Adsorptive Stripping Measurements of Chromium and Uranium at

the end of each run. The same hemispherical surface is thus used over a prolonged period of over five weeks, performing hundreds of runs with RSDs low...
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Anal. Chem. 1997, 69, 1657-1661

Adsorptive Stripping Measurements of Chromium and Uranium at Iridium-Based Mercury Electrodes Joseph Wang,* Jianyan Wang, Baomin Tian, and Mian Jiang

Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003

Iridium-based mercury electrodes are shown to be very suitable for adsorptive stripping measurements of chromium and uranium in the presence of the DTPA and propyl gallate complexing agents. The well-adhered mercury hemispherical electrode offers remarkable durability to withstand various manipulations expected under field deployment and the “pure” mercury surface essential for efficient adsorptive accumulation of the corresponding metal chelates. An electrochemical “cleaning” step ensures complete removal of the adsorbed metal chelate at the end of each run. The same hemispherical surface is thus used over a prolonged period of over five weeks, performing hundreds of runs with RSDs lower than 10%. Detection limits of 0.4 µg/L uranium and 0.5 µg/L chromium are obtained following a 10 min adsorptive accumulation. The electrode responds rapidly to the “switching” between solutions of low and high concentrations of chromium or uranium. Proper choice of the constant current used for stripping potentiometric measurement of the uranium-propyl gallate complex results in an effective elimination of the oxygen background contribution. Various experimental parameters relevant to the mercury plating, adsorptive accumulation, and surface “cleaning” steps are explored and optimized. Applicability to groundwater and soil samples is demonstrated. The highly stable and favorable adsorptive stripping response of the iridium-based mercury electrode makes it very attractive for on-site measurements of trace uranium and chromium, as well as of other environmentally relevant metals. Stripping analysis is one of the most powerful techniques for trace metal measurements.1 Its remarkably low detection limits are attributed to its “built-in” preconcentration step, during which the target metal is accumulated onto the surface of the working electrode. The use of stripping analysis has been expanded greatly during the 1990s, owing to the growing needs for on-site environmental monitoring or clinical testing of trace metals.2,3 Such decentralized metal testing can greatly benefit from the development of adsorptive stripping procedures.4-7 Such schemes, which rely on the formation and interfacial accumulation of appropriate (1) Wang, J. Stripping Analysis: Principles, Instrumentation and Applications; VCH Publishers: New York, 1985. (2) Wang, J. Analyst 1994, 119, 763. (3) Noble, D. Anal. Chem. 1993, 65, 265A. (4) Kalvoda, R.; Kopanica, M. Pure Appl. Chem. 1989, 61, 97. (5) Paneli, M.; Voulgaropoulos, A. Electroanalysis 1993, 5, 355. (6) Wang, J. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 16, pp 1-88. (7) van den Berg, C. M. G. Anal. Chim. Acta 1991, 250, 265. S0003-2700(96)01175-4 CCC: $14.00

© 1997 American Chemical Society

surface-active complexes, have greatly expanded the scope of stripping analysis toward numerous trace elements that cannot be measured by conventional (electrolytic) stripping schemes. Two such adsorptive stripping protocols, aimed at monitoring trace chromium8,9 and uranium,10,11 are of particular interest to the U.S. Department of Energy (DOE) due to the extensive use and release of these metals by the nuclear industry during the 50-year-long cold war. Unlike conventional anodic stripping measurements that utilize both mercury film and mercury drop electrodes, adsorptive stripping experiments usually rely on the use of the hanging mercury drop electrode (HMDE). The widespread use of the mercury drop electrode in adsorptive stripping measurements is attributed to the needs for a smooth uniform surface for optimal adsorption (in contrast to the large-area-to-volume requirement of conventional stripping analysis). Surface renewability, with the dispension of a new drop in each run, represents another advantage of the drop when “memory” effects, due to incomplete desorption of reaction products, are concerned. Yet, such extensive use (and associated disposal) of toxic mercury represents a serious drawback for most field applications. In addition, conventional mercury drop electrodes are not stable mechanically during various steps (e.g., medium exchange, use of open circuit, rinsing or shaking of the electrode) and may thus not be compatible with different on-site or in-situ applications. These problems, along with the growing interest in field analysis of uranium and chromium, have prompted the investigation of new electrode systems for adsorptive stripping voltammetry. The main purpose of this work is to characterize and test iridium-based mercury electrodes for adsorptive stripping measurements of trace uranium and chromium. Recent studies12-16 have shown that conventional (electrolytic) stripping analysis (particularly of lead, cadmium, and copper) can greatly benefit from the use of mercury-plated iridium electrodes. Comparing different materials as potential substrates for mercury electrodes, Kounaves and Buffle12 concluded that iridium is the best owing to its very low solubility in mercury and good wettability. In the present study, these characteristics of iridium-based mercury (8) Golimowski, J.; Valenta, P.; Nurnberg, H. W. Fresenius Z. Anal. Chem. 1985, 322, 315. (9) Olsen, K.; Wang, J.; Setiadji, R.; Lu, J. Environ. Sci. Technol. 1994, 28, 2074. (10) Wang, J.; Wang, J.; Lu, J.; Olsen, K. Anal. Chim. Acta 1994, 292, 91. (11) Wang, J.; Setiadji, R.; Chen, L.; Lu, J.; Morton, S. Electroanalysis 1992, 4, 161. (12) Kounaves, S. P.; Buffle, J. J. Electroanal. Chem. 1987, 216, 53. (13) Golas, J.; Galus, Z.; Osteryoung, J. Anal. Chem. 1987, 59, 389. (14) Tercier, M. L.; Parthasarathy, N.; Buffle, J. Electroanalysis, 1995, 7, 55. (15) Belmont, C.; Tercier, M. L.; Buffle, J.; Fiaccabrino, G.; Koudelka-Hep, M. Anal. Chim. Acta 1996, 329, 203. (16) Kounaves, S. P.; Deng, W. Anal. Chem. 1993, 65, 375.

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hemisphere electrodes result in the “pure” mercury surface essential for efficient adsorptive accumulation of metal chelates of chromium and uranium and in the durability needed for withstanding various manipulations expected under field deployment. Such an attractive adsorptive stripping behavior is elucidated and illustrated in the following sections. EXPERIMENTAL SECTION Reagent. Stock solutions (1000 mg/L, atomic absorption standard, Aldrich) of uranium(VI) and chromium(VI) were diluted as required. The 0.05 M propyl gallate (Aldrich) solution was prepared with 1:1 (volume) ethanol/double distilled water. The acetate buffer solution (pH 4.5), used for uranium measurements, was prepared from anhydrous sodium acetate (Aldrich) to form a 0.05 M electrolyte supporting solution; glacial acetic acid was used to adjust buffer solution to pH 4.5. A solution containing 0.05 M diethylenetriaminepentaacetic acid (DTPA) (Aldrich), 0.2 M sodium acetate (Aldrich), and 2.5 M sodium nitrate (Aldrich), prepared with double-distilled water (final pH 5.6), was used for the chromium experiments. The mercury solution, used for preparing the iridium-based electrode, was prepared by dissolving the desired amount of mercury acetate (Aldrich) in 0.1 M perchloric acid. All solutions were prepared from double-distilled water. All chemicals used were of analytical grade. The groundwater and soil samples were collected at Hanford site (Richland, WA). The soil sample was microwave digested, and the leachate was filtered and diluted in accordance with ref 10. Apparatus. A Tracelab potentiometric stripping unit (PSU20, Radiometer), and an IBM PS/2 55SX computer were used to obtain the potentiograms (dt/dE - E). An EG&G PAR 264A voltammetric analyzer and a PAR 0073 X-Y recorder were used to obtain the voltammograms. The iridium electrode was prepared by heat-sealing iridium wire (1 cm long and 150 µm diameter) into a 7-cm-long glass pipet. The resulting pipette tip was polished first with Carbimet paper disk and then nylon polishing cloth with 3-µm alumina powder. Finally, the shiny iridium disk was polished again with 0.15-µm alumina powder. The polishing quality was inspected with a video microscope (×10-300, Nikon A4-B). The 3-mm-diameter glassy carbon disk electrode was obtained from BAS Inc., along with the Ag/AgCl reference electrode (Model RE-1); a platinum wire was used as the counter electrode. The EG&G PAR Model SMDE 303 was used as the capillary HMDE. Procedure. Adsorptive Stripping Voltammetry for Uranium. The iridium-based mercury electrode was prepared prior to experiments by preplating mercury at -0.4 V for 25 min from a 4000 mg/L Hg2+/0.1 M HClO4 solution. The mercury-coated glassy-carbon electrode (r ) 1.5 mm) was prepared by preplating mercury at -0.9 V for 15 min using a 800 mg/L Hg2+/0.1 M HCl solution. A 10 mL sample of 0.02 M acetate buffer solution (pH 4.5), containing the desired level of the propyl gallate chelating agent, was pipetted into the voltammetric cell and deaerated by nitrogen for 8 min. Following the cleaning step (at -1.35 V for 45 s), an accumulation potential of -0.05 V (vs Ag/AgCl) was applied to the working electrode for the selected time using a quiescent solution. After the accumulation step, the blank potentiogram was recorded by applying a constant negative current (-0.1 µA for Hg/Ir, -5.0 µA for Hg/GCE). Alternately, the background voltammogram was recorded by applying a negative-going differential pulse potential scan (20 mV/s) from -0.05 to -0.80 V. Following this, an aliquot of the uranium 1658 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

Figure 1. Comparison of typical differential pulse voltammograms of 40 µg/L uranium (A) and 20 µg/L chromium (B) at the HMDE (a), MFE/GC (b), and Hg/Ir electrode (c) after a 1-min adsorptive accumulation from stirred (solid line) and quiescent (dotted line) solutions. Uranium measurements with a 0.02 M acetate buffer (pH 4.5) solution, containing 1 × 10-5 M propyl gallate; 0.04 M sodium acetate, and 0.5 M sodium nitrate containing 0.01 M DTPA (pH 5.6) is used for the chromium experiment. Pulse amplitude, 50 mV; pulse interval, 0.2 s; scan rate, 20 mV/s. Cleaning at -1.35 (A) or -1.60 (B) V for 45 or 30 s, respectively. Other conditions are described in the Experimental Section.

standard solution was added and the cleaning/accumulation/ stripping cycle was repeated. Except when stated otherwise, nitrogen was passed over the solution throughout the experiments. All experiments were carried out at room temperature. Adsorptive Stripping Voltammetry for Chromium. The iridiumbased mercury electrode was prepared prior to experiments by preplating mercury at -0.4 V for 40 min using a 1000 mg/L Hg2+/ 0.1 M HClO4 solution. A 10 mL sample of solution containing 0.01 M DTPA, 0.04 M sodium acetate, and 0.5 M sodium nitrate (pH 5.6) was pipetted into the voltammetric cell. Following the cleaning step (at -1.60 V for 30 s), an accumulation potential of 0.00 V (vs Ag/AgCl) was applied to the working electrode in a quiescent solution for a certain period of time. After the accumulation step, the potential was switched to -0.9 V, and the background voltammogram was recorded by a negative-going differential pulse potential scan (20 mV/s). The measurement cycle was repeated after spiking an aliquot of the chromium standard. All experiments were carried out at room temperature. RESULTS AND DISCUSSION Established stripping protocols for chromium and uranium, using DTPA8 and propyl gallate10 as complexing agents, were used to characterize, demonstrate, and test the suitability of iridiumbased mercury electrodes for adsorptive stripping analysis. Figure 1 compares typical adsorptive stripping voltammograms for 40 µg/L uranium (A) and 20 µg/L chromium (B), obtained at the conventional HMDE (a), mercury-film glassy-carbon electrode (MFE/GC; b) and iridium-based mercury (Ir/Hg) electrode (c). All the electrodes offer convenient quantitation of these low levels of uranium and chromium following a short (1-min) adsorptive

Figure 2. Response of the Hg/Ir electrode upon switching between solutions of low (10 µg/L, dotted line) and high (50 µg/L, solid line) concentrations of uranium (A) or chromium (B). Electrodes were rinsed with water upon transfer from the 50 µg/L solution to the 10 µg/L one. Medium, 0.02 M acetate buffer (pH 4.5) containing 1 × 10-5 M propyl gallate for uranium; 0.04 M sodium acetate and 0.5 M sodium nitrate containing 0.01 M DTPA (pH 5.6) for chromium; accumulation time, 15 (A) and 60 (B) s; pulse wave form, as in Figure 1. Other conditions are described in Experimental Section.

accumulation period. While offering different sensitivities (due to their different surface areas), these mercury electrodes result in similar signal-to-background (S/B) characteristics. (Greatly improved S/B characteristics will be illustrated below in connection with adsorptive stripping potentiometry.) Also shown, as dotted lines in Figure 1, are analogous voltammograms obtained following accumulation from quiescent solutions. Because of its smaller dimension (150-µm diameter), approaching the microelectrode domain, the Ir/Hg electrode results in larger ratios of peak currents obtained in quiescent and stirred solutions (ip,q/ ip,s). For example, ip,q/ip,s values of 0.26, 0.32, and 0.49 can be estimated from the chromium response at the HMDE, MFE/GC, and Ir/Hg electrodes, respectively. All subsequent work employed quiescent solutions. While the S/B of the three mercury electrodes are similar, the main advantages of the Ir/Hg electrode for adsorptive stripping applications (particularly those performed in the field) are its remarkable durability and mechanical stability. Poor mechanical stability has always been a major drawback of conventional (capillary) HMDEs.1 Both the classical Kemulatype HMDE and the modern SMDE are not stable during mediumexchange (electrode transfer) experiments, upon exposure to open circuit, mild movement/shaking, or under vigorous solution stirring or flow. These conditions, which are commonly experienced in various field operations, result in a rapid dislodgment of the drop. In contrast, the remarkable durability of Ir/Hg electrodes greatly facilitates their use under medium-exchange adsorptive stripping experiments (and in connection to other manipulations of the electrode). This was tested from carry-over experiments, involving sequential measurements of four replicates of 10 and 50 µg/L uranium solutions, utilizing the same mercury surface (Figure 2A). The sensor responds rapidly to sudden changes in the uranium level. The peak rises rapidly from the 10 µg/L baseline upon transferring the electrode to a 50 µg/L

uranium solution and decays sharply upon returning to the original low-concentration medium. Apparently, such medium-exchange operation (with its associated open-circuit conditions, electrode movement, and rinse) has no deleterious effect upon the adsorptive stripping response of the Ir/Hg electrode. Similarly, minimum carry-over effects and a fast response are indicated from the experiment of Figure 2B involving four replicates at 10 µg/L chromium, followed by replicates at the 50 µg/L chromium level, and a return to the original 10 µg/L solution. Individual peaks from this series are also displayed. Once again, the electrode responds rapidly to the “switching” between the low- and highchromium solutions. Such a quick response indicates great promise for on-site probing of dynamic processes. The carry-over data of Figure 2 also illustrates that despite the use of the same mercury surface, the iridium-based sensor does not suffer from history effects. Unlike conventional stripping analysis, where the measurement (stripping) step serves also for “cleaning” the surface from the electrodeposited metals, adsorptive stripping procedures have commonly relied on the use of fresh mercury drops for erasing memory effects. In order to continuously use the same Ir/Hg surface, we examined various protocols for desorbing the accumulated metal chelate at the end of each run. This investigation led to the incorporation of a simple and short electrochemical cleaning step at potentials negative to the metal-chelate peak. For example, 45 s at -1.35 V or 30 s at -1.60 V were found useful for removing the adsorbed complexes of uranium or chromium, respectively, during measurements over the 1-400 µg/L range. Longer periods may be required for the quantitation of higher (mg/L) levels. The rapid response to changes from high to low metal concentrations (e.g., Figure 2), as well as other data reported throughout this paper, clearly illustrates the efficiency of the electrochemical cleaning step. The remarkable physical and chemical stability of the Ir/Hg electrode, coupled with the electrochemical cleaning capability, results in a highly stable adsorptive stripping response. The shortterm stability was demonstrated from the response to 100 repetitive measurements of 20 µg/L chromium, carried out with the same mercury surface over an unbroken 4-h period (Figure 3A). The chromium adsorptive signal remains nearly unchanged throughout this prolonged series; the relative standard deviation (RSD) of the complete series of 100 runs is 6.2%. Such remarkable reproducibility over a prolonged period illustrates again the efficiency of the electrochemical cleaning step. Even more impressive are the long-term stability data obtained with the same Ir/Hg surface over a 41-day period (Figure 3B). This prolonged experiment involved 200 runs during the first day (RSD ) 10%), 12 runs in each subsequent day (up to the 8th one), and 12 replicates in each second or third day (up to the 41th day). Between experiments, the Ir/Hg electrode was stored in the double-distilled, nondeaerated water solution under open-circuit conditions. The data points of Figure 3B represent the average of the individual measurements in each day. The RSD for each of these 12 daily runs ranges from 3.2 to 9.6%. While the response decreases gradually during the first four days (to 50% of its original value), it remains highly stable over the subsequent 37 days. A RSD of 8.8% was estimated for the 195 runs over this 37-day-long period. The exact reason for the gradual decrease in the response during the first four days is not fully understood. In view of the stable response for prolonged series of repetitive runs, we believe that such decrease is related to the transfer and overnight storage Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

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Figure 3. Short (A) and long (B)-term stability of the response of the Ir/Hg electrode to 20 µg/L chromium. (C) Stability of the adsorptive stripping medium-exchange voltammetric response of the MFE/GC (9) and Ir/Hg (O) electrodes to 40 µg/L uranium. Accumulation time, 60 s. Other conditions are as in Figure 1 and the Experimental Section.

in the water solution. Because of the good precision obtained daily, a short (5-10 min) calibration experiment, in the middle of each day, would be sufficient for reliable quantitation. The most important message of Figure 3B, for laboratory and particularly field applications, is that the same Ir/Hg electrode can be used over several weeks while performing hundreds of adsorptive stripping measurements (in a manner analogous to anodic stripping measurements of lead and cadmium14). The implications of these data for minimizing the handling and disposal of mercury (vs capillary HMDEs) are obvious. Figure 3C illustrates the advantages of the Ir/Hg electrode over the MFE/GC one in connection with adsorptive stripping medium-exchange experiments. The iridium-based sensor offers a stable uranium response over the unbroken 200-min period. In contrast, the medium-exchange protocol promotes a gradual degradation of the MFE/GC electrode. A gradual decrease of its uranium peak (to 30% of its original value) is observed within the first 150 min. Such loss in sensitivity is attributed to the exposure to open circuit and/or loss of loosely bound mercury droplets during the transfer to the blank solution. Apparently, the Ir/Hg electrode is not prone to such effects. Special attention to the preparation of the Ir/Hg electrode is required for obtaining the attractive adsorptive stripping performance described in this paper. Besides proper sealing and polishing,14 one should carefully control the mercury plating. We have found that adsorptive stripping measurements of uranium and chromium require different plating conditions (probably due to the extreme potential of the chromium peak, and its nearby hydrogen evolution process). Conditions optimized first for adsorptive stripping measurements of uranium (25 min of Hg preplating from a 4000 mg/L mercuric acetate solution) were not suitable for the detection of chromium. Most favorable chromium response was achieved after a 40-min deposition from a 1000 mg/L mercury solution. The charge used for the formation of this electrode was 5 mQ. The mercury deposition was monitored also using a video microscopy. These images indicated the formation of a hemispherical cap, and a complete coverage of the iridium substrate using plating periods longer than 20 min (in connection with a 4000 mg/L solution). Yet, exposed iridium surface (not 1660 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

Figure 4. Adsorptive stripping voltammograms (A, C) and potentiograms (B) for 40 µg/L uranium (A, B) and 20 µg/L chromium (C) following different accumulation times: 30 (a), 60 (b), 90 (c), 120 (d), and 150 (e) s. Stripping current, -0.1 µA. Other conditions are given in Figure 1c and in the the Experimental Section.

visible under optical microscopy) may account for the inferior chromium response following 25-min deposition from the 4000 mg/L mercury solution. Figure 4 shows the adsorptive stripping response of the iridium-based mercury electrode for 40 µg/L uranium (A, B) and 20 µg/L chromium (C) following different preconcentration times (30-150 s, a-e). As expected, the longer the accumulation period the more metal complex is adsorbed, and the larger is the adsorptive stripping signal. The adsorbed uranium-propyl gallate complex can be readily quantified using a pulse voltammetric wave form (A) or in connection with a constant-current chronopotentiometric operation (B). The latter offers effective correction of background contributions and hence greatly enhances the performance of the Ir/Hg electrode. Indeed, even a 1-s adsorption period offered a well-defined potentiometric response for this 40 µg/L uranium solution (not shown). Attempts to perform similar adsorptive stripping potentiometric measurements of trace chromium were not successful due to the large background noise associated with the extreme potential of the Cr-DTPA peak. In addition to improved detection of uranium, the potentiometric stripping mode can be used to address the problem of large oxygen background contribution encountered in adsorptive stripping analysis. For example, the large oxygen background current obscured completely the voltammetric stripping signals for 40 and 80 µg/L uranium (not shown; 20-s accumulation). A similar oxygen interference was observed in connection with the stripping potentiometric mode using a constant current of -0.1 µA. However, with a proper selection of the stripping current (-0.8 µA), such oxygen contribution was completely eliminated and the uranium peaks were readily detected. The exact reason for this attractive behavior is currently under investigation. It appears to be related to the different rates of the oxygen and metal-chelate reduction processes. Such elimination of the oxygen intereference

Figure 5. Adsorptive stripping response for groundwater (A) and soil (B) samples. (A) Potentiograms for a 20-fold diluted of groundwater sample (a) and following additions of 20 µg/L of uranium (bd). (B) Voltammograms for a 100-fold diluted digested soil sample (a) and following additions of 10 µg/L of chromium (b-d). Accumulation time, 60 s. Other conditions are described in the Experimental Section.

drastically reduces the analysis time and greatly simplifies onsite operations. Stripping potentiometric and voltammetric measurements obtained at the iridium-based mercury electrode for 10 successive concentration increments of 20 µg/L (ppb) uranium and chromium, respectively, yielded well-defined calibration plots in connection with short [60 (U) and 30 (Cr) s] preconcentration periods (not shown). The resulting calibration plots were characteristics of adsorptive stripping procedures, with deviations from linearity reflecting the surface saturation with the adsorbed complex. The response was linear up to 80 (U) and 50 (Cr) µg/L, with a curvature at higher levels. The slopes of the initial linear portions were 0.5 ms‚L/µg (U) and 8 nA‚L/µg (Cr). Extension of the linear range can be achieved by using shorter accumulation periods. Detection limits were estimated from the response characteristics (S/N) for 2 µg/L chromium and uranium solutions (not shown). For a 10-min accumulation period, values of 0.4 µg/L uranium and 0.5 µg/L chromium were thus estimated. Such values are slightly higher than the corresponding ones at hanging mercury drop electrodes.9,10 (17) Palecek, E. Anal. Biochem. 1988, 170, 421.

Figure 5 demonstrates the suitability of the iridium-based mercury electrode for the determination of uranium (A) and chromium (B) in groundwater and soil samples, respectively, relevant to the DOE cleanup mission. Three standard additions of 20 µg/L uranium to the highly contaminated diluted groundwater sample from the Hanford Site (WA) result in well-defined potentiometric stripping peaks (A, traces b-d). The uranium peak in the original sample (A, trace a) can thus be readily quantitated by means of the resulting standard additions plot (also shown) to yield a uranium value of 422 µg/L. Similarly, the contaminated soil sample was digested and diluted to give a well-defined chromium stripping peak (B, a) that can be readily quantified by means of standard additions (B, b-d and inset); the chromium level thus found corresponds to 1.41 mg/L. ICP spectroscopic assays of these samples yielded uranium and chromium values of 412 µg/L uranium and 1.67 mg/L chromium. Accumulation periods longer than the 60 s used in Figure 5 would be required for direct measurements in unpolluted natural samples. In conclusion, we have demonstrated that iridium-based mercury electrodes are very suitable for adsorptive stripping measurements of trace chromium and uranium and for addressing various steps and manipulations anticipated under field deployment. While the concept is illustrated for the detection of chromium and uranium, it could be readily extended to adsorptive stripping schemes for detecting other environmentally important metals. Similarly, adsorptive stripping measurements of organic and biological compounds4,6 should also benefit from the attractive features of iridium-based mercury electrodes. For example, trace measurements of nucleic acids commonly involve an electrode transfer (medium-exchange) protocol,17 and are currently being performed using a “loosely” attached Kemula-type hanging mercury electrode. Smaller iridium microelectrodes should further enhance the accumulation from quiescent solutions. The adsorptive stripping protocols can also provide useful speciation (oxidation state) information.9 The greatly reduced amount of used mercury should be of great importance to both field and centralized (laboratory) applications of adsorptive stripping protocols. Research in this and other15,16 laboratories is progressing toward the adaptation of mercury-coated iridium electrodes in various field applications ranging from hand-held analysis to online monitoring or remote sensing. ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy (DOE) Grant DE-FG07-96ER62306. Received for review November 20, 1996. February 6, 1997.X

Accepted

AC961175N X

Abstract published in Advance ACS Abstracts, March 15, 1997.

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