Anal. Chem. 1994,66, 3151-3157
Nature and Stability of Mercury Thin Films on Glassy Carbon Electrodes under Fast-Scan Anodic Stripping Voltammetry H. Plng Wu YSI Incorporated, Research Center, P.O. Box 279, Yellow Springs, Ohio 45387
The surface structureof mercury thin films (MTFs) on a glassy carbon electrode (GCE) and the changes of MTFs associated with anodic stripping voltammetry (ASV) were studied using ex situ optical microscopy and linear scan voltammetry. The results of microscopic examinations showed that, in contrary to tbe early observations of large scattered microdroplets on GCE surfaces, the original state of MTFs was close to a thin film, The surface coverage of the mercury microdroplets was continuousmicroscopically,and the size of the droplets ranged from less than 0.1 pm (unidentifiable under 400X magnification) to about 1 pm, depending on the total amount of mercury on the GCE surface. Under the condition of ASV scanning (2 V/s) with in situ MTFs, the stability of a mercury coating was shown to depend on the total amount of mercury on GCE. It was also shown that the severe noise spikes and the erratic current behavior were associated with the surface structural changes from a state of fine and smooth microdroplets to a state of large scattered microdroplets. The transformation from one state to another is believed to be driven by the surface perturbation of rapid potential changes under the ASV condition. We report here the surface conditions of mercury coatings on a glassy carbon electrode (GCE) and the surface changes associated with anodic stripping voltammetry (ASV). ASV is a sensitive tool for the analyses of trace amounts of heavy metals.’-’ Perhaps the most widely used electrode form is the mercury thin film (MTF) on a glassy carbon substrate because of the versatility, the ease of preparation, and the wide potential window for a n a l y s e ~ . ~ -In ~ l particular, ~ ASV with in situ formed MTF provides high sensitivity, low detection limit, and high resolution of neighboring peaks, as demonstrated by Fl~rence.~ Since their initial usage, the nature of mercury coatings on GCE has been the subject of many s t ~ d i e s ~ - ~because ,~,~J~ of the importance of a stable and reproducible mercury coating to ASV and the interesting properties of mercury thin films as a form of electrode. The model of homogeneous thin film has been used in theoretical treatments9-13 of metal diffusion (1) Vydra, F.; Stulik, K.; Julakova, E. Electrochemicol Stripping Analysis; Wiley: Chichester, Sussex, U.K., 1976. (2) Wang, J. Stripping Analysis-Principles, Instrumentation ond Applications; VCH: Deerfield Beach, FL, 1985. (3) Frenzel, W. Anal. Chim. Acta 1993, 273, 123. (4) Florence, T. M.J. Electroonol. Chem. 1970, 27, 273. (5) Wojciechowski, M.;Balcerzak, J. Anal. Chem. 1990, 62, 1325. (6) Perone. S.P.; Davenport, K. K.J . E/ectroaM/. Chem. 1966, 12, 269. (7) Stulikova, M.J . Electrounal. Chem. 1973, 48, 33. (8) Florence, T. M.Anal. Chim. Acta 1980, 119, 217. (9) de Vries, W. T.; van Dalen, E. J . Electround. Chem. 1964, 8, 366. 0003-2700/94/0366-3151$04.50/0 0 1994 American Chemical Society
in and out of the mercury coating and in experimental applications in ASV,435 with the film thickness ranging from 1 to 10 nm for in situ formed MTFs4 and from 0.1 to 1 pm for ex situ formed M T F s . ~ On the other hand, mercury coatings on the GC surface have been shown, by various optical microscopic and SEM i m g a g e ~ , ~ to J ~be J ~inhomogeneous with scattered microdroplets. Despite these reports, the nature of mercury coatings is still not at all certain because of the dynamics involved in stripping processes and the inconsistent behavior of ASV with various MTFs. Thus, to what degree these mercury coatings can be considered as thin films remains unanswered. The objectives of the present investigation of mercury coatings on GCE have been to understand the nature of mercury coatings under various conditions and to study the relationship between the nature of the stripping current, especially the erratic behavior, and the surface structural changes. For mainly practical reasons, particular attention was given to the in situ formed MTFs. We first describe the surface conditions of mercury coatings as a function of mercury concentrations and deposition times under conditions of varying mercury deposition rate. We then reveal the surface structural changes of mercury coatings by a combination of ex situ optical microscopy and linear scan voltammetry. Factors affecting the stability and reproducibility of in situ formed MTFs are discussed.
EXPER I MENTAL SECTION Reagents and Solution Preparations. H N 0 3 (double distilled) and KNO3 (99.999% pure) were purchased from GFS Chemicals (Columbus, OH). AA standard solutions of Pd, Cd, Cu, and Hg of 1000 ppm were obtained from Fisher. Standard solutions of appropriate concentrations were diluted from the AA standard solutions. An Eppendorf pipet (0.101.00 mL) and an Oxford pipet (1.0-5.0 mL) equipped with disposable pipet tips were used for the appropriate dilutions. All solutions were prepared with water from NANOPURE water system of Barnstead/Thermolyne Corp. and stored in polyethylene bottles. All ASV experiments were carried out (10) de Vrics, W. T. J. Electroonal. Chem. 1965, 9, 448. (1 1) Kounaves, S. P.; O’Dea, J. J.; Chandresekhar, P.; Osteryoung, J. Anal. Chem. 1986, 58, 3199. (1 2) Kounaves, S. P.; O’Dea, J. J.; Chandresekhar, P.; Osteryoung, J. A M / .Chem. 1987, 59, 386. (13) Copeland, T. R.; Christie, J. H.; Osteryoung, R. A.; Skogerbor, R.K. Anal. Chem. 1973,45, 2171. (14) Brainina, K. Z.; Vilchinokaya, E. A.; Khanina, R. M.Analyst 1990, 115, 1301. (15) Wang, J.; Tien, B. Anal. Chem. 1992, 64, 1706.
AnalvticaIChemistry, Vol. 66,No. 19, October 1, 1994 3151
in nitrate medium with acidity ranging from 25 to 50 mM nitric acid. Electrochemical Experiments. All analyses were carried out in a 10-mL cell. Sample injections were made with a 25-pL micropipet syringe (YSI, Yellow Springs, OH) with a dilution factor of 400 so that a sample injection would not affect the bulk concentration of the analytes. ASV experiments and data acquisition were conducted with a CS-1090 Electrochemical system (Cypress Systems, Lawrence, KS). Solutionstirring was done with a Thermolyne Stir-mate, Model S-7805 stir-plate (SYBRON/Thermolyne, Dubuque, IA), and the stirring was set at scale 3 during the deposition time. The substrate electrode was a glassy carbon electrode of 1 mm diameter (GC-EEOOSP, Cypress Systems, Lawrence, KS). The reference electrode was a Ag/AgCl electrode in 3 M NaCl solution (BAS, West Lafayette, IN), and the counter electrode was a Pt wire. Amounts of Hg at various deposition times on the GCE surface were estimated by charges of the mercury stripping peak (by electronic integration of the stripping current) from staircase voltammograms at 0.25 V/s. The wave form for ASV scans was staircase with 1 mV/step. All ASV experiments were conducted in solutions without purging of the oxygen. The deposition time for ASV was 2 min with stirring and 30 s without stirring (rest condition). Before each experiment, the GCE was polished with 0.05 pm alumina on a microcloth polishing pad (Buehler, Evanston, IL). The GCE was rinsed with nanopure water and cleaned by sonication for 15 s in nanopure water following polishing. Noise analyses of voltammetric data by Fourier transform (FT) were carried out by HYPERPLOT (JHM International, Columbus, OH). DC components of the data were depressed with the matched end function of HYPERPLOT. Microscopic Examination. All microscopic examinations were carried out with a Unitron optical microscope (Unitron, Japan). Various mercury coatings on the GCE were obtained from solutions of 1,0.4, and 0.1 mM Hg2+,which were diluted from the Fisher AA standard solution (Hg2+,HON3). The electrochemical cell was turned off before the electrode was taken out from the cell for microscopic examination. The electrode was rinsed with nanopure water by running the water down the side of the electrode. A water bead was left at the electrode surface deliberately to preserve the integrity of the mercury coating (see text for more detailed discussion). Pictures were taken with Polaroid film, Type 667. At 400X magnification, 1 mm in the scale of all pictures represents 2.5 pm in the actual size.
RESULTS AND DISCUSSION Rate of Mercury Deposition. The investigation began with the estimation of the rate of mercury deposition. Charges of mercury deposited on a GCE were plotted against deposition times, with the results listed in Table 1 for cases of 0.1 and 0.2 mM Hg2+ concentrations. It can be seen from Table 1 that the amount of Hg on the substrate increased linearly with the deposition time (the linearity is indicated by R = 0.999 for both cases). Also, the slope with 0.2 mM Hg2+ is twice that with 0.1 mM Hg2+,indicating that the amount of mercury on the substrate is directly proportional to the mercury concentration. Thus, the slope of a calibration curve (in pC/ s) can be used to describe the rate of mercury deposition. 3152
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Table 1. Characterlstlc Measurementsof Hg Flhn Deporitlona slope deposition Hgzt chargeb density (rate) intercept charge densit
(mM) 0.1 0.2
(rc) (rC/mm2) 115.1 231.6
90.4 181.9
(m) (pc) 0.72 1.44
3.03 5.27
R
wwd
0.9998 0.9991
0.92 1.83
e Range of deposition times was 5 s to 2 min; see text for details of measurements. Stripping charge of mercury film after 2-min deposition (stirring) and 30-srest. Deposition charge density, slope/area; I-mm GCE disk.
*
It is known that the rate of mercury deposition is a function of deposition potential, stirring rate, and mercury ion concentration. Given the fact that all depositions in this study were carried out at -1.0 V vs Ag/AgCl (a potential for diffusion-controlled deposition) and a constant stirring, the rate of deposition is only a function of the mercury ion concentration. The results in Table 1 show that the deposition rate is indeed a linear function of the Hg ion concentration. Assuming a thin-film condition, the amount of 90 p C (obtained with 0.1 mM Hg2+ at 2-min deposition and 30-s rest) gives a mercury film on the order of 9 nm on a 1-mm disk of GCE. This is comparable to the film thickness estimated by Florence! Surface Conditions of in Situ Formed MTFs. Shown in Figure 1 are pictures of mercury coatings on GCE at different Hg2+ concentrations and deposition times. In Figure la, a solution of 0.1 mM Hg2+ at 2-min stirring and 30-s rest (with a stirring condition corresponding to a deposition rate of 0.72 p C / s ) gives a surface of very finely distributed and densely populated mercury droplets, with the size of the droplets unidentifiable under 400X magnification. The texture of the mercury surface did not show much change with increasing Hg2+concentrations up to 1 mM, at which point the texture seems to be slightly coarser, as shown in Figure 1b. At a deposition time of 5 min with 1 mM Hg2+, the microdroplet size is bigger, as shown in Figure IC. At an even longer deposition time (15 min), the droplets are much larger, as shown in Figure Id, than those in Figure IC. Among them, the bigger ones are presumably formed by the coalescence of small droplets. Figure le shows a surface condition of an ex situ MTF (5-min deposition with 1 mM Hg2+at -1 .OV) after it had been used for 10 ASV stripping cycles in a nitrate solution. Finally, Figure If shows the result of a transformation from a surface like that in Figure ICduring a microscopic examination. Thesizeof thedroplets of the transformed MTFs depends on the amount of the mercury deposited on the GC surface. Going from Figure l a to 1d, thedroplet size is progressively larger because of the increasingly larger loading of mercury on the GCE surface. The average size of the droplets in Figure IC is about 0.5 pm as seen from the picture, except for some bigger droplets. Assuming that the linearity of mercury deposition with Hg2+ concentration and time determined earlier extends to 1 mM Hg2+,the average size of droplets in the case of Figure l b is about 0.2 pm (2.5 times shorter deposition time than that in Figure IC). Furthermore, the average size of droplets in Figure l a is then estimated to be 20 nm since the initial Hg2+concentration is 10 times smaller than that in Figure lb. The surface conditions of the first three pictures indicate that MTFs can be in a state close to a thin film
+
+
Flgure 1. 400X Optical micrographs of MTF at GCE from (a) 2-min deposition/30-s rest at -1.0 V in 0.1 mM Hg2+ 0.1 M KNO3 30 mM HN03;(b) 2-min deposition/30-s at -1 .O V in 1.O mM Hg2+0.1 M KN03 57 mM HN03; (c) 5-min deposition/30-s rest at -1 .O V in 1.O mM Hg2+ 0.1 M KN03 57 mM HN03; (d) 15-min deposition/30-srest at -1 .O V in 1.O mM Hg2+4- 0.1 M KN03 57 mM HN03; (e) ex situ MTF (from 5-min deposition/30-s rest at -1.0 V in 1.0 mM Hg2+ 0.1 M KN03 57 mM HN03)after it has been used for 10 ASV cycles in 0.1 M KNO3 50 mM HN03. (f) Transformation resulting from micrographc during microscopic examination. No purge of solutions during deposition. The scale shown is in millimeters.
+ +
+
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microscopically, especially in Figure la. This finding is seemingly contrary to the early observations,7J4J5 which indicated that the surfacestructure consisted of large, scattered microdroplets (2-5 pm). We believe that this difference may be due to the point in time at which the surface was examined
+
+
microscopically. Thus, pictures a, b, c, and d of Figiire 1 represent the original state of mercury coatings on GCEs at various conditions for the following two reasons. First, it is conceivablethat mercury is selectivelydeposited on the active sites of glassy carbon surface as proposed in ref 7. However, Ana&ticalChemistry, Vol. 66, No. 19, October 1, 1994
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the scale of the active sites on G C (the edge sites of carbon crystallites) has been shown to be typically on the order of 10 nm.16J7 Thus, the density of the active sites on GCEs is consistentwith that of the mercury droplets onGCEs in Figures la, b, and c but is much higher than the density of the scattered mercury droplets reported p r e v i o ~ s l y . ~ JSecond, ~ J ~ there is a fundamental difference between the surface structures in parts a, b, and c and in parts e and f of Figure 1 and in the microscopic pictures of the early r e p ~ r t s . ~ JAt ~ J 400X ~ magnification, the mercury coverage in the former cases is shown to be continuous and ordered microscopically, while that in the latter cases is clearly discontinuous and random, even though the mercury coverage in Figure l a might also be discontinuous nanoscopically. Thus, it is possible that the surface conditon of scattered droplets on a GCE in the early reports might have resulted from a transformation of an original thin film such as observed in our work. In our hands, this transformation was very facile, as evidenced by Figure 1, and might be induced by the evaporation of water (see later discussion). This change would be even more likely if SEM were used to examine the surface conditions because of the likelihoodof the change of mercury surface under high vacuum. During the process of our microscopic examination, the fine mercury droplets were found to transform quickly into a surface of large scattered droplets. It was also found that by leaving a layer of water deliberately on the electrode surface, we could preserve the integrity of the surface structure. (The stability of the mercury films with proper handling allowed us to take several pictures without any structural changes.) After several trials, it was noticed that the transformation appeared to be triggered by the drying-out of the water layer on top of the mercury surface. Once the last trace amount of water evaporated, the whole surface collapsed into the surface of scattered droplets very rapidly (ca. 1 s). The final droplet size was dependent on the total amount of mercury on the surface. The tendency of MTFs to transform from one state to another seems to illustrate that without careful handling during microscopic observation, the condition in Figure If will likely be the one being observed, even though it was not the one originally present. One of the advantages of in situ MTFs for ASV is the new surface generated at each analysis. Instead of the mercury being wiped off with a wet tissue, the MTF can be stripped off electrochemically from the GCE surface and re-formed repeatedly. Shown in Figure 2 are linear scan ASV voltammograms of Cd, Pb, and Cu using an in situ MTF. A scan rate of 2 V/s was chosen to enhance the sensitivity and to avoid the interference of oxygen. (Fast scan voltammetry has been employed as a general strategy to avoid the oxygen interference in the un-deaerated solution^.^.^^) The excellent features of these voltammograms, which include highly reprodicible background currents in the potential range from -1.0 to +0.1 V, narrow peak widths, and base line resolution of the Cd and Pb peaks at a fast scan of 2 V/s, can only be achieved with very thin and reproducible mercury films, where a completely new surface was formed at each deposition cycle. The slightly varying background currents in the potential range of +O. 1 to +0.4 V and the small peak around 0.3 V in the later (16) Clem, R. G.; Litten, G.; Omelas, L. D. Anal. Chem. 1973, 45, 1306. (17) Clem, R. G Anal. Chem. 1975,47, 1778.
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cu
P b , ,
25
0.6
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,
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Potential (Volt vs. Ag/AgCI) Flgue 2. Linear scan (2 VIS) ASV voltammograms of Cd, Pb, and Cu with in situ MTF in 0.1 mM Hg2+ 0.1 M KNO:, 30 mM HN03: (a) background scan: (b) 50 nM of each metal Ion (5.6 ppb Cd, 10.3 ppb Pb, 3.2 ppb Cu); (c) 100 nM; (d) 150 nM: (e) 200 nM; (f) 250 nM; (9) 300 nM; (h) 350 nM. No purge of solution during ASV experiments, Bmin depositionl30-s rest.
+
+
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Frequency (Hz) Figure 3. (A) Linear scan (2 VIS) background voltammograms of In situ MTFs in (a) 0.1 mM Hg2+ 0.1 M KNO:, 30 mM HNO,; (b) 0.4 mM Hg2+ 0.1 M KNOB 25 mM HNO,; (c) 1.0 mM Hg2+4- 0.1 M KN03 50 mM HNO,; (d) 1.4 mM Hg2+ 0.1 M KNO:, 50 mM HN03. Voltammograms b, c, and d have been displaced from original positions appropriately for demonstratlon. (8) FT of the background voltammograms above. Noise spectra a, b. and c have been displaced from original positions appropriately for demonstration.
+
+
+
+
+
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+
scans appeared to be caused by the trace amount of chloride gradually leached out from the reference electrode (see later discussion for more detailed explanations). Stability of in Situ Formed MTF GCE. Attempts were made to determine the factors which contributed to a stable MTF. Shown in Figure 3 are four linear scanvoltammograms of in situ MTFs at different Hg2+concentrations along with their corresponding noise spectra (FT of thevoltammograms). The four voltammograms in Figure 3 seem to indicate that
MTFs become unstable, with many spikes generated during a scan, when the Hg2+concentration is higher than 0.1 mM under the present deposition and stripping conditions. The instability of the in situ MTFs was also observed at a slower scan rate of 0.5 V/s. Note that a higher Hg2+concentration ultimately leads to a larger total amount of mercury at the GCE surface. The pattern of spiking, however, seems to be random, as indicated by the noise spectra (Figure 3) of these voltammograms. At the severe situation of 1.4 mM Hg2+, erratic currents (the upward-going currents in Figure 3d) occur. Note that the erratic currents have the tendency of going cathodically (in the cases of voltammograms c and d) despite the anodic scanning. Osteryoung and co-workershave reported that similar spiking currents are caused by the combination of mercury droplets in a mercury deposition step.20 The number and the magnitude of spikes, however, are much larger in our work. The investigation was extended in order to attempt to understand the relationshipbetween the current behavior and the surface conditions. In the experiment, 0.4 mM Hg2+ solution was chosen for the study because it results in an unstable condition. After forming an MTF at 2 min of stirring and 30 s of rest, the electrode was taken out of the cell for a microscopic examination. As shown in Figure 4a, the new mercury coating is again in a state of fine droplets. Four linear voltammetric scans from -1 .O to +0.2 V were applied to the mercury film electrode during the study, with the voltammograms shown in Figure 5 . Two pictures of the MTF surface condition, one after the second scan and one after the last scan as shown in Figure 4b and 4c, were taken ex situ in an attempt to record the intermediate states during the voltammetric perturbations. As was seen earlier, the first voltammogram in Figure 5 exhibits many spikes. The picture taken after the first scan shows that the MTF surface texture has changed from Figure 4a to 4b, which exhibits some relatively large droplets among the still fine droplets. The second scan of Figure 5 (recorded after picture Figure 4b was taken) shows the same noise characteristics as the first scan. When the MTF surfacewas further perturbed by voltammetric scans, erratic currents were obtained, as shown in Figure 5c and 5d. The final result was that the surface changed from Figure 4b to 4c. Compared to the transformation described during microscopic examinations, the following two aspects seems to be similar: (1) a fine and smooth surface was obtained initially in both cases and (2) transformations in both cases were triggered by a surface perturbation (evaporation and shrinking of water layer and rapid change of potential at 2 V/s). There thus seems to be a good correlation between the current behavior and the structural changes. First, the noise spikesare associated with the small scaleof structural changes of MTF surfaces as confirmed by several separate microscopic examinations. Second, we believe that the erratic currents were caused by the large-scale changes of surface structure as a result of the tendency of MTFs to transform into a more stable condition. The cathodically going erratic currents record the transition from one form of connection of mercury (18) Bowling, R. J.; Packard, R. T.; McCreery, R. L. J. Am. Chem. Soc. 1989,111, 1217. (19) Jenkins, G.M.; Kawamura, K. Nature 1971, 231, 175. (20) Colas, J.; Galus, Z.; Osteryoung, J. Anal. Chem. 1987, 59, 389.
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Figure 4. 400X optical micrographs of MTF at W E from (a) 2-min deposition/30-s rest at -1.0 V in 0.4 mM Hg2+ 0.1 M KN03 30 mM HN03; (b) same deposition as in a, taken after a scan from -1.0 to 0.2 V had been applied to MTF; (c) same deposition as in a, taken after three additional scans were applied to the MTF (see Figure 5 for details of voltammograms). The scale shown is in millimeters.
+
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droplets with the substrate to a completely different form of connection. Stability of ex Situ Formed MTFs. A separate study was carried out to determine the behavior of an ex situ formed MTF. A 5-min deposition in 1 mM Hg2+solution was chosen AnalyticalChemistry, Vol. 66, No. 19, October 1, 1994
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-6
r
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r
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Flgure 7. (A) Linear scan (2 V/s) background voltammograms of In situ MTFs in 0.1 mM Hg2+ 0.1 M KNOI 30 mM HN03: (a) no chloride added; (b) 0.3 mM KCI added; (c) 0.6 mM KCI added; (d) 0.9 mM KCIadded; (e) 1.2 mM KCIadded; (f) 1.5 mM KCI. Voltammograms b, c, d, e, and f have been displaced from originalposttlonsappropriately for demonstration. (B) FT of the background voltammograms above. Noise spectra a, b, c, d, and e have been displaced from original positions appropriately for demonstration.
+
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
Potential (Volt vs. Ag/AgCI) Flgure 6. Llnear scan (2 VIS) ASV voltammograms of Cd, Pb, and Cu with ex situ MTF in 0.1 M KN03 30 mM HNO3: (a) Initial background scan: (b) second background scan; (c) 10 ppb of each metal Ion (89 nM Cd, 48 nM Pb, and 157 nM Cu); (d) 20 ppb; (e) 30 ppb; (f) 40 ppb; (9) 50 ppb; (h) 00 ppb; (i) 70 ppb. 2-min deposition/30-s rest, no purge of solution.
+
to form the MTF, which is comparable to the condition in the literature for ex situ MTF. A surface transformation was deliberately triggered by the drying and shrinking of the water layer front during the microscopic examination after the deposition to form a surface similar to that in Figure If. The ex situ MTF was then put back into a nitrate solution for ASV experiments. The first background scan (after themicroscopic examination and induced transformation) in a nitrate solution gives a smooth but somewhat erratic voltammogram, while the current in the second background voltammogram is basically flat, as shown in Figure 6a and 6b. The behavior in Figure 6a indicates that there may still be some structural change. The ASV voltammograms in Figure 6 show that stable stripping scans were obtained during the ASV experiments after the first scan. However, the background currents were different for each scan. Figure 1e, taken after 10stripping cycles, shows that the mercury surface is no longer in the state of Figure 1f, confirming that further surface structural changes have occurred. In another ASV experiment with an ex situ MTF, no spikes or erratic currents were observed throughout 10 stripping cycles. It appears that for ex situ formed MTFs, the transformation took place in the conditioning step after the 5-min mercury deposition. Compared to the performance of in situ formed MTFs, ex situ MTFs give ASV signals with 3156
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AnalytlcalChemlstry, Vol. 66, No. 19, October 1. 1994
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broader peak widths, less reproducible background currents, and lower sensitivity. This is probably because the ex situ MTFs are normally thicker than the in situ MTFs and the surface of the ex situ MTF changes slightly after each deposition/stripping cycle.
Effect of Chloride on in Situ MTF Behavior. A study of the chloride effect on the stability of in situ MTFs was conducted because of the observed ill behavior of the stable working system of 0.1 mM Hg2+ caused by a trace amount of chloride. The effect of chloride on the stability of MTFs has been well documented by 0thers.~>~J,~J5 ASV with in situ formed MTFs in chloride medium was not at all stable, with many noisy spikes, as reported by Wojciechowski et al.' The study was carried out by adding a fixed amount of chloride to a stable working system of 0.1 mM Hg2+ solution. The linear scan voltammograms after each chloride addition are shown in Figure 7A, along with their noise spectra (FT of the linear scan voltammogram) in Figure 7B. As usual, the voltammogram without chloride is smooth and well-behaved, as shown in a of Figure 7A. The noise spectrum a in Figure 7B reveals that, except for the system noise at about 236.67 Hz, noise at other frequencies is at a minimum. After the addition of 0.3 mM chloride, however, small spikes are seen on top of the background current, along with the oxidation peak right after +0.2 V. This peak has been considered to
be due to Hg(I).Z1 As more and more chloride is added to the system, four apparent consequences are seen from the voltammograms of the in situ formed MTFs: (i) the Hg(1) peak after +0.2 V becomes increasingly larger (the peak current saturated the AID converter at that particular current setting after addition of 0.6mM chloride), which narrows the potential window of ASV; (ii) the background currents near -1 .O V become increasingly higher cathodically as more and more chloride is added to the system, probably because of the lowering of the hydrogen overpotential; (iii) more and stronger spikes are obtained, and, as a result, the noiselevel is increased, as shown by the noise spectra; and (iv) the peak at about 0.0 V (due to the trace amount of Cu) is buried in the background current. A microscopic examination at 400X magnification revealed that, after the addition of 1.5 mM chloride to the system of 0.1 mM Hg2+ solution, the mercury surface was still in a fine and smooth state without any large droplets. Such a microscopic view implies that the initial surface condition of in situ MTFs at a chloride-added solution was the same as that without added chloride even though they might be different nanoscopically. Apparently, the addition of chloride to the stable system of 0.1 mM Hg2+ has made the in situ MTF increasingly unstable with respect to the ASV scan. The formation of a calomel film has been suggested to be the probable reason for such a behavior.* The tolerable amount of chloride which can be present without generating spikes and erratic currents is about 0.3 mM according to this study, even though the lowering of hydrogen overpotential and the formation of Hg(1) will inevitably result.
CONCLUSION In summary, it has been shown that mercury coatings on GCEs are in a state of fine and smooth microdroplets, very (21) Dieker, J.; van der Linden, W. E. Fresenius' Z.Anal. Chem. 1975, 274, 97.
close to a thin-film state, when the total mercury deposition is about 115 pC/mm2. As the mercury on the surface is increased, the droplet size also increases. Thin mercury coatings on GCEs have the tendency to transform into a more stable state, a change which is triggered by various surface perturbations. An unstable in situ MTF during an ASV scan is characterized by many current spikes and sometimes by erratic currents in a severe situation. These noisy and erratic currents were found to correlate with the structural change of the surface condition. It has also been shown that, whereas the initial surface condition of MTFs is in a state of fine microdroplets, the ex situ formed MTFs are in a state of less densely populated and larger droplets, a seemingly more stable condition when more mercury than the in situ MTF is loaded on the GCE surface. Structurally, the surface condition exhibited by in situ MTFs is a better form of electrode for ASV than that exhibited by ex situ MTFs in terms of sensitivity, consistency, and peak resolution. Chloride was shown to destabilize the mercury coatings for ASV with in situ formed MTFs. With a careful control of chloride content in the solution, the background currents from a stripping voltammogram of in situ MTF can be reproducible from run to run with little hysteresis.
ACKNOWLEDGMENT The author thanks Herb Silverman and John McDonald for their review of and inputs into the work. The comments of Larry Anderson on the manuscript are also appreciated. Recelved for review Aprll 2, 1994. Accepted June 22, 1994." *Abstract published in Aduonce ACS Absrrucrs, August 1 , 1994.
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