Antimony Film Electrode for Electrochemical Stripping Analysis

In this work, an antimony film electrode (SbFE) is reported for the first time as a possible alternative for electrochemical stripping analysis of tra...
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Anal. Chem. 2007, 79, 8639-8643

Antimony Film Electrode for Electrochemical Stripping Analysis Samo B. Hocevar,*,† Ivan S ˇ vancara,†,‡ Bozidar Ogorevc,† and Karel Vytrˇas‡

Analytical Chemistry Laboratory, National Institute of Chemistry, P.O. Box 660, SI-1001 Ljubljana, Slovenia, and Department of Analytical Chemistry, The University of Pardubice, Na´ meˇ stı´ C ˇ s. Legiı´ 565, CZ-532 10 Pardubice, Czech Republic

In this work, an antimony film electrode (SbFE) is reported for the first time as a possible alternative for electrochemical stripping analysis of trace heavy metals. The SbFE was prepared in situ on a glassy carbon substrate electrode and employed in combination with either anodic stripping voltammetry or stripping chronopotentiometry in nondeaerated solutions of 0.01 M hydrochloric acid (pH 2). Several key operational parameters influencing the electroanalytical response of SbFE were examined and optimized, such as deposition potential, deposition time, and composition of the measurement solution. The SbFE exhibited well-defined and separated stripping signals for both model metal ions, Cd(II) and Pb(II), surrounded with low background contribution and a relatively large negative potential range. The electrode revealed good linear behavior in the examined concentration range from 20 to 140 µg L-1 for both test metal ions, with a limit of detection (3σ) of 0.7 µg L-1 for Cd(II) and 0.9 µg L-1 for Pb(II) obtained after a 120 s deposition step, and good reproducibility, with a relative standard deviation (RSD) of (3.6% for Cd(II) and (6.2% for Pb(II) (60 µg L-1, n ) 12). When comparing the SbFE with the commonly used mercury film electrode and recently introduced bismuth film electrode, the newly proposed electrode offers a remarkable performance in more acidic solutions (pH e 2), which can be advantageous in electrochemical analysis of trace heavy metals, hence contributing to the wider applicability of electrochemical stripping techniques in connection with “mercury-free” electrodes. Among different electrochemical techniques, stripping voltammetry and chronopotentiometry play an important role and have long been recognized as one of the most powerful tools in trace and ultratrace analysis of heavy metals and some organic compounds, due to their unique preconcentration capabilities in conjunction with different electrode materials.1 Among them, despite its well-known toxicity and difficulties associated with its handling, mercury has been commonly used as an electrode material during the last 5 decades in combination with different * Corresponding author. Phone: +386 1 4760 214. Fax: +386 1 4760 300. E-mail: [email protected]. † National Institute of Chemistry. ‡ The University of Pardubice. (1) Wang, J. Analytical Electrochemistry, 3rd ed.; Wiley-VCH: Hoboken, NJ, 2006. 10.1021/ac070478m CCC: $37.00 Published on Web 10/20/2007

© 2007 American Chemical Society

electrochemical (stripping) protocols and electrode geometries, e.g., static or hanging mercury drop electrode, dropping mercury electrode, and mercury film electrode. In particular, the mercury film electrode has been often used due to its relatively simple in situ or ex situ preparation and convenience for its application in flow systems.2 Many other electrode materials have been suggested as substitutes for mercury, e.g., different modifications of carbon, gold, platinum, silver, iridium, several alloys and amalgams, but none of them approached mercury’s excellent electroanalytical performance.3-8 Recently, the bismuth film electrode was introduced as one of the most promising alternatives to its mercury counterpart9 and is, by now, widely accepted in numerous electroanalytical laboratories worldwide. There are ongoing efforts orientated toward measuring other potentially interesting analytes in combination with the bismuth film electrode and its employment in new challenging applications.10-15 In parallel, also other interesting types of metal film electrodes have appeared, e.g., with mixed Ag/Bi16 and Cu/Bi17 films or thin Pb film electrodes.18 However, due to the decreased popularity of electrochemical techniques resulting from mercury’s toxic character and its tedious handling, there is still growing interest for new, “mercury-free” electrode materials that exhibit advantageous electrochemical stripping characteristics and can be satisfactorily employed for the reliable (2) Economou, A.; Fielden, P. R. Talanta 1998, 46, 1137-1146. (3) Achterberg, E. P.; Braungardt, C. Anal. Chim. Acta 1999, 400, 381-397. (4) Wang, J.; Tian, B. Anal. Chem. 1993, 65, 1529-1532. (5) Nolan, M. A.; Kounaves, S. P. Anal. Chem. 1999, 71, 3567-3573. (6) Wang, J.; Hocevar, S. B.; Deo, R. P.; Ogorevc, B. Electrochem. Commun. 2001, 3, 352-356. (7) Mikkelsen, Ø.; Schrøder, K. H. Electroanalysis 2001, 13, 687-692. (8) Fischer, J.; Barek, J.; Yosypchuk, B.; Navratil, T. Electroanalysis 2006, 18, 127-130. (9) Wang, J.; Lu, J. M.; Hocevar, S. B.; Farias, P. A. M.; Ogorevc, B. Anal. Chem. 2000, 72, 3218-3222. (10) Kefala, G.; Economou, A. Anal. Chim. Acta 2006, 576, 283-289. (11) Hutton, E. A.; Hocevar, S. B.; Mauko, L.; Ogorevc, B. Anal. Chim. Acta 2006, 580, 244-250. (12) Wang, J.; Lu, D.; Thongngamdee, S.; Lin, Y.; Sadik, O. A. Talanta 2006, 69, 914-917. (13) Lin, L.; Thongngamdee, S.; Wang, J.; Lin, Y.; Sadik, O. A.; Ly, S. Y. Anal. Chim. Acta 2005, 535, 9-13. (14) Svancara, I.; Baldrianova, L.; Tesarova, E.; Hocevar, S. B.; Elsuccary, S. A. A.; Economou, A.; Sotiropoulos, S.; Ogorevc, B.; Vytras, K. Electroanalysis 2006, 18, 177-185. (15) Demetriades, D.; Economou, A.; Voulgaropoulos, A. Anal. Chim. Acta 2004, 519, 167-172. (16) Skogvold, S. M.; Mikkelsen, Ø.; Schrøder, K. H. Electroanalysis 2005, 17, 1938-1944. (17) Legeai, S.; Bois, S.; Vittori, O. J. Electroanal. Chem. 2006, 591, 93-98. (18) Korolczuk, M.; Tyszczuk, K.; Grabarczyk, M. Electrochem. Commun. 2005, 7, 1185-1189.

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measurement of trace heavy metals and other species of interest associated with environmental monitoring, clinical and industrial testing, biomedical applications, tailoring of disposable electrochemical sensors, biosensors, and microsensors.19,20 The use of antimony as an electrode material for potentiometric pH measurement was first reported in 1923 by Uhl and Kestranek21 followed by other contributions suggesting, e.g., application of an antimony-based electrode for measuring pH in whole blood.22 As an electrode material, antimony/antimony oxide have been successfully used for decades for pH measurements, as a solid metal/ metal oxide ion-selective electrode in laboratory and biomedical (e.g., dental) applications23-26 exploiting a well-known Sb/Sb2O3 surface equilibrium. Hitherto, there has not been any work presented regarding the application of an antimony electrode in electrochemical stripping analysis for measuring, e.g., trace metal ions. In this work, we have demonstrated the introduction and initial characterization of the in situ prepared antimony film electrode (SbFE) and its successful application for measuring trace heavy metals, i.e., cadmium(II), lead(II), bismuth(III), and mercury(II), in the presence of dissolved oxygen and exposed its remarkable stripping performance at low pH values. EXPERIMENTAL SECTION Apparatus. Anodic stripping voltammetric (ASV), stripping chronopotentiometric (SCP), and cyclic voltammetric (CV) measurements were performed using a modular electrochemical workstation (Autolab, Eco Chemie, Utrecht, The Netherlands) equipped with PGSTAT12 and ECD modules and driven by GPES software (Eco Chemie). A three-electrode configuration was employed, consisting of an antimony (or bismuth or mercury) film modified glassy carbon electrode (d ) 2 mm), Ag/AgCl/KCl(saturated), and a platinum wire as working, reference, and counter electrodes, respectively. All electrochemical experiments were carried out in one-compartment voltammetric cells (10-20 mL) at conditioned room temperature (23 ( 1 °C). All potentials in this work are referred to Ag/AgCl/KCl(saturated) as the reference electrode. In voltammetric and chronopotentiometric stripping measurements, during the electrochemical deposition step, a magnetic stirrer (approximately 300 rpm) was employed. Reagents and Solutions. Standard stock solutions of antimony(III), cadmium(II), and lead(II) (1000 mg L-1, atomic absorption standard solutions) were obtained from Merck, whereas standard stock solutions of bismuth(III) and mercury(II) (1000 mg L-1, atomic absorption standard solutions) were provided by Aldrich and Alfa Aesar, respectively, and diluted as required. Unless otherwise stated, a 0.01 M solution of hydrochloric acid (pH 2) served as the supporting electrolyte. All other chemicals were of analytical grade purity. Water used to prepare all solutions (19) Yosypchuk, B.; Novotny, L. Crit. Rev. Anal. Chem. 2002, 32, 141-151. (20) Hutton, E. A.; Hocevar, S. B.; Ogorevc, B. Anal. Chim. Acta 2005, 537, 285-292. (21) Uhl, A.; Kestranek, W. Monatsh. 1923, 44, 29-34. (22) Brinkman, R.; Buytendiyk, F. J. J. Biochem. Z. 1928, 199, 387-391. (23) Brown, D. H.; Gould, C. T.; Tatevossian, A. Arch. Oral Biol. 1974, 19, 601603. (24) Alexander, P. W.; Joseph, J. P. Anal. Chim. Acta 1981, 131, 103-109. (25) Kinoshita, E.; Ingman, F.; Edwall, G.; Thulin, S.; Glab, S. Talanta 1986, 33, 125-134. (26) Wang, M.; Ha, Y. Biosens. Bioelectron. 2007, 22, 2718-2723.

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throughout the work was first deionized and then further purified via an Elix 10/Milli-Q Gradient unit (Millipore, Bedford, MA). Preparation of the Antimony Film Electrode (SbFE). Before measurement, a substrate glassy carbon electrode was assiduously polished on a polishing pad using 0.05 µm alumina slurry and subsequently thoroughly rinsed with double-distilled water. The substrate electrode was then transferred into the measurement solution containing usually 0.01 mol L-1 hydrochloric acid and 1 mg L-1 of antimony(III) together with the model heavy metal ions, i.e., cadmium(II), lead(II), bismuth(III), and mercury(II). During the electrochemical deposition step, typically at -1.2 V, the SbFE was formed in situ together with electrochemical deposition of analytes. For comparison, the bismuth film and mercury film electrode were prepared using the same proceeding. Procedures. Anodic stripping voltammetry and SCP were performed as follows: Following the electrochemical deposition step and 15 s of equilibration, a square-wave voltammogram was recorded by applying a square-wave potential scan toward more positive potentials, and in the case of SCP measurements, similar deposition and equilibration period protocols were used, followed by a stripping step using a constant current of usually 1 µA. Before each measurement, the “cleaning” step was performed by keeping the working electrode at +0.3 V for 30 s. Cyclic voltammety was performed as follows: measurements were performed by cycling the potential from +0.5 to -1.0, -1.2, or -1.4 V in a solution of 0.01 M hydrochloric acid containing 10 mg L-1 antimony(III). RESULTS AND DISCUSSION Since preliminary experiments revealed the pertinence of the antimony-based electrode for trace heavy metal detection in connection with its operation in relatively strong acidic medium (pH e 2) under ASV mode, we conducted further investigation with the aim of attaining more insights into its interesting electroanalytical behavior. Figure 1 depicts a comparison between mercury film (dashed line), bismuth film (thin line), and antimony film (thick line) electrodes for measuring 50 µg L-1 of both cadmium(II) and lead(II) as test metal ions. The electrodes were prepared in situ simultaneously with deposition of both analytes in nondeaerated solutions of 0.01 M hydrochloric acid (pH 2) containing 1 mg L-1 of either mercury(II), bismuth(III), or antimony(III) ions. Notably, the newly introduced SbFE exhibits very similar electrochemical stripping behavior to the bismuth film electrode, in particular with respect to the position of stripping signals corresponding to both model metal ions, i.e., at -0.78 V for cadmium(II) and at -0.53 V for lead(II). In comparison with the mercury film electrode, signals for cadmium(II) obtained at antimony film and bismuth film electrodes are shifted similarly toward more negative potentials for approximately 100 mV offering a wider applicable potential window between signals corresponding to cadmium(II) and lead(II) (although the signal belonging to lead(II) at SbFE appears at 28 mV more negative potential than that obtained at its bismuth counterpart). This behavior implies there are similar processes associated with the formation of metal alloys with bismuth and can apparently be predicted also for antimony. In addition, the SbFE provides slightly broader signals for both model analytes in comparison with the bismuth and mercury film electrodes, which may indicate some specific surface

Figure 1. Anodic stripping voltammograms of cadmium(II) and lead(II) at in situ prepared antimony film (thick line), bismuth film (thin line), and mercury film (dashed line) electrodes. Solution: 0.01 M hydrochloric acid (pH 2) containing 50 µg L-1 cadmium(II) and lead(II) together with 1 mg L-1 antimony(III), bismuth(III), or mercury(II). Deposition at -1.2 V for 120 s, equilibration period of 15 s, and “cleaning” step of 30 s at +0.1 V. Square-wave voltammetric stripping scan with a frequency of 25 Hz, potential step of 4 mV, and amplitude of 25 mV.

conditions at the SbFE, perhaps due to partial hydrolysis, affecting the deposition and reoxidation processes. With respect to the bismuth film electrode, SbFE exhibits practically the same sensitivity toward lead(II) and slightly higher sensitivity toward cadmium(II). Both electrodes provide improved sensitivity for cadmium(II) in comparison with the mercury film electrode, while the latter exhibits a higher response for the same concentration of lead(II). Also, when compared with the bismuth film electrode, SbFE provides more favorable hydrogen evolution, a behavior similar to that of the mercury analogue. Apparently, the most remarkable feature of the SbFE is also evident from Figure 1, showing voltammetric reoxidation of the antimony film. As can be seen, this process gives rise to a very small signal for antimony when compared with oxidation processes for both mercury and bismuth. Such an undeveloped response, typical for a solution of 0.01 M HCl, which has been proven to be the most suitable for the optimal performance of SbFE, is undoubtedly advantageous with respect to markedly lower background in the proximity of the anodic potential limit and resulting improvement of the overall signal-to-noise characteristics of the SbFE. The origin of this phenomenon has not been fully explained yet. In order to understand the electrochemical behavior of SbFE, we studied the reduction/oxidation (deposition/dissolution) pattern of the antimony film on a glassy carbon substrate electrode. We performed CV measurements in a model solution of 0.01 mol L-1 hydrochloric acid containing 10 mg L-1 antimony ions as shown in Figure 2, while following the effect of different cathodic vertex potentials upon CV behavior of antimony (see the second potential scan). It is evident, that the growth of the antimony film starts at potentials more negative than ca. -0.25 V with two closely overlapping reduction peaks at approximately -0.60 V, which are followed by the commencement of hydrogen evolution at relatively favorable potentials more negative than approximately -1.2 V. Formation of the two aforementioned overlapping reduction peaks was also observed in earlier works, which is likely due to the experimental conditions and not fully explained.27 After changing the direction of potential scan, strong oxidation, i.e., dissolution/

stripping of antimony, can be observed at +0.05 V, which as expected, depends on the pH of the measurement solution (not shown), accompanied with a shoulder at +0.15 V. When scanning the potential toward more negative values than -1.2 V, attenuation of the oxidation/stripping signal can be observed together with the appearance of a single oxidation peak at a potential of around +0.05 V. This decrease in the oxidation/stripping signal is due to the beginning of hydrogen evolution, which affects deposition of antimony onto the electrode surface, also consistent with a previous work in connection with lead-acid batteries regarding accumulation of antimony in the potential region from -950 to -1150 mV and “antimony purging” observed at more negative potentials than -1320 mV versus Hg/Hg2SO4 reference electrode.28 Increasing vertex potential to more negative values was also associated with a slight shift of the second reduction signal toward more negative potentials (see Figure 2) during the second

(27) Metikosˇ-Hukovic´, M.; Babic´, R.; Brinic´, S. J. Power Sources 2006, 157, 563570.

(28) Bohnstedt, W.; Radel, C.; Scholten, F. J. Power Sources 1987, 19, 301314.

Figure 2. Cyclic voltammograms of antimony(III) in the potential range from +0.5 to -0.8 V (a), +0.5 to -1.0 V (b), +0.5 to -1.2 V (c), and +0.5 to -1.4 V (d). Solution: 0.01 M hydrochloric acid (pH 2) containing 10 mg L-1 antimony(III). Cyclic voltammetric scan with a scan rate of 100 mV s-1, potential step of 10 mV, and initial potential of +0.5 V.

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Figure 3. Anodic striping voltammograms obtained at an in situ prepared SbFE (A) together with the corresponding background voltammogram (thin line) at BiFE (B) and MFE (C). Solutions: 1.0 M hydrochloric acid containing 100 µg L-1 cadmium(II), 50 µg L-1 lead(II), 50 µg L-1 bismuth(III) (A and C), and 50 µg L-1 mercury(II) (A and B) together with 1 mg L-1 antimony(III) (A), bismuth(III) (B), and mercury(II) (C). Other conditions are as in Figure 1.

potential scan. When scanning the potential in a negative direction to -0.8 V only (a), a single oxidation signal can be observed at ca. +0.00 V followed by a single reduction peak at ca. -0.60 V during the second potential scan toward negative potentials. However, since this feature did not directly affect the stripping measurements under conditions chosen, further details were not investigated within this initial characterization of the SbFE. With the aim at presenting the usefulness of the newly developed SbFE we proceeded with measurements under extremely acidic conditions in combination with the introduction of two additional analytes characterized by their stripping signals being in close proximity to the antimony stripping signal. Figure 3 clearly illustrates the comparison of the SbFE with bismuth and mercury film electrodes together with its attractive operation in the solution of 1 M HCl for simultaneous measurement of four analytes, i.e., 100 µg L-1 of cadmium(II), 50 µg L-1 of lead(II), 50 µg L-1 of bismuth(III), and 50 µg L-1 of mercury(II). The stripping voltammogram obtained at the SbFE (Figure 3A) shows its favorable electroanalytical performance exhibiting well-defined and separated stripping signals for all four analytes along with a low background contribution. It can be clearly seen that in addition to cadmium(II) and lead(II), also bismuth(III) and mercury(II) can be conveniently measured at the same time, without their signals being overlapped at microgram per liter concentration levels. Furthermore, it is evident from the stripping voltammogram which was recorded in the solution containing only 1 mg L-1 antimony(III) without any analytes (Figure 3A, thin line) that the stripping signal belonging to antimony(III) has only a negligible effect upon the bismuth(III) stripping signal and no effect upon the signal corresponding to mercury(II) at the examined concentration level. As can be seen from Figure 3A, the sensitivity of the SbFE toward cadmium(II) and lead(II) is higher than that observed at the bismuth analogue and similar to the sensitivity of the mercury film electrode. In addition, the signal for 50 µg L-1 of mercury(II) obtained at the SbFE exhibited almost double sensitivity in comparison with the bismuth film electrode. With respect to the potential separation between the stripping signals 8642 Analytical Chemistry, Vol. 79, No. 22, November 15, 2007

Figure 4. Effect of deposition potential (A) and deposition time (B) upon the stripping voltammetric response of cadmium(II) and lead(II) at an in situ prepared antimony film electrode. Solutions: 0.01 M hydrochloric acid (pH 2) containing 40 µg L-1 cadmium(II) (A) and 70 µg L-1 cadmium(II) and lead(II) (B), along with 1 mg L-1 antimony. Deposition for 120 s (A) at -1.0 V. Other conditions are as in Figure 1.

corresponding to cadmium(II) and lead(II), the antimony film and bismuth film electrodes exhibited very similar characteristics, whereas the mercury film electrode revealed lower separation for approximately 50 mV. Close inspection of electrochemical characteristics in the upper negative potential region unveiled favorable overpotential for hydrogen ion reduction at the SbFE, which is more alike to the mercury counterpart and significantly improved with respect to the bismuth film electrode. To enhance the electroanalytical performance of the SbFE, we optimized several key operational parameters, including deposition potential and deposition time. Figure 4A shows the dependence of the stripping signal for 40 µg L-1 cadmium(II) on the deposition potential, and Figure 4B depicts the dependence of the stripping signal for 70 µg L-1 of both cadmium(II) and lead(II) on the deposition time. As expected, at potentials less negative then -1.0 V, we observed a poorly developed signal for cadmium(II) (see Figure 4A), whereas deposition at -1.0 V and at more negative potentials up to -1.3 V resulted in significantly higher stripping currents. After applying potentials more negative than -1.3 V, we observed slight attenuation of stripping signals, due to commencement of hydrogen evolution (also see Figure 2) and consequently hindered deposition of the antimony film together with the corresponding analyte. For further measurements, a value of -1.2 V was chosen as the optimum deposition potential. In addition, as seen in Figure 4B, the signals for both model metal ions increase almost linearly with longer deposition times with only a slight negative curvature from linearity at deposition times longer than 180 s. This indicates that in this case SbFE practically did not suffer from the saturation effect, which often complicates measurements with other metal film based electrodes.9

Figure 5. Constant current stripping chronopotentiogram of cadmium(II) and lead(II) at an in situ prepared antimony film electrode. Solution: 0.01 M hydrochloric acid (pH 2) containing 60 µg L-1 cadmium(II) and lead(II) along with 1 mg L-1 antimony. Deposition for 120 s at -1.2 V and stripping current of 1.0 µA. Other conditions are as in Figure 1.

In addition, we examined SCP as an advantageous method when employing microelectrodes or samples with a more complex matrix. It is evident from Figure 5 that the SbFE also exhibits excellent electroanalytical performance under chronopotentiometric stripping mode for measuring in solutions with relatively low acidity (pH 2), in the presence of dissolved oxygen. Two sharp and well-defined stripping signals can be observed for 60 µg L-1 of both metal ions at potentials of -0.72 and -0.48 V for cadmium(II) and lead(II), respectively, surrounded with low background and with no adverse effect of dissolved oxygen in the measurement solution, when an oxidative stripping current of 1 µA was selected as the optimum. In comparison with stripping voltammetry, practically the same separation of approximately 240 mV between the signal for cadmium(II) and lead(II) was obtained; however, a higher sensitivity for lead(II) with respect to cadmium(II) was observed, which is opposite to the stripping voltammetric mode. This feature can be explained by different electrode reaction kinetics for cadmium(II) and lead(II) ions taking place at the surface of the antimony film when oxidation is induced with a constant current. To provide more information about electroanalytical performance, we followed the ASV signals for both metals, while simultaneously increasing their concentrations. As can be seen from Figure 6, the SbFE revealed good linear behavior for both metal ions, i.e., R2 of 0.999 and 0.996 for cadmium(II) and lead(II), respectively, in the examined concentration range from 20 to 140 µg L-1 after a 120 s deposition step, without any cross disturbance during simultaneous measurements of both analytes. The calculated limit of detection (3σ) of 0.7 µg L-1 for cadmium(II) and 0.9 µg L-1 for lead(II) was obtained following a 120 s deposition time, while repetitive measurements yielded a relative standard deviation (RSD) of (3.6% for cadmium(II) and (6.2% for lead(II) (60 µg L-1, n ) 12). These results have proven the auspicious electroanalytical characteristics of the in situ prepared SbFE in combination with advanced electrochemical stripping techniques and its appropriateness for measuring trace heavy metals in solutions with low pH values and in the presence of dissolved oxygen, thus obviating the need for time-consuming deaeration of a measurement solution.

Figure 6. Anodic stripping voltammograms (ASV) for successive additions of cadmium(II) and lead(II) in 10 µg L-1 steps together with background response obtained at an in situ prepared antimony film electrode. The inset depicts the corresponding calibration plot. Solutions: 0.01 M hydrochloric acid containing increasing levels of cadmium(II) and lead(II) from 20 to 140 µg L-1 along with 1 mg L-1 antimony(III). Other conditions are as in Figure 1.

CONCLUSION We demonstrated the application of the SbFE, which was prepared in situ on a glassy carbon substrate electrode, for measuring trace heavy metals, i.e., cadmium(II) and lead(II). The SbFE revealed favorable electroanalytical performance similar to that of bismuth- and mercury-based electrodes in combination with stripping voltammetry and SCP. The most promising characteristic of the SbFE is its convenient operation in acidic solutions of pH 2 or lower (which is superior to that reported for BiFEs) in the presence of dissolved oxygen. Furthermore, the SbFE exhibits a very small signal for the reoxidation/stripping of antimony under chosen conditions, a feature which has not been fully explained yet; however, it provides markedly lower background characteristics in the vicinity of the anodic potential limit opposite to those of bismuth- and mercury-based electrodes. In addition, the SbFE yields highly reproducible and well-defined stripping signals for both test metal ions, hence holding great promise in electrochemical stripping analysis as another type of “mercury-free” electrode, thus expanding the scope and utility of electroanalysis in those cases where the application of mercury-based electrodes or even the bismuth electrode is not convenient or is even impossible. Due to the early stage of our investigations, it is inevitable to conduct further work oriented toward the study of interferences, the ex situ prepared SbFE and its operation under adsorptive stripping protocol, and expanding the application of the SbFE to other potentially interesting inorganic and organic analytes. ACKNOWLEDGMENT Financial support from the Slovenian Research Agency (P10034 and Z1-6370) is gratefully acknowledged. The Czech authors are indebted to the Ministry of Education, Youth, and Sports of the Czech Republic (MSM0021627502).

Received for review March 8, 2007. Accepted September 7, 2007. AC070478M Analytical Chemistry, Vol. 79, No. 22, November 15, 2007

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