Anal. Chem. 1997, 69, 4660-4664
The Silver Electrode in Square-Wave Anodic Stripping Voltammetry. Determination of Pb2+ without Removal of Oxygen Mordechai Brand, Inna Eshkenazi, and Emilia Kirowa-Eisner*
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
A silver rotating disk electrode is used for the determination of lead in concentrations from 2 × 10-9 to 3 × 10-7 M by square-wave anodic stripping voltammetry (SWASV) without removal of oxygen. The repeatability of consecutive SWASV runs in synthetic solutions covering the entire concentration range is better than 2%. The calibration curve is represented by a correlation coefficient of at least 0.999. The detection limit for a 2-min electrodeposition is 0.5 nM. Up to 600 runs can be carried out on synthetic solutions without any pretreatment of the electrode. A 4-fold excess of Cd2+ and a 10 000-fold excess of Cu2+ do not interfere with the determination of part-per-billion concentrations of Pb2+. Surfactants present in tap water distort the SWASV. Improvement of the analytical response in tap water is achieved by pretreatment of the samples: irradiation at 254 nm or digestion with HNO3. During anodic stripping voltammetry in the concentration range studied, a uniformly distributed submonolayer of lead, occupying 0.02%-1% of the real surface of the electrode, is formed by underpotential deposition. Linearity in calibration plot is achieved up to 1% electrode coverage; in terms of the experimental parameters of the deposition step (rate of rotation and time of electrolysis), this condition for linearity is CPb2+N1/2td e 2.2 × 105 nM rpm1/2 s. This study focuses on characterizing the silver electrode as a tool for determination of trace lead ions. There are only limited reports on the use of silver electrodes for stripping analysis, with the majority in cathodic stripping.1-3 Miwa et al.3 used silver-plated glassy carbon electrodes for anodic stripping voltammetry. The parameters of polarographic analysis in the millimolar concentration range are applicable to anodic stripping voltammetry with mercury electrodes. With solid electrodes, however, where submonolayers are formed in trace analysis of metals, phenomena related to underpotential deposition (UPD) are of utmost importance. Although underpotential deposition was discovered in connection with anodic stripping studies at solid electrodes,4 little use is made today of the vast amount of UPD data. In systems in which UPD phenomena are present, simple guidelines could be used for selecting electrodes and conditions for ASV: (i) the width and form of the dissolution peak of the UPD cyclic voltamogram, (1) Shain, I.; Perone, S. P. Anal. Chem. 1961, 33, 325-329. (2) Ishiyama, T.; Tanaka, T. Anal. Chem. 1996, 68, 3789-3792. (3) Miwa, T.; Nishimura Y.; Mizuike, A. Anal. Chim. Acta 1982, 140, 59-64. (4) Rogers, L. B.; Stehney, A. F. J. Electochem. Soc. 1949, 95, 25-32.
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(ii) the efficiency of the dissolution step, i.e., how close to unity is the ratio of stripped charge and deposited charge, and (iii) suitability of the potential range of deposition and dissolution with respect to interfering materials. On the basis of these guidelines and according to UPD data,5-8 silver is a potentially suitable electrode for the determination of Pb2+ and Tl+; it is, however, not expected to be very sensitive with respect to Cd2+, while gold is not suitable for the anodic stripping analysis of Pb2+. These considerations, however, may not apply to potentiometric stripping analysis, where the dissolution of the analyte is usually performed by a chemical and not electrochemical step. EXPERIMENTAL SECTION Preparation of the Silver Disk Electrode. The silver electrode is a home-made disk electrode embedded in Teflon. Silver of 99.99% purity is used. An electrode with an area of 4.91 mm2 is used as a rotating disk, and an electrode with an area of 1.77 mm2 is used in solutions stirred with a magnetic stirrer. A new electrode is rinsed in ethanol and then polished successively with 600-grit emery paper and with 0.3- and 0.05-µm alumina powder on a Texmet polishing cloth (Buehler Catalog No. 407618), to a mirror-like finish. In routine practice, when a deterioration in the proper functioning of the electrode is observed, it is repolished with 0.05-µm alumina. The electrode is then rinsed with water, immersed in an ultrasonic bath for 5 min, and rinsed again. Several anodic stripping voltammograms of the background are to be carried out after polishing, under conditions suitable for the determination of lead, in order to improve and stabilize the background. An electrode used in synthetic solutions, prepared in high-purity water and free of surface-active species, can be used for over 500 stripping runs without requiring any mechanical pretreatment. However, an electrode used in untreated drinking waters may need frequent polishing. The electrode is mounted on a home-made rotator with an adjustable rate of rotation in the range 400-10000 rpm. While not in use, the electrode is stored in a solution of 10 mM KCl, 10 mM HNO3. Silver Quasi-Reference Electrode (AgQRE). A silver wire (diameter, 1.0 mm; area, 0.7 cm2) directly dipped into the analyte, containing at least 10 mM chloride ions, serves as a quasi(5) Popov, A. Electrochim. Acta 1995, 40, 551-559. (6) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11. (7) Kolb, D. M.; Przasnyski, M.; Gerischer, H. J. Electroanal. Chem. 1974, 54, 25-38. (8) Astley, D. J.; Harrison, J. A.; Thirsk, H. R. J. Electroanal. Chem. 1968, 19, 325-334. S0003-2700(97)00420-4 CCC: $14.00
© 1997 American Chemical Society
reference electrode. The large area of the reference electrode compared to that of the working electrode enables a two-electrode configuration. The potential of AgQRE in 10 mM KCl is 98 ( 5 mV vs SCE. The electrode is characterized by long-term stability of the potential. The electrode is stored in a 10 mM KCl, 10 mM HNO3 solution and is used for months without any pretreatment. The advantage of this electrode is that it causes less contamination than conventional electrodes, which have a relatively large surface area and a porous separator that may adsorb and desorb impurities. Instrumentation. A Radiometer polarographic analyzer, Pol 150, a polarographic stand, MDE 150, and a 10-mL cell are used in a two-electrode configuration. Reagents and Solutions. All solutions are prepared in Type 1 reagent grade water. A stock solution of 1.00 mM Pb(NO3)2 in 10 mM HNO3 is calibrated with EDTA by titrating. A 0.1 mM Pb(NO3)2 in 10 mM HNO3 is used for daily preparation of the standard solutions. HNO3 65% (Merck, p.a.), NaCl (Merck, Suprapur), and KCl (Merck, Suprapur) are used for preparations of samples. Prior to use, plastic- (polyethylene and Teflon) and glassware were cleaned by immersion in a 1:1 aqueous solution of HNO3, followed by copious rinsing in deionized water. Anodic Stripping Procedure. The analysis is performed without removal of oxygen in a 5.00-mL solution, containing 10 mM HNO3 and 10 mM KCl. The conditions for the preconcentration step are Ed ) -0.7 V vs AgQRE; 15 e td e 120 s; 400 e N e 10000 rpm. After a rest period of 10 s (interruption of stirring), a square wave (SW) voltammogram is recorded. The peak signal is independent of the value of the deposition potential in the range from -750 to -600 mV vs AgQRE. Conditions for the SW mode are as follows: pulse amplitude, 25 mV; step amplitude, 5 mV; step duration, 40 ms; Ein ) -0.7 V; Ef ) -0.05 V vs AgQRE. Evaluation of Results. Three modes for quantifying the analytical signal are tested using Radiometer software, TM5: (a) the stripping peak current, (b) the stripping peak area, and (c) the average slope at the two inflection points of the peak. The mode using the slope yielded the best linear response vs concentration of analyte and is, therefore, adapted for the analytical determinations. The peak current is used for semiquantitative estimations. The quantitative determinations are performed using the standard additions method. RESULTS AND DISCUSSION Comparison between the Ag Disk Electrode and the Hanging Mercury Drop Electrode. The working potential window of the silver electrode is narrower than that of the hanging mercury drop electrode (HMDE) due to low overpotential of hydrogen evolution. The square-wave anodic stripping voltammetry (SWASV) peak of Pb2+ at the silver electrode is well-defined (Figure 1). It is nearly 8 times higher than that obtained at the HMDE. The sensitivity is enhanced since the deposit is retained on the surface of the solid electrode and all of it is available during the stripping step. In the case of the HMDE, diffusion into the mercury electrode decreases the amount of deposit available for oxidation during the stripping step. Peak Potentials. At the mercury electrode, the peak coincides, as expected, with the standard potential of the Pb2+/Pb(Hg) couple (both reduced and oxidized forms are soluble either in the solution or in the mercury, and the electrode process is
Figure 1. SWASV of Pb2+ at the Ag electrode and at the HMDE with and without removal of oxygen. Solution: 42 nM Pb2+, 0.01 M HNO3, and 0.01 M KCl. Conditions of electrodeposition: Ed, -0.7 V; td, 120 s; magnetic stirring of solution. SW mode: pulse amplitude, 25 mV; step amplitude, 5 mV; step duration, 0.04 s.
reversible). At the silver electrode, where the reduced form is insoluble, the peak potential should have appeared in the vicinity of the reversible potential of the Pb2+/Pb couple, were it a result of bulk deposition. According to this, the peak at the silver electrode under the conditions in Figure 1 ([Pb2+] ) 36.8 nM and Erev,Pb2+/Pb - E0Pb2+/Pb ) -223 mV) would be expected to be shifted by 223 mV more negative with respect to the peak at the mercury electrode. The peak potential at the silver electrode coincides, however, with the standard potential and not with the reversible one. This is attributed to underpotential deposition and will be discussed below. Characterization of the Lead Deposit on the Silver Electrode. Estimate of the Amount of Deposited Lead. The deposited amount of lead from nanomolar Pb2+ concentrations can be estimated from the deposited charge or from the stripped charge. The deposited charge is calculated using the Levich equation, and the stripped charge is determined by performing the stripping step with linear scan voltammetry. Linear scan is preferred to SW because the integral of the peak is the value of the stripped charge, while in the case of SW, it only relates to it. The charge for the stripping peak resulting from a 1-min electrodeposition at 2100 rpm of 107 nM Pb2+ is 8.2 µC/cm2 ( 10% (cf. Figure 2). This charge is independent of sweep rate in the range of 200-1000 mV/s. The deposited charge, 8.6 µC/ cm2, is calculated from the mass transfer-controlled current of Pb2+ at the silver rotating disk electrode, 144 nA/cm2, using the Levich equation (with DPb2+ ) 6.6 × 10-6 cm2) and the time of electrolysis. The stripped charge compares favorably with the deposited charge. One monolayer of a compact deposit of Pb corresponds to roughly 250 µC/cm2 (p 132 in ref 6). Thus, assuming an Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
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Figure 2. Estimation of the coverage of Pb formed during the electrodeposition step of ASV at the silver rotating disk electrode. Solution: 107 nM Pb2+, 0.01 M HNO3, and 0.01 M KCl, oxygen removed. Ed, -0.7 V; td, 60 s; rotating rate, 1200 rpm. Conditions for the stripping step: linear scan at 500 mV/s.
Figure 3. Cyclic voltammogram of Pb2+ on the Ag electrode. Solution: 0.35 mM Pb2+, 0.01 M HNO3, and 0.01 M KCl, oxygen removed. Scan rate, 10 mV/s. Arrows indicate the direction of the potential scan.
atomically flat electrode surface, the amount of uniformly distributed deposit of 8.6 µC/cm2 corresponds to about 3% of a monolayer. Taking into account a roughness of 2.4 for the electrode surface (cf. next paragraph), the coverage obtained by 1-min electrodeposition from 107 nM Pb2+ is estimated to be about 1% only. The surface coverage in the concentration range of interest in this work (2-200 nM) is 0.02%-2%. The deposit constitutes a uniformly deposited submonolayer obtained at underpotential electrodeposition (discussed below). Underpotential Deposition of Pb2+ on Ag. Cyclic voltammetry in millimolar concentrations of Pb2+ yields two pairs of peaks (cf. Figure 3). The plot has the features of underpotential and bulk deposition (p 141 in ref 6). A monolayer of a metal can be deposited and stripped from an inert metal substrate at positive potentials with respect to the reversible Nernst potential. This underpotential deposition is a well-studied phenomenon.5-8 Underpotential deposition results from a strong interaction between the atoms of the metal being deposited and the substrate. The more positive pair in Figure 3 corresponds to underpotential deposition and stripping at -481 and -448 mV vs AgQRE, respectively. The second pair of peaks corresponds to bulk deposition and bulk stripping at -620 and -560 mV, respectively. The underpotential shift, ∆Ep, and the half-width of the monolayer stripping peak, δ1/2,UPD, are somewhat smaller than those reported 4662 Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
by Kolb et al. in 1 M NaClO4.7 The difference may be attributed to the specific adsorption of chloride ions in our samples ([Cl-] ) 10 mM), which are assumed to reduce the bond strength between metal adatom and the substrate.7 The UPD stripping peak obtained under the experimental conditions shown in Figure 3, at Pb2+ concentrations of 5 mM and higher, corresponds to a charge of 600 µC/cm2. This limiting value for the charge in conjunction with the commonly used value of ∼250 µC/cm2 for a compact monolayer of Pb provides an estimate of 2.4 for the roughness factor of the silver electrode surface. From the above experiment, it is concluded that the SWASV peak obtained at the silver electrode in trace concentrations of lead (Figure 1) corresponds to underpotential deposition of lead. The SWASV peak potential in trace concentrations is more positive by ∼70 mV than the calculated value from the peak potential obtained from underpotential deposition experiments at millimolar concentrations (as calculated on the basis of a shift 30 mV/decade of [Pb2+]/coverage, p 153 in ref 6). Such broad-range extrapolation (over more than 4 orders of concentration) is, however, not sufficiently reliable, and more work on the subject is needed. It should be mentioned that, if the SWASV peak potential in trace concentrations would be associated with bulk deposition, the peak potential should be a few hundred millivolts more negative than the experimental Ep value. Analytical Characteristics of SWASV of Pb2+ in Synthetic Solutions. Proper functioning of the electrode is defined by three parameters: (a) The width of the peak at half-height, 46 ( 2 mV. (b) The reproducibility, expressed as the standard deviation of subsequent runs performed in the same solution. For the 2-300 nM concentration range of Pb2+, the reproducibility is below 2%. (c) The correlation coefficient of a calibration curve, at least 0.999. The detection limit for a 2-min electrodeposition at 5000 rpm is ∼0.5 nM. This value is determined as 3 times the standard deviation of the current at the background in the potential region of the peak. There is evidence, based on the stability and reproducibility of the background, that the detection limit could be reduced by a factor of 10 by prolonging the time of electrolysis and the speed of rotation. Work in the subnanomolar concentration range is in progress. Short- and Long-Term Stability of the Ag Disk Electrode. For short periods (the time required to record a 10-point calibration curve, or to perform a standard additions determination), the stability of the electrode is excellent. Electrodes used in synthetic solutions containing up to 0.1 mM Cu2+, 10 nM Cd2+, 200 nM Pb2+, and 0.5 M NaCl do not require any pretreatment for over 600 measurements. The shape and the reproducibility of the lead stripping peak remains unchanged. Over long periods, the sensitivity of the signal fluctuates by about 10%. Exposure of the electrodes to untreated drinking water may drastically affect its behavior, distorting the shape of the current/ potential plots and reducing the stability and the sensitivity of the signal. In such cases, the proper functioning of the electrode is restored by polishing, as described in the experimental part. Analytical Signal-Concentration Relationship. Stripping curves of lead are presented in Figure 4. Under proper experimental conditions, linearity between the analytical signal and concentration is observed in a relatively large concentration range (2-300 nM) (cf. Figure 5). The range of linearity depends on the time
The sensitivity, SPb, (cf. eq 1) on a freshly polished electrode slope is (a) with respect to slope measurements, SPb ) (1 ( 0.1) × 10-2 nA mV-1 nM-1 cm-2 s-1 rpm-1/2, and (b) with respect to ip peak current measurements, SPb ) 0.37 ( 0.04 nA nM-1 cm-2 s-1 rpm-1/2.
SPb )
Figure 4. SWASV of Pb2+ on the Ag rotating disk electrode without removal of oxygen in 0.01 M HNO3, 0.01 M KCl. Concentration of Pb2+ from bottom to top: 0, 1.96, 3.85, 5.66, 7.41, 9.09, and 10.7 nM. Ed, -0.7 V; td, 120 s; rotating rate, 4900 rpm. The respective calibration plot is included in the inset.
Figure 5. Calibration plots of SWASV of Pb2+ on the Ag rotating disk electrode without removal of oxygen in 0.01 M HNO3, 0.01 M KCl. Ed, -0.7 V; rotating rate, 2100 rpm; time of electrolysis marked on plots.
of electrolysis and the rate of rotation. Deviation from linearity, as observed from plots of signal vs (i) CPb2+, (ii) time of electrolysis, and (iii) rotation rate, occurs at a constant value of the product, CPb2+N1/2td, equal to 3.0 × 105 nM rpm1/2 s. This product, according to the Levich equation, is directly related to the amount of lead deposited per unit area. It can be seen from Figure 5 that, while the concentration of Pb2+ at the point of the break changes by a factor of 4, the product CPb2+N1/2td fluctuates within only 10%. The lead coverage of the electrode at the point of the break corresponds to about 1% of a monolayer (the roughness factor of the electrode, 2.6, taken into account). It might be speculated that, beyond the point of the break, the surface conditions are such that the peak widens progressively, causing a respective decrease in peak current and peak slope. If this were correct, then the calibration plot of charge vs [Pb2+] would remain linear. A test to verify this assumption cannot be performed with lead, since charge measurements are not accurate due to the tailing of the stripping peak. This subject is currently under investigation with Tl+. The product CPb2+N1/2td at the point of the break is useful for estimating the upper concentration limit at given N and td, for which the signal is linear with concentration. The quality of the calibration curve is excellent. The correlation coefficient is at least 0.999.
analytical response AAgCPb2+telectN1/2
(1)
Analytical Signal-Deposition Time and Speed of Rotation Relationships. Linear relationships between analytical signal and (i) time of deposition and (ii) speed of rotation of the silver electrode are observed for the tested range of time 15-240 s and of rotation 400-10000 rpm, for conditions under which the product CPb2+tdN1/2 e 3.0 × 105 nM rpm1/2 s. The correlation coefficients of both relationships are at least 0.999. Interferences. Effect of Dissolved Oxygen. On both silver and mercury electrodes, the dissolved oxygen is reduced in two waves. At the first wave, O2 is reduced to H2O2, and at the second, H2O2 is reduced to H2O. The reduction waves obtained at the silver electrode at a slow linear sweep in a stirred solution are ill-defined. The half-wave potentials correspond to -0.44 and -0.60 V vs AgQRE (supporting electrolyte, 10 mM HNO3 and 10 mM KCl). The respective potentials at mercury in the same medium are -0.08 and -1.0 V. The effect of dissolved oxygen on the stripping voltammograms of Pb at HMDE and at Ag are shown in Figure 1. At the silver electrode, there is no change in the form of the front part of the peak and in the peak height, but the tail is distorted. That, however, is of no importance, since the analytical signal is determined from the slope of the peak. At the HMDE, the peak current decreases by 13%, and both the foot and the tail of the peak are somewhat distorted. During the electrodeposition step (at potential -0.7 V vs AgQRE), oxygen is reduced to H2O at the silver electrode and to H2O2 at the mercury electrode. The concentrations of O2 and of H2O2 formed during the stripping step in the diffusion layer are negligible at the silver electrode. At the HMDE, the concentration of H2O2 is about the same as that of oxygen in the bulk of the solution, i.e., 0.25 mM. Thus, during the square-wave stripping step at the mercury electrode, the deposit is partially oxidized by the H2O2.9 Thus, the removal of oxygen affects the stripping peak of Pb at the HMDE. It is possible to determine lead by anodic stripping voltammetry at a silver electrode in the presence of dissolved oxygen, since (i) oxygen is depleted at the electrode during the preconcentration step and (ii) oxygen from the bulk of the solution does not reach the electrode during the short duration of the square-wave stripping step.9 SWASV of Cd2+. Cd2+ displays an ill-defined SWASV at the silver electrode (Figure 6). The peak is wider than that of Pb, and, more surprisingly, its area is about one-fourth that of Pb at equimolar concentrations. (At equimolar concentrations, the amounts of deposit of the two elements should be similarsthere is a marginal difference in their diffusion coefficientssand one would expect, therefore, similar areas of the dissolution peaks.) There is a linear relationship between peak signal and concentra(9) Wojciechowski, M.; Go, W.; Osteryoung, J. Anal. Chem. 1985, 57, 155158.
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Figure 6. SWASV of Pb2+ on the Ag rotating disk electrode in the presence of 80 nM Cd2+ without removal of oxygen in 0.01 M HNO3, 0.01 M KCl. Concentration of Pb2+ from bottom to top: 0, 19.6, 38.5, 56.6, 74.1, 90.9, and 107 nM. Ed, -0.7 V; td, 60 s; rotating rate, 2100 rpm. Table 1. Determination of Pb2+ at the Ag Electrodea Pb, nM
a
additional comments
amt taken
amt found
0.5 M NaCl 8 nM Cd2+, Ed ) -0.7 8 nM Cd2+, Ed ) -0.57 0.2 mM Cu2+
1.96 19.6 19.6 18.9 18.9 19.6
2.05 19.3 19.1 17.0 17.7 20.1
All solutions contain 10 mM HNO3 and 10 mM KCl.
tion of Cd2+ in the tested range of 20-100 nM. The sensitivity of -3 and the signal with respect to Cd2+ is Sslope Cd ) (2.5 ( 0.2) × 10 ip -2 2+ SCd ) (10 ( 1) × 10 . Equimolar concentrations of Cd do not interfere with the determination of Pb2+. The proper functioning and the stability of the silver electrode are unaffected by the cadmium deposit. Data on determination of Pb2+ in synthetic solutions containing an excess of Cd2+ are shown in Table 1. Effect of Cu2+. Peak location, peak sensitivity, and peak width of lead are unaffected by the presence of a 10 000-fold excess of Cu2+. The precision of the determination of Pb2+ in synthetic solutions containing a large excess of Cu2+ is the same as that in its absence (cf., Table 1). The stripping peak of Cu2+ at the Ag electrode is displayed at about 0 V vs AgQRE. The lack of interference by a large excess of Cu2+ (×104) in the determination of nanomolar concentrations of Pb2+ will be dealt with in future publication. (10) Achterberg, E. P.; van den Berg, C. M. G. Anal. Chim. Acta 1994, 291, 213-232. (11) Kubiak W.; Wang, J. J. Electroanal Chem. 1989, 258, 41-48. (12) Economou A.; Fielden, P. R. Analyst 1993, 118, 1399-1404.
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Effect of Chlorides. SWASV of Pb2+ at the Ag rotating disk electrode was recorded for different concentrations of chlorides. Small concentrations of chlorides (∼10 mM) improve the shape of the stripping peak of lead. It renders it sharper. This has been ascribed to a decrease in the bond strength Pb-Ag, due to a modified silver surface with specifically adsorbed chlorides.7 As the concentration of chlorides increases, the following is observed: the peaks remain well-defined; the peak is shifted to more negative potentials; the peak width increases; the sensitivity, slope SPb , drops by a factor of about 2, in the transition from 10 to 100 mM, and by a factor of almost 3, in the transition from 10 mM to 0.5 M. The Effect of Surfactants Present in Drinking Water. In samples of tap water, the first SWASV run of Pb2+ has usually a normal shape, but the following runs are sometimes heavily distorted, depending on the source of water. The electrode surface is probably affected by surfactants present in tap water. The proper functioning of the electrode is restored by polishing (cf. experimental part). It is of interest to note that, under identical conditions, the SWASV signal at the HMDE is not distorted. This is due to the fact that each run is performed on a fresh drop. In order to analyze tap water using the silver electrode, several approaches were tested: UV digestion pretreatment,10 performed in Prof. van den Berg’s laboratory, and wet digestion with nitric acid. Both made possible the determination of lead in tap water. Addition of fumed silica in aqueous11 or ethanolic suspensions12 failed to improve the electrode behavior. More work is required in order to provide a general procedure for analysis of lead in water samples using the silver electrode. Determination of Pb2+ in Synthetic Solutions. Pb2+ is determined in synthetic solutions using the standard additions method (three additions). The determinations are performed using -0.7 V for the deposition step. In the presence of Cd2+, the proximity of the two peaks impairs the accuracy of the determination. This problem is easily solved by changing Ed to -0.57 V. The results are summarized in Table 1. ACKNOWLEDGMENT Fruitful discussions with Prof. E. Gileadi, Dr. J. Penciner, and Dr. A. Vaskevich are acknowledged. Special thanks to Prof. C. M. G. van den Berg for the opportunity to carry out experiments in his laboratory. Thanks to our friends B. Agam and N. Lavie for their important contribution in glass and mechanical craftsmanship.
Received for review April 23, 1997. Accepted August 13, 1997.X AC970420F X
Abstract published in Advance ACS Abstracts, October 1, 1997.
