W. Anal. Chem. 1980, 52, 1994. Roston, D. A.; Brooks, E. E.; Heineman, W. R. Anal. Chem. 1979, 51, 1728. HanekamD, H. B.; Voogt, W. H.; Bos, P.; Frei, R. W. Anal. Chlm. Acta 1980, 1 1 8 , 81. Wang, J.; Dewald, H. D. Anal. Chim. Acta 1982, 136, 77. Wang, J. Electfochim. Acta 1981, 26, 1721. Strohl, A. N.; Curran, D. J. Anal. Chem. 1979, 5 1 , 1050. Blaedel, W. J.; Wang, J. Anal. Chem. 1979, 5 f , 799. Shuman, M. S.; Woodward, G. P., Jr. Anal. Chem. 1976, 48’, 1979. Copeland, T. R.; Osteryoung, R. A.; Skogerboe, R. K. Anal. Chem . 1974, 4 6 , 2093. Yarnitzky, C.; Ouziel, E. Anal. Chem. 1976, 48. 2024.
RECEIVED for review October 25, 1982. Accepted February 7 , 1983. This work was supported by a Grant from the U.S. Department of the Interior, through the New Mexico Water Resources Research Institute.
