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Anal. Chem. 1003, 65, 2089-2092
Electrochemical Determination of Trace Amounts of Gold( I I I) by Anodic Stripping Voltammetry Using a Chemically Modified Electrode Iva Turyan and Daniel Mandler' Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
An electrochemically pretreated glassy carbon electrode (GCE)modified with an aza crown ether, 8,9,17,18-dibenzo-1,7-dioxa10,13,16-triazacyclooctadecane (DDTC), has been used for the very sensitive and selective analysis of trace amounts of gold(II1). A detection limit of 4.2 X 10-lo M was obtained by applying anodic stripping voltammetry. The parameters that affect the sensitivity and possible interferences by other ions have been examined in detail. The modified electrodes exhibit high stability and, therefore, have been used repeatedly without mechanical regeneration. Finally, the method has been successfully applied to determine gold traces in a geological sample. INTRODUCTION Electrochemical analysis of low levels of analytes can be frequently improved using chemically modified electrodes.' Modification of the electrode surface is directed toward introducing selectivity as well as sensitivity. A common approach to increase selectivity is to attach host molecules which selectively interact with specific guest molecules. The properties of crown ethers and other macrocycles as host molecules for metal ions have been widely explored.2 As a consequence,crown ethers and their azaand thiaaza analogues have been used for electrode modification as a means of preconcentrating various metal ions followed by their electrochemical determination.- For example, a derivative of 18-crown-6 and 14-crown-5 incorporated in a Nafion film coated on a glassy carbon electrode has been utilized for the determination of trace amounts of silver? thallium: lead? and mercury.@ Gold is one of the metals which has been concentrated and determined on electrode surfaces previously modified with anion exchangers,' dithizone? and polyacrylamide thiocyanate.9 Although gold has been successfullyanalyzed by these electrodes, most of these procedures are rather cumbersome, and the lowest detection limit reported has been 5 X le7M applying cathodic stripping voltammetry (CSV).' The application of anodic stripping voltammetry (ASV) made it possible'0-22 to determine lower concentrations of gold(2.5 X 10-8M). Nevertheless, three major reasons severely limit the ~~
(1) Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987,59, 379A-90A. (2) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S. Chem. Rev. 1991, 91, 1721-2085. (3) Dong, S.; Wan& Y. Anal. Chim.Acta 1988,212, 341-7. (4) Wang, Y.; Dong, 5. Fenxi Huaxue 1988,16, 216-9. (5) Dong, S.; Wang, Y. Talanta 1988,36, 819-21. (6) Gao, Z.; Li, P.; Zhao, Z. Microchem. J. 1991,43, 121-32. (7) Kalcher, K. Anal. Chim. Acta 1986,177, 175-82. (8)Kalcher, K. Freseniua 2.Anal. Chem. 1986,325, 181-5. (9) Su, Z.; Li, H.; Zhang, Y.; Li, Zh.Fenxi Huaxue 1986,14,886-90. (10) Lintem, M.; Mann, A.; Longman, D. Anal. Chim.Acta 1988,209, 193-203. 0003-2700/93/0365-2089$04.00/0
application of ASV for routine determination of gold.10 The reported precision and detection limits have been relatively poor due to the fact that the electrodes employed, such as carbon paste'"'8 or impregnated graphite"22 are chemically or mechanically unstable. The preparation of the reported modified electrodes have been laborious. Finally, many ions which are generally present in samples containing gold, e.g., Cu2+, Ag+, and Fe3+, interfere with the electrochemical determination of gold. We wish to present here a method that successfully addresses these shortcomings by applying a solid electrode modified with an aza crown ether. 8,9,17,18-Dibenzo-1,7dioxa-l0,13,16-triazacyclooctadecane[DDTC(I)I has been first synthesized by Formanovskii,23 and its properties for metal extraction and recovery have been explored by Beklemishev and his co-workers.24 These studies showed that DDTC dissolved in organic solutions acted as a superior agent for gold extraction. Thus, we examined the characteristics of solid electrodes modified by DDTC as a voltammetric probe for gold(II1). Our findings demonstrate that an electrochemically pretreated glassy carbon electrode (GCE) modified by adsorbed DDTC offers a simple and highly reproducible method for the very sensitive and selective determination of low levels of gold(II1).
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EXPERIMENTAL SECTION Voltammetric experiments were performed on a BAS-100B electrochemical analyzer. Working electrodes were assembled (11) Zhang, Y.; Lu, F. Gaodeng Xuexiao Huaxue Xuebao 1987, 8, 125-7. (12) Hutin, M. F.; Netter, P.; Burnel, D.; Faure, G.; Cussenot, F.; Delagoutte, J. P.; Pourel, J.; Gaucher,A. Ann. Med. Nancy Est 1982,21, 41-2. (13) Nghi, T. V.; Vydra, F. J. Electroanal. Chem. 1976, 64,163-73. (14) Luong, L.; Vydra, F. Collect. Czech. Chem. Commun. 1976, 40, 1490-503. (15) Vasil'eva, L. N.; Vinogradova, E. N.; Koroleva, T. A,; Yustus, Z. L. Zavod. Lab. 1976,41,1199-200. (16) Markova, N. V.; Yakubtaeva, T. V.; Sumakova, N. S.Zavod. Lab. 1974,40,938-40. (17) Monien, H. Freseniua 2.Anal. Chem. 1968,237,409-19. (18) Jacobs, E. S. Anal. Chem. 1963,35, 2112-5. (19) Zakharchuk, N. F.; Bikmatova, G. S.;Yudelevich, I. G. Zavod. Lab. 1971,37, 530-3. (20) Vasil'eva, L. N.; Koroleva,T.A.Zh. Anal. Khim. 1971,26,1682-5. (21) Stromberg, A. G.; Kolpakova, N. A.; Kaplin, A. A.; Nemtinova, G. M.; Belousova, N. I. Metody Anal. Khim. Reakt. Prep. 1971,20,78-81. (22) Kadin. A. A.: Pichunina. . V. M.; Filichkina, 0. G. Zavod. Lab. 1988,54,4-s. . (23) Formanovskii, A. A.; Mikhura, I. V.; Sokolovskii, S. A.; Murakhorskaya, A. S.;Terent'ev, P. B.; Sharbatyan, P. A. Khim.Geterotsikl. Soedin. 1988,8, 1128-35. (24) Beklelmishev, M. K.; Formanovskii,A. A.;Kuunin, N. M.; Zolotov, Yu. A. Zh. N e o g . Khim. 1986,31, 2617-22. 0 1993 American Chemlcal Society
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by cutting a short rod (3-mm diameter) of glassy carbon (AtomergicChemetals,Farmingdale,NY) and sealing it in a glass tube with epoxy (Torr Seal, Varian, Lexington, MA). The electrodes were first polished by using emery paper, which was followed by fine polishing with alumina paste (1,0.3, and 0.05 pm). The electrodes were finally sonicated for 5 min in clean water prior to their electrochemicalpretreatment. An Ag/AgCl (KCLtO reference electrode was used for all experiments, and thus potentials are quoted vs this reference electrode. All experiments (except those in which the effect of pH was examined)were conducted in buffer acetate solutions (0.1 M, pH 3.7 or 3.2) that consisted also of sodium bromide (0.1 M). Deionized water (Milli-Q,Millipore)was further double distiled. Hydrochloric and hydrobromic acids (BDH, England) and all other chemicals (Aldrich, Milwaukee, WI) were of analytical grade. Gold(II1) stock solution (lo00 mg-L-l) was prepared in 0.1M HClO, and 0.1M HNOa from chloroauric acid. The solution was standardized by atomic absorption spectrometry. More diluted solutions were prepared daily from the stock solution. DDTC was generously provided by M. K. Becklemishev (since the toxicity of DDTC has never been explored, caution is recommended in handling this substance). DDTC was dissolved (10-2 g-L-1) in 0.1 M acetate buffer solution (pH 3.0). Electrochemicalpretreatment of glassy carbon electrodes was carried out in 0.1 M acetate buffer solution (pH 3.7) which consisted also of 0.1 M NaBr. The polished electrodes were first anodized at 1.75 V for 5 min and then cathodized at -1.55 V for 1 min according to the procedure described by Nagaoka and Yoshino.26 Modificationof GCEs by DDTC was accomplished by leaving the electrochemically pretreated electrodes in a stirred aqueous solutions of DDTC unbiased or under a constant potential for 5 min. The electrodes were then rinsed with water prior to immersing in the electrochemicalcell. Osteryoungsquare wave voltammogram (OSWV)from the potential of depositionto +0.45 V followed the deposition of gold. The pulse amplitude was 50 mV, and the scan rate was 90 m V d (step potential 6 mV, frequency 15 Hz). Regeneration of the modified electrodes was completed by biasing the electrodes for 2 min at 0.3 V or by electrocycling them between -0.7 to 0.3 V. The dependence of the excess of surface coverage of DDTCmodified electrodes on the time and potential of modification was obtained from the OSWV recorded from -0.2 to 0.9 V in 0.1 M acetate buffer (pH 3.7). A geological sample obtained as a waste product from a gold milling process was received from Freegold North Metallurgy (South Africa). A sample of 10g containing 5-8 ppm of gold was treated, according to the procedure developed by Dadgar,%with 100 mL of solution consisting of 0.1% Brz and 1% KBr for 6 h at room temperature. The resulting solution was centrifuged and diluted (1:400) in a buffer solution where gold was analyzed using the standard addition method.
RESULTS AND DISCUSSION Figure 1 shows the anodic stripping Osteryoung square wave voltammogram (OSWV)recorded with an electrochemically pretreated modified glassy carbon electrode (GCE) in a solution containing 1.21 X 10-8 M Au(1II). A clear anodic peak associated with the oxidation of predeposited gold is observed at 0.20 V. Control experiments showed that the electrochemical pretreatment as well as the modification of the electrode are crucial for detecting these trace amounts of gold. GCEs have been pretreated in various ways to improve their r e s p o n ~ e . ~In ~ ~particular, ~7 the electrochemical pretreatment of GCE has drawn the attention of many electrochemists.26 Although electrochemically pretreated GCEs usually exhibit higher charging current, their electron-transfer properties are generally improved. Our results totally depend on this pretreatment process. The electrochemical pretreat(25) Nagaoka, T.; Yoshino, T. Anal. Chem. 1986,58, 1037-42. (26) Dadgar, A. JOM 1989,41,37-41. (27) Taylor, R. J.; Humffay, A. A. J. Electroanal. Chem. 1973, 42, 347-54.
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Figure 1. Anodic OSteryOung square wave stripping voltammogram of a glassy carbon electrochemically pretreated electrode, modified with DDTC in a solution of 0.1 M acetate buffer (pH 3.7), 0.1 M NaBr, and 1.21 X 10" M Au(II1). Deposition potential, -0.7 V; depositlon time, 1200 s; ac amplitude, 50 mV; frequency, 15 Hz; and potentlai step, 6 mV.
ment affects the structure and roughens the surface of the GCE as confirmed by microprobe techniques such as STM.28 Thus, it is conceivable that the adsorption of the aza crown ether has been enhanced, increasing the sensitivity of the method. Moreover,the resulta have been highly reproducible only after the electrodes have been electrochemically pretreated. The appearance of an anodic peak, attributed to the oxidation of gold, could be detected only on pretreated electrodes which were further modified with the aza crown ether. To investigate the mechanism by which DDTC(1) is attached to the electrode surface during modification, we studied the parameters that control this process, i.e., the time and potential of modification. The OSWV recorded from -0.2 to 0.9V of an electrochemically pretreated GCE modified with DDTC reveals a wave at 0.7 V, which corresponds to the oxidation of DDTC. As a result, the dependence of the excess of surface coverage of DDTC on the time and potential of modification could be studied. Figure 2A shows the dependence of the charge, associated with the oxidation of DDTC, on the potential of modification. It can be seen that the adsorption of DDTC exceeds a maxima a t potentials between 0.0 to -0.2 V. This potential range lies within the range of the potential of zero charge (pzc) for GCE,B suggesting that DDTC is adsorbed via a nonspecific adsorption process. This conclusion is also supported by the dependence of the excess of surface coverageof DDTC on the time of adsorption (Figure 2B). Under open circuit potential, the adsorption of DDTC follows a typical isotherm. The asymptotic value is reached within a few minutes, and therefore, electrodes were immersed for 5 min into DDTC solution under open circuit conditions for all further experiments. The fact that the excessof surface coverage of DDTC has been highly reproducible and controllable can be explained by the electrochemicalpertreatment of the electrodes, i.e., identical pretreatment provided very (28) Wang, J.; Martinez, T.; Yaniv, D. R.; McCormic, L. D. J. Electroanal. Chem. 1990,278, 379-86. (29) Randin, J. D.; Yeager,E. J.Electroanal. Chem. 1975,58,313-22.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993 2091
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reproducible surfaces. In addition, we found that DDTC was strongly and irreversibly attached to GC unless it was electrochemically oxidized (E> 0.5 V). To establish the optimum conditions for the detection of gold, we studied the different parameters that affect the response of DDTC-modified electrodes toward gold(II1). Figure 3A shows the anodic stripping peak current of gold as a function of potential of deposition. The electrodes were biased for 10 min at a constant potential before the OSWV was recorded. It can be seen that the anodic stripping peak current increases linearly as the deposition potential varies
from -0.1 to -0.65 V. A constant stripping peak current was obtained when potentials more negative than -0.7 V were applied. Thus, a deposition potential of -0.7 V was selected for all experiments. This potential lies sufficiently negative to ensure a diffusion controlled process, yet not causing significant interference due to solvent reduction or codeposition of impurities. It is worth noting that the cathodic irreversible wave of gold is shifted to negative potentials from 0.5 to ca. 0.15 V as a result of the modification by DDTC. This implies that a complex, which stabilizes the gold ions, is formed in the presence of DDTC. The role that DDTC plays in the preconcentration process has been further addressed performing the preconcentration step under open circuit potential. Preconcentrated gold was determined by CSV in a gold-free solution. The fact that a cathodic wave was observed even at low concentrations of gold(II1) strongly indicates that gold ions have been preconcentrated by complexation with DDTC. Moreover, a time saturation effect has been observed after 10 min of preconcentration. Nevertheless, the cathodic waves (under saturation conditions) have been substantially smaller that those obtained by ASV. Figure 3B depicts the dependence of the anodic peak current on the time of deposition. It is evident that the peak current increases as a function of time of deposition. Comparison of the charge, associated with the oxidation of gold (after 20 min of deposition), with the moles of DDTC which are expected to form a monolayer (based on 100& per DDTC molecule) reveals that more gold ions were preconcentrated on the electrode surface than the number of DDTC molecules. The fact that no time saturation effect was seen as compared to CSV suggests that DDTC serves as a catalyst, i.e., complexedgold is released upon reduction. Thus, we can conclude that the overall mechanism of the process involves a chemical preconcentration step supported by electrochemical reduction. In other words, DDTC assists to preconcentrate gold onto the GCE, where it is being reduced. The fact that trace amounts of gold, as low as 0.2 ppb, were detectable only with a modified electrode by applying ASV supports this hypothesis as well. The third parameter that governs the efficiency of the process is the pH. Figure 4 shows the effect of the pH of the deposition solution on the anodic stripping peak current. It can be seen that the best conditions for the determination of gold(II1)were found at pHs between 3.7 and 7.0. These values were obtained upon applying -0.7 V for 10 min in a 7.55 X 10-9 M Au(II1) solution. Similar results were reported by Beklemishev while studying the effect of the pH on the extraction of gold(II1) by the same aza crown ether.2' We speculate that a t pH 7, the hydrolysis of gold interferes with the complexation.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993 25
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To conclude, optimum conditions for the determination of gold(II1)using glassy carbon electrode modified with DDTC have been achieved by biasing the modified electrode at -0.7 V, for 5-25 min in a buffer acetate solution a t 3.7 < pH < 7.0. Under these conditions, a good linear dependence between the anodic stripping peak current and the gold(II1) concentration in the range of 1.62 X 10-9-1.59 X 10-8 M was obtained (Figure 5). The relative standard deviation for 1.21 X 10-9 M Au(II1) and a deposition time of 10 min was 3%. The detection limit, calculated from the standard deviation of the background (signal equals 3a of the background noise), is 4.2 X 10-l0 M, which corresponds to 83 ppt. To the best of our knowledge, this is the lowest detection limit ever reported for gold(II1)applying an electrochemical technique, in particular with solid electrodes. This detection limit competes with most sensitive methods for gold determination such as graphite furnace atomic absorption (-10-9 M). Moreover, it should be pointed out that the modified electrodes exhibited remarkable stability and, therefore, could be utilized for tens of experiments. Although no detectable change has been observed in the performance of the modified electrodes during 1day of work, electrodes have been modified daily. The modified surface was electrochemically regenerated by soaking the electrode after every experiment in a buffer solution for 2 min a t 0.3 V. This procedure ensured that all the deposited gold was anodically removed wihtout affecting the modified surface.
Although high sensitivity is the dominant requirement from any analytical method, it must be accompanied by high selectivity as well. Hence, we examined possible interferences from common metal ions and anions capable of complexing with DDTC or gold(II1). We found that the determination of 10-9 M gold(II1) ions was not affected by the addition of 1od M Cu2+,Pb2+, Hg2+,and FeS+ and 1o-S M Ag+. Anions, such as acetate, chloride, bromide, and thiocyanate also had no effect on the electrochemical response of the modified electrode. The high sensitivity obtained by this simple method made it possible to determine the concentration of gold in a geologicalwaste sample produced from a gold milling process. Gold was determined by the single-standard addition method. The concentration of gold in the original sample was 4.0 f 0.2 ppm as compared to 5 f 1 ppm obtained by atomic absorption. The fact that the sample had to be diluted 400 times before it was electrochemically analyzed means that much lower levels of gold can be, in principle, detected. To conclude, our results indicate that the application of glassy carbon electrodes modified with DDTC for gold analysis is very promising. The modified electrodes do not only offer considerably higher sensitivity than other reported electrochemical methods but also exhibit high selectivity and reproducibility. Since the prepared electrodes show excellent reproducibility without any noticeable change in their response for at least 1day, they can be applied as a simple and reliable method for the routine analysis of gold in natural samples.
ACKNOWLEDGMENT Professor Yu. A. Zolotov and Dr. M. K. Becklemishev from the Moscow State University and Professor N. M. Kuzmin from the V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry in Moscow are acknowledged for providing us with DDTC samples. This research is supported by the Israeli Minister of Science and Technology (Contract 3735-1-91).
RECEIVED for review November 18, 1992. Accepted April 21, 1993.
