402
Anal. Chem. 1986, 58,402-407
Cellulose Acetate Coated Mercury Film Electrodes for Anodic Stripping Voltammetry Joseph Wang* and Lori D. Hutchins-Kumar Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
The response characterlstlcs and analytical advantages of cellulose acetate coated mercury fllm electrodes for anodlc strlpplng measurements of trace metals are descrlbed. The coating provldes an effectlve barrler on the mercury surface, thus elknlnatlng the effects of various organic surfactants. For example, up to at least 100 ppm gelatln does not alter the response. The dlagnostic power of rotatlng disk measurements Is used to evaluate the transport toward the mercury surface. The response Is limited by the permeablllty of the fllm, thus allowlng strlpping measurements In systems with poorly controlled mass transport. Base hydrolysls of the film Is used to manlpulate the permeablllty. Scanning electron micrographs show the microstructures of the fllms followlng dlfferent hydrolysis times. The dlscrlmlnatlve propertles of these coatings can be used also to Improve the resolutlon between two adjacent strlpplng peaks. The response of the modlfled electrode is directly proportlonal to the analyte concentratlon and Is reproducible. WHh a 10-mln deposltlon tlme, detection IlmHs are 7 X lo-’’ M lead and 1.3 X lo-’ M cadmlum. Varlous metal ions and organic surfactants are tested. The performance of thls novel electrode system Is compared to that of a conventlonal mercury fllm electrode.
Stripping voltammetry is a powerful electroanalytical technique for trace-metal measurements ( I ) . The technique offers excellent sensitivity, coupled with multielement, speciation, and on-line capabilities, all with low-cost instrumentation. However, in order to meet the new challenges of today’s analytical problems, improved stability and selectivity are desirable. This is particularly due to problems associated with sorption of surface active compounds and overlapping stripping peaks that are encountered when complex (environmental and clinical) matrices are analyzed. The adsorption of organic surfactants on the working electrode surface may affect both the deposition and stripping steps, leading to a decrease or increase in the peak current and a shift in peak potential. Various approaches, based mainly on different sample pretreatment procedures (e.g., UV irradiation, acid digestion, ozone oxidation, and preferential masking or ionexchange separation) have been suggested to minimize the surfactant sorption and overlapping peaks problems, respectively. In addition to being time-consuming, some of these steps may result in severe contamination especially when ultratrace measurements are concerned. Strategies based on new working electrodes, or modification of existing surfaces, have not been explored for circumventing the above problems. The most practical electrodes for stripping voltammetry are the hanging mercury drop electrode and the mercury film electrode. Because of its greater sensitivity, the mercury film electrode is commonly used for ultratrace measurements. Although this electrode has found widespread use for about two decades (2,3), its chemical modification-that may improve its performance-has not been reported. The field of chemically modified electrodes has been the focus of considerable research activity. While this area of electrochemical
research offers unusual analytical possibilities, progress in this direction has been very slow. Most reports on the use of chemically modified electrodes for electroanalysis have been concerned with improved electrochemical detection for liquid chromatography using electrocatalytic surfaces ( 4 , 5 )and the development of new preconcentration schemes (using various ligands or ion exchangers) for analytes that cannot be accumulated electrolytically (6-10). The work presented here describes the analytical advantages and transport characteristics of cellulose acetate coated mercury film electrodes fop anodic stripping voltammetry. The analytical potential of cellulose acetate coated electrodes was first demonstrated by Sittampalam and Wilson (1I ) using amperometric detection of hydrogen peroxide at a surfacemodified platinum detector. In a recent work, we demonstrated that controlled hydrolysis of cellulose acetate films in alkaline media can be used to manipulate their permeability and thus to improve the selectivity and stability of glassy carbon detectors used in liquid chromatography/flow injection systems (12).While the above studies have demonstrated the potential of this concept for amperometric detection at bare solid electrodes, voltammetric studies at mercury surfaces coated with cellulose acetate polymeric films have not been reported. The utility of base-hydrolyzed cellulose acetate coating for trace-metal measurements is examined in the present study. The results demonstrate that by controlling the access to the mercury film surface, stripping voltammetry can be improved with respect to surfactant interferences and overlapping peaks. Additionally, the unique transport characteristics of these modified mercury film electrodes offer the advantage of mass-transport-independent response. These, and other, characteristics and advantages are discussed below. EXPERIMENTAL SECTION Apparatus. A Bioanalytical Systems Model VC-2 electro-
chemical cell was employed in most experiments. The working electrode (glassy carbon disk, Model MF 2012, Bioanalytical Systems), reference electrode (Ag/AgCl, Model RE-1, Bioanalytical Systems), platinum wire auxiliary electrode, and the nitrogen delivery tube joined the cell through holes in its cover, which was made of Teflon. The cell was placed on a magnetic stirrer (Sargent-Welch,no. 76490), and a 1 cm long magnetic bar was placed in the cell bottom. Some experiments used a rotating disk electrode (Model DDI 15, Pine Instruments) in conjuction with a 100-mL glass cell. A removable end disk electrode, made of a Plexiglas sleeve and a 3 mm diameter glassy carbon disk was machined to interface to the rotated shaft. (The disk was expoxied to the Plexiglas sleeve.) Such “homemade” design was required because the cellulose acetate film did not adhere in a stable manner to the Teflon sleeve of the commercial electrode. (No difficulties were observed with Plexiglas and Kel-F supports.) Current-voltage data were recorded with a Princeton Applied Research Model 174 polarographic analyzer and Houston Omniscribe strip-chart recorder. Scanning electron microscopy was done with a Phillips 5OlB-SEM electrodes were prepared for this investigation in the usual procedure (see below) with an additional evaporative coating of gold for better resolution. Reagents. Stock solutions (IO4 M) of metal ions were prepared by dissolving the pure metal or its salt in nitric acid and diluting
0003-2700/86/0358-0402$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1988 GC
~
403
ion
Figure 1. Schematic depiction of the transport of metal ion, M", during the deposition and stripping steps at the cellulose acetate coated mercury film electrode.
> ~-.o-o-o-o=
A-A-A-A-AB
to volume with water. All samples were prepared in 0.1 M acetate buffer (pH 4.5). Cellulose acetate (39.8% acetyl content) was purchased from Aldrich Chemical Co. Solutions (5000 ppm) of the organic surfactants were prepared weekly by dissolving the reagent (or practical) grade materials at room or elevated temperature. All solutions were prepared with double-distilled water. Coating the Glassy Carbon Disk with Mercury and Cellulose Acetate Films. Because of the nature of the procedure, preplated mercury films were used. Prior to the mercury plating, the glassy carbon surface was polished with 1-pm a-alumnia particles and rinsed with diluted nitric acid, followed by copious amounts of deionized water. The mercury plating solution, 5 X M mercuric nitrate in 0.1 M acetate buffer (pH 4.51, was deaerated with nitrogen for 8 min. Then a potential of -0.70 V was applied for 30 min, under forced convection conditions-400 rpm stirring or 900 rpm rotation speed-depending on the electrode used. (Shorter deposition periods and/or lower mercuric ion concentrations yielded unstable films due to the subsequent exposure to air associated with the modification procedure.) Following this, the potential was switched to 0.0 V and held there for 1 min. The potentiostatic control was then disconnected; the electrode was rinsed with double-distilled water and immersed in a quiescent 5% cellulose acetate solution (the preparation of which was described previously (12)). The dip-coating procedure proceeded for 30 s, following which the electrode was inverted and allowed to dry in air for 2 h in a dust-free area. Prior to the hydrolysis of the film the electrode was immersed for 10 min in the supporting electrolytesolution, and background voltammograms were recorded to test its performance. (Approximately two out of three electrodes yielded a satisfactory background response.) Hydrolysis proceeded with such electrodes in a deaerated (and stirred) 0.07 M KOH solution for the desired time. The electrode was then stored in the acetate buffer solution until the stripping experiment (as well as between experiments). Procedure. Following an 8-min deaeration, background and sample measurements were carried out successively as follows. A deposition potential was imposed on the working electrode under forced convection conditions (solutionstirring or electrode rotation). The potential was chosen according to the cations to be determined and was maintained for a period depending on their concentration level. The stirring (or rotation) was then stopped, and after a 15-s rest period the metals were stripped from the mercury film by applying a differential pulse anodic potential ramp with a 5 mV/s scan rate and 25 mV amplitude. The scan was stopped at +0.15 V, and after 60 s the system was ready for the next deposition-stripping cycle. Both (mercury and cellulose acetate) films were removed at the end of the day by wiping the electrode face with a soft tissue wetted with double-distilled water. RESULTS AND DISCUSSION Response Characteristics. The successful analytical utility of cellulose acetate coated mercury film electrodes depends upon understanding their fundamental behavior. The diagnostic power of the rotating disk electrode was used to evaluate the transport characteristics of these electrodes. As illustrated in the reaction shceme (Figure l),the coating on
o-o-o-o-oA
0
,
IO
50
30 df2, (rprn,'f2
Flgure 2. Peak current vs. (rotation rate)"' for stripping of lead at the rotating disk electrode; electrode coating with 0- (A), 20- (B), and 40-min (C) hydrolysis; bare electrode (D); 2 X M lead in 0.1 M acetate buffer (pH 4.5); deposition for 2 min at -1.1 V differential pulse
stripping mode with 5 mV/s scan rate and 25-mV amplitude.
the mercury surface may be viewed as a polymer layer that is interposed between the mercury surface and the Levich layer. The metal ion must be transported across both layers to reach the mercury surface where it undergoes reduction. Similarly, the stripped metal ion rapidly diffuses across both layers, as indicated from the absence of memory effects (see below). Figure 2 gives the stripping peak current vs. rotation speed data for 2 X lo-' M lead a t the coated electrode with 0-(A), 20-(B), and 40-min (C)hydrolysis, as well as a t the unmodified electrode (D).Electrode rotation speeds were varied over the 0-3600 rpm range. The unmodified electrode exhibits the expected mass-transport-dependent Levich behavior (slope of the 400-3600 rpm data, 0.400 MArpm-1/2; correlation coefficient, 0.999), with some natural convection contribution at 0 rpm. Films of different hydrolysis times demonstrate different permeabilities, as expected from the increased porosity of cellulose acetate (12). The nonhydrolyzed film (0-min data (A)) excludes the lead ions from the mercury surface; i.e., no peak is observed for all rotation speeds. The film permeability increases upon increasing the hydrolysis time. Well-defined lead peaks are obtained for the 20- and 40-min films. With a 20-min hydrolysis time the peak current is independent of rotation speed over the 0-3600 rpm range. The 40-min film exhibits only a slight dependence on rotation speed over the 0-900 rpm range; rotation-speed-independent response is observed over the 900-3600 rpm range. Thus, transport through the film is the major contributor to the total diffusional resistance over the entire range of rotation speeds examined. This feature would be particularly useful for making trace-metal measurements in systems with poorly controlled mass transport (e.g., on-line industrial and environmental monitors), as complications from external mass transport are eliminated. This situation is analogous to that encountered for disk electrodes covered with bulk membranes (e.g., Millipore VC, Cuprophane, Spectrapor), for which the response becomes independent on external mass transport at high rotation speeds (13-15). The increased diffusional resistance associated with the film coating results in some loss in sensitivity (2-6 times over the 400-3600 rpm speed range; a t 0 rpm the modified electrode yields a 2.5-fold larger response). AS will be shown later, such diminished sensitivity appears to be of no concern, considering the inherent sensitivity of stripping voltammetry.
404 ANALYTICAL CHEMISTRY. VOL. 58. NO. 2. FEBRUARY 1986
4 2.0
I
F@w 9. capulsonof supping w*ammgnms rsmd.d at tho bare(a)Mdcoatad@)maaryRn&c!mde3: 2 X t O - ' M c a & & n . lead. and copper in 0.1 M acetate bulief (pH 4.5); 4OmIn hydroiysis; s t M q rate. 400 rpm: other condMons as in Flpm 2.
Table 1. Characteristics of the Stripping P a b at the Bare and Coaled Mercury Film Electrodes"
metal ion cadmium lead copw
.WaL .oatantid. V ordinary modified electrode electrode -0.66 -0.50 -0.11
4.69 -0.53 -0.14
oeak half-width. mV ordinary modifled electrode electrode 50 51
63 60
63
71
Figure 3 compares stripping voltammograms for a 2 X Icr? M cadmium, lead, and copper solution, obtained under identical conditions at the unmodified electrode (a) and the coated electrode (b). At both electrodes well-defined peaks are observed, following a 2-min deposition. The peak p tentials and widths at half-height are summarized in Table I., The fdte resistance of the film results in 0.03-V negative shifts in peak potentials, an well an in some peak broadening (3-14 mV increases in widths at half-height). Such effecta are not substantial, as compared to bulk membranea that often exhibit large electrical resistivity and require special arrangement, with all three electrodes isolated behind the membrane (14, 16). The analytical utility of this novel electrode system requirea knowledge of the effect of the hydrolysis time &e., film permeability) on the stripping response. Figure 4 shows the dependence of the stripping peak current on the hydrolysis time for different metal ions. The nonhydrolymd film (0-min data) excludes the copper, lead, and cadmium ions from the surface; only the bismuth and indium ions yield small peaks under these conditions. A gradual increase of the lead and cadmium peak is observed upon hydrolyzing the film for 10, 20, and 30 min. A slight decrease of the indium and bismuth peaks is observed for the 10- and 20-min films. With hy-
Flwa 5. Scamhe elsclron W o g a p h s of calubse acemte ooahps hydrolyzed fa 15 (A) and 40 IBI min: mgnificaton. (A) lOOOOX and (E) 640X; acwbrating voltage. I A l 7 2 k V and (8)3.6kV.
drolysis times longer than 30 min. the copper, bismuth. and indium peaks rapidly increase. while the cadmium and lead peaks continue their gradual growth. The trend in permeability observed under these conditions, In > Bi > P b > Cu > Cd is in good agreement with the variations in sue among these ions, Cd > Cu > P b > In > Bi (17). The transport of organic compounds thmugh cellulose acetate coatings has been shown to depend primarily on molecular size. Under the acidic conditions used in the present study the f h coating is neutral, and the ionic charge should not play a major role in the transport. Simple explanation for the trends observed for coatings with short hydrolysis times (l(t30 min) is not available. Overall, these data illustrate that the transport of heavy metal ions toward the mercury surface is drastically increased by the hydrolysis process. A hydrolysis time of 40 min wan used in most experiments described below as it offers the best compromise between sensitivity and film integrity. Shorter hydrolysis times were used in some experiments aimed M improve the resolution between two metals. Examination of the hydrolyzed cellulose acetate coating by seanning electron microscopy is shown in Figure 5. The photomicrograph of the l b m i n coating (A) exhibits small pores with diameters ranging from 200 to 600 nm. Substantially larger pores, with diameters ranging from 2 to 10 pm, are observed in the 40-min film (B). (Notice the different
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
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,
I
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M Figure 6. Effect of gelatin on the Stripping peaks of 2 X cadmium (a), lead (b), and copper (c) at the coated (A) and bare (B) mercury film electrodes: stirring rate, 400 rpm; hydrolysis time (A), 40 min; other conditions as in Figure 2.
magnifications employed.) While the filmswere removed from the mercury surface prior to this examination, mercury droplets can be seen under the surface of both films (and within some pores of the 40-min coating). No pores were observed at the nonhydrolyzed film (not shown). While the appearances of these films is not homogeneous, the micrographs in Figure 5 clearly illustrate the morphological differences between coatings hydrolyzed for different periods. Thus, the transport properties (described earlier) are closely related to the film microstructre. Additional experiments were performed to further characterize the behavior of the coated mercury film electrode. The dependence of the peak current on the metal concentration was evaluated from a series of eight successive standard additions of cadmium and lead; each addition corresponded to a 1 X M increase in concentration (conditions as in Figure 3). This experiment yielded linear calibration plots, with slopes of 7.110 (Pb) and 8.019 (Cd) PA PM-~,and correlation coefficients of 0.997 (Pb) and 0.999 (Cd). The analogous experiment a t the bare mercury film electrode yielded slopes of 32.979 (Pb) and 33.778 (Cd) pA PM-I, with correlation coefficients of 0.999 (Pb) and 0.998 (Cd). Measurements of 2 x M cadmium and lead were used to estimate the detection limit (deposition for 10 min with 900 rpm rotation speed and film hydrolyzed for 40 min; other conditions as in Figure 2). Based on a signal-to-noise ratio of 2, values of 7 X M lead and 1.3 X lob9M cadmium were calculated. The precision of the results at the coated electrode was estimated from a series of six successive measurements of 2.2 X lo-' M lead and cadmium (other conditions as in Figure 3). These experiments yielded mean peak currents of 3.14 (Cd) and 3.20 (Pb) pA, with relative standard deviations of 2.4 (Cd) and 0.5% (Pb). Changes in peak currents, ranging from l to 20%, were observed in day-to-day results due to the use of different mercury and polymeric coatings, as well as glassy carbon surface conditions. Such changes cause no problem as daily calibration provides accurate quantification. Even with such changes, the trend is maintained according to ionic size, and the rejection of organic surfactants (the main advantage) is maintained. (Some day-to-day changes are common also a t conventional electrodes, due to differences in the glassy carbon and mercury surfaces). Long-term stability studies in the presence of organic surfactants will be described below. These stability, precision, and calibration studies indicate rapid diffusion of the stripped metal ion back to the bulk solution, prior to the subsequent deposition-stripping cycle. Analytical Utility. The presence of surface-active substances may have a marked effect on the stripping response, owing to the adsorption of such compounds at the mercury
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Dodecyl sodium sulfate conc.[ppml
Figure 8. Effect of dodecyl sodium sulfate on the stripping peak8 of 2 X M cadmium (a), lead (b), and copper (c) at the coated (A) and bare (B) mercury film electrodes: stirring rate, 400 rpm; hydrolysis time (A), 40 min; other conditions as in Figure 2. surface. Buffle et al. (18) emphasize the need for a reproducible voltammetric electrode capable of minimizing organic adsorption. This important matrix effect can be largely minimized or eliminated through the use of cellulose acetate coatings. We examined the effects of several common surfactants on the stripping response at both the bare mercury film electrode and the cellulose acetate coated mercury film electrode. The variation in the stripping peak currents of cadmium, lead, and copper with the concentration of gelatin, agar, and dodecyl sodium sulfate is illustrated in Figures 6, 7, and 8. Gelatin, a known maximum suppressor in polarography and a representative colloidal protein, has a pronounced effect on the response at the bare mercury film electrode (Figure 6B). The addition of 30 ppm gelatin results in 80 and 57% reduction of the copper and cadmium peaks, respectively; further additions up to 100 ppm cause no further decrease. The lead peak first increases (- 15%) upon additions of 10 and 20 ppm gelatin, following which it decreases gradually to values similar to those observed without gelatin. In contrast, the cellulose acetate coating excludes the large gelatin molecules (MW, 75 000) from reaching the mercury surface, resulting in a highly stable response for the three metals over the entire gelatin concentration range, 0-100 ppm (Figure 6A). While the bare electrode exhibits 20-40-mV shifts in peak potentials and broadening of the copper peak, no such effects were observed a t the coated electrode (not shown). Agar was used as a model compound for the polysaccharides excreted into natural waters by plankton. At the bare electrode, all three metals exhibit similar profiles upon increasing
408
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
Table 11. Stability of the Response in the Presence of 25 ppm Gelatin"
analyte lead cadmium copper
RSD,b % bare modified electrode electrode 10.4 23.2 60.1
7.8 8.2 33.1
L
I
5.OpA (a)
(4- i 3 2 ) / i l bare electrode
modified electrode
0.12 0.50 0.74
-0.07 -0.11 0.39
0.5pA (b)
Conditions as in Figure 3. Relative standard deviation of 32 successive measurements over an 8-h period (using the same surface and solution). (I
the agar concentration, with approximately 45% reduction in peak current at the 100 ppm surfactant level (Figure 7B). At the coated electrode no change in peak currents is observed for cadmium and copper up to 100 ppm agar (Figure 7A). A gradual increase in the lead peak (up to -25%) is observed upon increasing the agar concentration; this could be the result of lead impurities present in the practical grade polysaccharide. No changes in peak potentials were observed at either electrode using agar additions. Successive additions of the anionic surfactant dodecyl sodium sulfate (MW, 288), a representative polar lipid, result in pronounced fluctuations of the peak currenta for all three metals at the bare mercury electrode (Figure 8B). In contrast, only small and gradual increases in the peak currents are observed at the coated electrode (Figure 8A); these again may be attributed to impurities in the practical grade surfactant. No changes in peak potentials were observed at either electrode when dodecyl sodium sulfate was added. The effect of varying the organic compound concentration in the 0-100 ppm range was evaluated also for camphor, Triton X-100, and humic acid (not shown; conditions as in Figure 6). At the bare mercury film electrode, camphor (MW, 157) had a pronounced effect (up to 40% depression) on the copper peak and only a slight effect on the cadmium and lead peaks. In contrast, no change in the peak current of these metals was observed at the coated electrode. Triton X-100 (MW, 5000) exhibited drastic effects on the peak current of the three metals at the bare electrode (93,80, and 40% depressions of the Pb, Cd, and Cu peaks, respectively, at 100 pprn). At the coated electrode Triton X-100 caused a gradual decrease (up to 40%) in the peak current of the three metals. As it is unlikely that Triton X-100 could diffuse through the cellulose acetate coating, this decrease may be attributed to interference with the mass transport of the ions, possibly by partial blockage of the film. Successive additions of humic acid resulted in similar profiles for lead and copper at both electrodes (up to 60 and 90%, respectively, decrease in sensitivity at 100 ppm) and a sharper decrease of the cadmium peak at the bare electrode. Such behavior is expected as humic acid is a natural chelating agent as well as a surface active compound. Thus, both complexation and sorption effects may occur simultaneously. It appears possible that the use of cellulosic coatings would allow distinction between these effects, and thus it would be possible to obtain important information on the interactions of trace metals with this and similar organic materials. For example, the similar profiles observed at both electrodes for lead and copper indicate that complexation has a dominant effect on the response of these metals in the presence of humic acid. In order to be practical for routine stripping applications, the surface modification should be stable, yielding the desired protection over a long period of time. The stability of the modified and bare mercury film electrodes was tested by checking their behavior-in the presence of 25 ppm gelatin-during a long run of 32 successive determinations
i
.O
- 0.6
E,V
- 0.2
Figure 9. Differential pulse anodlc stripping voltammograms for 4 X M Indium and 2 X lo-' M lead at the bare (a) and coated (b) mercury film electrodes: rotation speed, 900 rpm; hydrolysis time (b), 15 min; other conditions as in Figure 2.
carried out over a total time of 8 h. The same surface and solution were used during each experiment. The results are summarized in Table 11. For the three metal ions tested, substantial improvement in the stability is observed at the coated electrode. A drastic decrease in sensitivity is observed at the bare electrode over this period (12,50, and 74% for lead, cadmium, and copper, respectively). In contrast, a slight increase (7 and 11%) in sensitivity is obtained for lead and cadmium respectively, at the coated electrode; only the copper peak exhibits a gradual decrease, up to 39%, over the 8-h period. These changes, and the resulting relative standard deviations, illustrate that the cellulosic coating maintains its fouling protection over long time frames. In a separate exeriment, in the absence of gelatin, the modified electrode exhibited a similar behavior (not shown). This indicates that the variation of the copper peak is not the result of electrode fouling but rather due to possible loss of copper ions on the cell walls; gradual absorption of copper ions is another possible explanation, since such polymers are known to interact with copper species (19). Besides their major advantage, i.e., protection from organic surfactants, the cellulose acetate coated mercury film electrodes can be used, in certain situations, to improve the resolution of stripping measurements. Such improvement is based on the hydrolysis time vs. peak current profiles shown in Figure 4. As indicated from these profiles, different metal ions exhibit different permeabilities through the cellulose acetate coating. A judicious choice of the hydrolysis time can thus be used to minimize the overlapping peaks problem when different binary systems are concerned. For example, trace measurements of lead often suffer from resolution problems, as additional metal ions (e.g., thallium, indium, tin, cadmium) yield stripping peaks within a narrow potential range around the lead peak. Because the permeability of the lead ion gradually increases at short hydrolysis times-while that of
407
Anal. Chem. 1986, 58,407-412
other ions does not-it is possible to enhance the selectivity of the measurement toward lead. Figure 9 illustrates the measurement of 2 X lo-' M lead in the presence of 4 X lo-' M indium at the bare (a) and coated (b) mercury film electrodes. The difference in the peak potentials for these two species is approximately 150 mV, and their peaks are partially merged. Due to the differences in permeability of the two ions at the 15-min hydrolysis coating, the degree of overlap is substantially reduced, resulting in an indium-blead peak ratio of 0.20 compared to 2.0 at the bare electrode. Similar improvement may be observed for other binary systems, e.g., measurements of bismuth in the presence of copper or of lead in the presence of cadmium (using 30- and 10-min hydrolysis times, respectively; see Figure 4). CONCLUSIONS The above results clearly demonstrate the potential utility of cellulose acetate coated mercury f i i electrodes for stripping voltammetry. The controlled permeability achieved by hydrolyzing this polymeric coating in alkaline media is exploited for minimizing interference such as adsorption of organic surfactants and overlapping stripping peaks. Since such problems are minimized via an in situ separation step-carried out a t the electrode surface-time-consuming sample pretreatment procedures may not be required. Because the response is not affected by external mass transport, stripping measurements can be performed with systems with poorly controlled mass transport (or in the absence of convective transport without any sensitivity loss). Even though the concept is presented here in terms of trace-metal measurements, it could be extended to determination of anions and organic analytes by cathodic stripping and adsorption voltammetric procedures. In future work we will examine the
utility of these electrodes to speciation studies, based on the possible discrimination between sorption interferences and complexation effects. Such discrimination would be extremely useful for the determination of complexing capacity of natural waters. Registry No. Hg, 7439-97-6;Pb, 7439-92-1;Cd, 7440-43-9;Cu, 7440-50-8; cellulose acetate, 9004-35-7. LITERATURE CITED (1) Wang, J "Stripping Analysis: Principles, Instrumentation, and Applications"; VCH Publishers: Deerfieid Beach/Weinheim, 1985. (2) Roe, D. K.; Toni, J. E. A. Anal. Chem. 1965, 3 7 , 1503. (3) Florence, T. M. J . Electroanal. Chem. 1970, 2 7 , 273. (4) Korfhage, K. M.; Ravichandran, K.; Baidwln, R. P. Anal. Chem. 1984, 5 6 , 1514. (5) Wang. J.; Freiha, B. Anal. Chem. 1984, 56, 2266. (6) Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, 7 7 , 393. (7) Cox, J.; Kuiesza, P. J. Anal. Chlm. Acta 1983, 754, 71. (8) Wang, J.; Greene, 8.; Morgan, C. Anal. Chim. Acta 1984, 758, 15. (9) Szentirmay, M. N.: Martin, C. R. Anal. Chem. 1984, 56, 1898. (10) Guadaiupe, A. R.; Abruna, H. D. Anal. Chem. 1985, 5 7 , 142. (11) Sittampalam, 0.; Wilson, G. S. Anal. Chem. 1983, 55. 1608. (12) Wang, J.; Hutchins, L. D. Anal. Chem. 1985, 5 7 , 1536. (13) Chien, Y. W.; Oisen, C. L.; Sokoloski, T. 0. J . Pharm. Sci. 1973, 6 2 , 435. (14) Gough, D. A.; Leypoidt, J. K. Anal. Chem. 1979, 57, 439. (15) Stewart, E. E.; Smart, R. B. Anal. Chem. 1984, 56, 1131. (16) Freese, J. W.; Smart, R. B. Anal. Chem 1982, 5 4 , 836. (17) Holtzclaw, H. F.; Robinson, W. R.; Nebergail, W. H. "General Chemistry"; D.C. Heath and Co.: Lexington, MA, 1984. (18) Buffie, J.; Cominoli, A.; Greter, F. L., Haerdi, W. Roc. Anal. Div. Chem. SOC. 1978, 75, 59. (19) Hayashita, T.; Takagi, M. Talanta 1985, 3 2 , 399.
RECEIVED for review August 5, 1985. Accepted October 1, 1985. This work was supported in part by the National Institutes of Health, Grant GM30913-02. The authors acknowledge H. P. Adams (EML, NMSU) for taking the scanning electron microscopy photographs.
Potentiometric Stripping Determination of Heavy Metals with Carbon Fiber and Gold Microelectrodes Andrzej S. Baranski* and Henry Quon
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0
The determination of Cd, Pb, and Cu wlth mercury film microelectrodes (4 X IO-' to 2 X lo-' cm2 surface area) was examined. I t was demonstrated that carbon fiber mlcroelectrodes are suitable for multicomponent trace analysis of very small (5-pL) samples. Varlous processes occurring at microelectrodes such as charging of the double layer, poisoning of the electrodes, and nucleation processes are also discussed in terms of their effect on the detection limit and precision of the method.
In recent years a considerable interest in the various applications of microelectrodes made of a carbon fiber or a thin metal wire has been observed (123). Due to the large edge effect, these electrodes have a unique mass transport characteristic ( 4 , 5 ) (large and steady-state flux) as well as low resistance polarization (6, 7). Under limiting current conditions the current density a t a stationary carbon fiber microelectrode (8 km in diameter) is comparable to one observed at a large disk electrode rotating with a speed of 500 rps (1). 0003-2700/86/0358-0407$01.50/0
In addition, if the rotating disk electrode is, for example, 2 mm in diameter, the resistance polarization of the carbon fiber electrode will be 250 times smaller than the first electrode. This illustrates the obvious advantages of small electrodes in stripping analysis. The enhancement of mass transport may lead to a decrease in the preconcentration time. The steady-state flux during the preconcentration step makes stirring the solution unnecessary. This eliminates one parameter that must be controlled and consequently one source of error. The low resistance polarization makes it possible to carry out determinations without the supporting electrolyte, which usually contains interfering impurities. The most serious disadvantage of microelectrodes under voltammetric conditions is a very small current that must be measured. Commercially available instruments do not operate in the proper current range. In addition a noise accompanying the signal of interest is troublesome ( I ) . In this context the potentiometric stripping technique developed by Jagner (8) seems to be an attractive alternative. This technique can accommodate electrodes of any size. The response of the electrode does not need to be amplified, thereby a lower noise 0 1986 American Chemical Society
