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Anal. Chem. 1984, 56, 2379-2382
Improved Resolution in Stripping Analysis Using the Formation of Intermetallic Compounds Joseph Wang,* Percio A. M. Farias: and Den-Bai Luoa Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
A new approach for Improving the selectivlty In stripping analysls, based on the preferentlal formatlon of an Intermetallic compound, Is described. For thls purpose, a third element is added to the sample solutlon to mask, In the mercury electrode, one of the metals in a mixture. Conditions that favor the formatlon of lntermetaillc compounds, Le., hlgh amalgam concentratlon, are explored and used. The method Is evaluated tor a number of dmerent mlxtures, wlth dlfferent degrees of peak overlap. For example, the addition of copper allows the selective measurement of lead or thaillum in the presence of Indium or cacbnlum. Different concentratlonratlos of the mlxture components can be tolerated dependlng upon the peak potential separatlon, from 90-fold excess of the Interferlng metal for A€, of 140 mV to a 1:l ratlo for A€, of 15 mV. The procedure Is simple and rapid. I n addltlon, the data provlde new inslghts into the formation of lntermetaillc compounds.
Stripping analysis is a particularly useful technique for measuring heavy metals in environmental, clinical, and industrial samples. A principal difficulty in its analytical utility is the problem of resolution of overlapping peaks. Within a relatively narrow potential window of about 1.5 V more than 15 metals may yield stripping (oxidation) peaks. Thus it is difficult to resolve metals which have stripping peak potentials in close proximity to one another. The resolution between two adjacent peaks depends on the peak potentials, peak widths, and concentrations. Chemical (1-6), mathematical (7-9),and instrumental (10-12)approaches have been used to minimize the problem of overlapping peaks in stripping analysis. The chemical approaches, which are the most commonly used, involve mainly a change in the supporting electrolyte to alter the peak potentials (1-3). This can be accomplished also using the medium-exchange method, Le., by performing the stripping step in a solution which has a more ideal composition from that of the sample (2,3). A second chemical approach is to mask one of the metal ions by chelation. For example, thallium has been determined in the presence of large amounts of lead following the addition of EDTA (4). It is possible also to add surfactant to suppress or shift one of the stripping peaks (5, 6). Usually, the closeness of the peaks limits the relative concentrations that each approach can tolerate. This paper presents a new chemical approach to improve the resolution in stripping analysis based on the preferential formation of intermetallic compounds. The formation of intermetallic compounds between metals deposited in the mercury electrode is usually regarded an inherent difficulty in stripping analysis (13-19). Such intermetallic interaction usually causes one of the stripping peaks to be depressed by Present address: De artment of Chemistry, Pontificia Universidade Catolica de Rio Janeiro, Rio de Janeiro, Brazil. Present address: Department of Chemistry, South-CentralInstitue for National Minorities, Wuan, People’s Republic of China.
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0003-2700/84/0356-2379$01.50/0
comparison with the peak height obtained in the absence of the second metal. A shift in the stripping peaks for the constituent metals, or a new peak, may also be observed. The formation of such compounds is different for different binary systems based on formation constant and solubility product considerations (14,18). Frequently noted examples are the interactions of copper and zinc, copper and cadmium, or zinc and nickel that result in depressions of the zinc or cadmium stripping peaks (14, 19). Under various experimental conditions, complete or nearly complete peak depressions have been reported (14, 17, 18). This work demonstrates that the formation of stable intermetallic compounds can be exploited for improving the resolution between two adjacent stripping peaks. For this purpose a third element is added to the sample mixture to mask-in the mercury electrode-one of the metals. This results in a useful means to suppress the electrochemical activity of the interfering metal. (The addition of a third element has been suggested to alleviate the intermetallic problem, e.g., gallium that preferentially combines with copper so that zinc may be determined without interference (19).) The resulting approach to improve the selectivity is simple and rapid and does not require extra manipulative steps or computerized instrumentation.
EXPERIMENTAL SECTION Apparatus and Reagents. Measurements were made in a 110-mL Pyrex glass cell, containing 100 mL of solution. The working electrode was a mercury film deposited on a 0.75 cm diameter glassy carbon disk (Model DDI 15, Pine Instruments Co., Grove City, PA); the electrode was mounted on a Pine Instruments Model PIR rotating disk assembly. An Ag/AgCl (3 M NaCl) reference electrode and a spectroscopic graphite counterelectrode completed the three-electrode system. Currentvoltage data were recorded with a Princeton Applied Research Model 174 polarographic analyzer and a Houston Omniscribe strip-chart recorder. All solutions were prepared from deionized water and analytical grade chemicals. Stock solutions M) of metal ions were prepared by dissolving the pure metal or its salt in nitric acid and diluting to volume with water. The 1X M mercury plating solution was prepared by dissolving mercury(I1)nitrate. The stock solutions were stored in polyethylene bottles and diluted as required for standard addition purposes. All samples were prepared in 0.1 M acetate buffer (pH 4.4). Procedure. The glassy carbon electrode was polished with 0.05-pm alumina slurry until a mirrorlike finish was obtained. The stripping data were obtained by coplating the mercury film and the trace metale on the working electrode in the following manner. The mercury(I1) solution (1mL) was diluted to 100 mL with the supporting electrolyte and deaerated by purging with nitrogen. During deaeration the working electrode was kept at 0.0 V. The nitrogen delivery tube was then raised above the solution, and a potential of -1.0 V was applied at the working electrode while it was rotated at 1600 rpm. After 10 min, the potential was switched to 0.0 V and held there for 2 min. Following this conditioning, the electrode was ready for use in the measurement cycles. Measurements were performed by applying the deposition potential for a selected time suitable for the concentration levels concerned. The rotation was then stopped, and after 15 s the @ 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
metals were stripped from the mercury film by applying a differential pulse anodic potential ramp. The scan was terminated at +0.1 V and after 90 s the system was ready for the next deposition-stripping cycle. The mercury film was removed at the end of the day by wiping the electrode face with a soft tissue wetted with 1 M nitric acid.
RESULTS AND DISCUSSION Response Optimization Cadmium, indium, thallium, lead, and tin yield stripping peaks within a narrow (-0.3 V) potential window. Accordingly, stripping analysis of mixtures containing some of these metals (e.g., environmental or clinical samples) suffers from major resolution problems. Hence, in the present work we attempt to improve the resolution by preferentially suppressing the stripping peak currents of some of these metals. This suppression is based on spiking the sample with a third element that forms a binary intermetallic compound with one of the interfering metals. Under these conditions, the bound metal in the amalgam does not contribute to the stripping response. In order to achieve complete or nearly complete masking we have used conditions that faoor the formation of intermetallic compounds, namely high amalgam concentration. The mercury film electrode is far more suitable for this purpose than the hanging mercury drop electrode; the concentration of metals in the film is usually 3 to 4 orders of magnitude higher than the concentrations in the drop. For the rotating mercury film disk electrode (used in this study), the amalgam concentration can be estimated from Faraday's law idtd
c,
=nFV In eq 1,id is the deposition current, td is the deposition time,
V is the volume of the mercury electrode, and n and F have their usual electrochemical significance. By substitution of the Levich equation for the deposition (limiting) current at the rotating disk electrode, the following expression is obtained for the amalgam concentration:
c,
=
0.62D2/3~-1/6Cb,'/2td 1
(2)
where D is the diffusion coefficient, v is the kinematic viscosity, Cb is the concentration of the metal in the bulk solution, w is the electrode rotation speed (during the deposition), and 1 is the film thickness. Heineman and co-workers (14) showed that severe peak depressions, due to the formation of intermetallic compounds, are observed at the in situ plated mercury film electrode, with low mercury(I1) concentration and long deposition times. Sufficient excess of the third element is another requirement for successful intermetallic masking. Copper has been chosen in the present study as the third element because it is known to form stable intermetallic compounds with various metals (13-19). Acetate buffer is employed as the medium as it is widely used in many practical applications. A detailed knowledge of the changes in stripping peak heights in the presence of the third element allows efficient utilization of the intermetallic masking approach. Figure 1 shows the dependence of the stripping peak currents for cadmium, indium, thallium, and lead on the copper(I1) concentration for a 4-min deposition. Obviously, the effect of copper on the stripping peak currents of these metals differs considerably. As the copper concentration increases, resulting in a larger copper concentration in the mercury film, the cadmium and indium peaks decrease substantially (88% and 78% suppressions at 5 X M copper up to 91% and 96% suppressions a t 1.7 X lo4 M copper). Slight suppressions of the lead and thallium peaks-up to 8% and 21% ,respectively, at 1.7 X lo4 M copper-are also observed. The different suppressions of the stripping peak currents are attributed to
4x100 '0
6ol\ 20.
-
b a
ku+21 ,pM Flgure 1. Effect of changing copper(I1)concentration on the indium (a), cadmium (b), thallium (c), and lead (d) stripping peak currents: deposition step, 4 min at -1.0 V and 1600 rpm; scan rate, 5 mV/s; amplitude, 50 mV; pulse repetition, 1 s; concentration of each metal, 1 X lo-' M acetate buffer (pH 4).
differences in the stabilities of the various intermetallic compounds formed in the mercury. Formation of Cu-Cd intermetallic compound has been frequently noted (14, 16) and attributed to exceeded solubility of copper in mercury. As a result, binary intermetallic compounds are formed when cadmium and copper are plated on the undissolved copper atoms. The Cu-In and Cu-TI intermetallic compounds have not been as widely reported. Neiman et al. (18) reported substantial suppression of the indium stripping peak with increasing copper concentration. According to these authors, the effect of copper on the metal peak current decreases in the order Ga > Sn > Zn r In > Cd > Pb, which is in agreement for the metals tested in Figure 1. The data of Figure 1indicate that the optimum copper concentration for measuring lead and/or thallium in the presence of cadmium and/or indium is 1X lo4 M. Such copper level results in 92%, 91%, 17%, and 5% suppressions of the indium, cadmium, thallium, and lead peaks, respectively. Thus, under these conditions the maximum response of the metals of interest (lead or thallium) is not achieved; however, this does not affect their quantitation, as will be described later in the paper. Similar effects are observed with other chemical approaches for minimizing the overlapping peaks problem (change of supporting electrolyte or addition of masking ligand) where the analyst does not have complete freedom to shift or mask the interfering peak without affecting the peak of interest (3,4). The effect of the deposition time on the cadmium stripping peak current was examined at various copper(I1) concentrations (not shown). The deposition time is an important factor in the degree of suppression of the cadmium stripping peak. As the deposition time is increased, Le., higher amalgam concentration (eq 2), the cadmium peak current is decreased. At copper concentration of 1X lo4 M, peak current reductions of 70%, 93%, and 95% were observed for 1, 4, and 6 min deposition times, respectively. Maximum cadmium masking (97%) was achieved by coupling 6 min deposition with 1.5 X M copper concentration. A 4-min deposition was used throughout the rest of this study as a compromise between effective masking and rapid measurement. Similarly, eq 2 predicts the achievement of higher amalgam concentration, Le., effective masking, using higher rotation speeds during the deposition step. Rotation speeds of 400,900, and 1600 rpm have resulted in 67%, 80%, and 90%, respectively, depressions of the 1 X lo-' M indium peaks in the presence of 1 X M copper as compared to the peaks in the absence of copper. At 1.25 X M copper these depressions have increased to 70%, 85%, and 94% for the 400,900, and 1600 rpm speeds (other conditions, as in Figure 1). Obviously, as the peak depression at the in situ plated mercury film electrode depends on the film thickness (eq 2) different masking profiles would be obtained with different mercury(I1) concentrations. Mercury(I1) concentrations ranging from 1to 5 X M are
ANALYTICAL CHEMISTRY, VOL. 56,NO. 13,NOVEMBER 1984
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i
i
-0.8
Figure 2. Dlfferentialpulse anodic strlpplng voltammograms for (a) 4 X M indium, M lead, (b) same as (a) but after adding 1 X (c) same as (b) but after addlng 1 X lo-' M copper. Conditions are given in Figure 1.
usually used for plating the mercury film. Herein we used a 1X lo6 M mercury level to enhance the masking effect. An adjustment of the procedure parameters-copper concentration, deposition time, etc.-will be required when using different electrochemical systems, possessing different characteristics (e.g., cell volume, mass transport). Analytical Applications. To experimentally examine the intermetallic masking approach, different binary systems with different degrees of peak overlap were studied. Figure 2 illustrates the measurement of 4 X lo4 M lead in the presence of 1X lo-' M indium. The difference in the peak potentials for these two species is approximately 140 mV. Conventional stripping measurements cannot tolerate the 25-fold excess of indium (peak ratio 1lA) and the two peaks are poorly resolved (curve b). In contrast, after addition of copper (curve c) a 92% depression of the indium peak is observed, as expected from Figure 1. As a result, the two peaks are well resolved and suffer virtually no overlap at any potential and the lead peak is measurable. No change in the lead peak is observed compared to the response before adding the indium and copper (curve a). In similar experiments, 2.8 X lo4 M lead was measured in the presence of a 90-fold excess of indium, and 5X M lead was measured in the presence of a 20-fold excess of cadmium (Up = 160 mV). The nearly complete masking of the metal present in large exceM permits adequate change in the current scale, Le., more accurate measurement of the sought-for peak. A larger degree of overlap is observed when the differential pulse anodic stripping measurements are attempted at faster scan rates desired for shortening the analytical procedure. This is due to the peak broadening associated with faster scan rates. For example, with a scan rate of 10 mV/s, the 2 X M lead peak could not be measured accurately in the presence of 1X lo-' M indium (not shown). However, a convenient quantitation was possible after spiking the solution with 1 X M copper. For the cadmium-thallium mixture the peak potentials separation is only 15 mV. This, and the broad thallium peak (one-electronprocess), results in severe overlap, as indicated from the combined peak shown in Figure 3b. Quantitative measurement of the thallium in such closely spaced 1:l mixtures is virtually impossible. The intermetallic masking approach can be effective for such mixtures (for which advanced computational approach have not been successful (7,
-0.6
L
-0.4
E .V Figure 3. Differentlal pulse anodic stripping voltammograms for (a) 1 X M cadM thallium, (b) same as (a) but after adding 1 X mium, (c) same as (b) but after adding 1 X lo-' M copper. Condltions are given In Flgure 1
i
1
-1.0
-0.8
-0.b
I
-0.4
E,V
Figure 4. Quantitatlon of thallium in the presence of cadmium: (a) 1 X lo-' M thallium and 7.5 X 10" M cadmium; (b) same as (a)but after adding 1 X lo-' M copper: (c)-(e) same as (b) but after successive M thallium. Conditions are given concentratlon increments of 1 X in Figure 1. The small peak at -0.52 V Is due to lead Ions present in the blank solution.
9)). The thallium peak, observed in the presence of cadmium and copper (Figure 3c) is 12% lower than that observed in the absence of these species (Figure 3a). This reduction is the net effect of the 17% reduction expected by the formation of the Cu-T1 intermetallic compound and the incomplete masking of the cadmium (see Figure 1for both effects). While the depression due to the Cu-T1 does not affect the thallium
2382
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 X 10"' M copper; conditions, as in Figure 1). The mean lead peak found was 23.2 pA with a range of 22.5-23.6 pA and a relative standard deviation of 2.1% (conditions, as in Figure 1). It is clear that the suppression of the interfering metal and the presence of excess of copper do not affect the reproducibility of the measurement.
l
CONCLUSIONS e
d
C
Flgure 5. Quantltatlon of lead in the presence of indium: (a) 5 X 10" M lead and 3 X lo-' M indium: (b) same as (a) but after adding 1 X lo4 M copper: (c)-(e) same as (b) but after successive concentration increments of 5 X IO-' M lead. Conditions are given In Figure 1.
quantitation (as will be shown later), the incomplete cadmium masking places rwtrictions on the cadmium concentration that can be tolerated. For example, a 10% error is estimated for the cadmium:thallium ratio of 1:l used in Figure 3. Similar errors have been reported for larger peak potentials separations (25-40 mV) using computational approaches such as multilinear least-squares regression (9) or semiderivative voltammetry (7).For such closely spaced systems, the error decreases as the species determined is in a concentration excess; an excess of the interfering metal cannot be tolerated. Quantitative evaluation of the species of interest can be obtained by using the standard additions method. Figure 4 shows the measurement of 1X lo-"M thallium in the presence of 7.5 X M cadmium (before (a) and after (b) adding copper). The four measurements shown in curves b-e are part of six concentration incrementa from 1 X lo-' to 6 X lo-"M thallium. Linearity between the stripping peak current and the thallium concentration is obtained. Least-squares treatment of these data yielded the equation i(pA) = (7.1 f 0.01)C(10-7 M) - 0.9 f 0.5 pA with r = 0.999 and S, = 0.56. When the peak potential separation is larger, successful quantitation is obtained when the interfering metal is in a considerable concentration excess. Figure 5 shows the quantitation of 5 X 10" M lead in the presence of 3 X lo-" M indium. A series of six concentration increments from 5 X lo4 M to 3 X 10-7 M lead (four of which are shown in curves b-e) yielded the equation i(pA) = (23.5 f 0.6)C(lO-"M) - 1.7 f 1.2 pA with r = 0.999 and S, = 1.26. The precision of the results was estimated by ten successive measurements of 1 X lo-" M lead (in the presence of 3 X lo-" M indium and 1
We have demonstrated that intermetallic compound formation can be exploited for improving the selectivity in stripping analysis. Different concentration ratios of the two metals can be tolerated, depending upon the peak potentials separation, from 90-fold excess of the interfering metal for hEpof 140 mV to 1:l ratio for hEpof 15 mV. The more stable the intermetallic compound between the interfering metal and the "third" metal, the more effective will be the masking. Neiman et al. (18)discussed the choice of the "third" element used for alleviating the problem of intermetallic compounds, based on the position of the metals in the periodic table. Similar considerations are appropriate for the choice of the metal used for improved selectivity by the intermetallic masking approach. Accordingly, improved selectivity may be obtained for other binary systems. For example, as the copper peak can be completely depressed by the addition of tellurium (181, bismuth or antimony may be measured in the presence of copper. Another aspect of intermetallic compounds that can be exploited for selectivity purposes is the shift in peak potential that may be associated with their formation. Work in this laboratory is continuing in this direction. Besides i k utility to the selectivity problem, the above data provide new insights into the formation of intermetallic compounds. Registry No. Cu, 7440-50-8; T1,7440-28-0;Cd, 7440-43-9;Pb, 7439-92-1;In, 7440-74-6; Cu-In, 50940-80-2;Cu-T1, 72048-19-2; Cu-Pb, 12731-48-5;Cu-Cd, 12685-29-9.
LITERATURE CITED (1) Desimoni, E.; Palmisano, F.; Sabbatini, L. Anal. Chem. 1080, 52, 1889. (2) Hu, A.; Dessy, R. E.; Gran6ll. Anal. Chem. 1082, 54, 320. (3) Anderson, L.; Jagner, D.; Josefson, M. Anal. Chem. 1082, 5 4 , 1371. (4) Bonneli, J. E.; Taylor, H. E.; Skogerboe, R. K. Anal. Chlm. Acta 1080, 118, 243. (5) Lukasrewskl, 2.; Pawlak, M. K.; Clzewskl, A. Talanfa 1080, 2 7 , 181. (6) Neeb, R.; Kiehnast, I. Nafurwlssenshaffen 1070, 57, 37. (7) Tomann, J. J.; Brown, S. T. Anal. Chem. 1081, 53, 1497. (8) Liu, J. H. Analyst (London) 1980, 105, 939. (9) Boudreau, P. A.; Perone, S. P. Anal. Chem. 1070, 51, 811. (IO) Florence, T. M. J . Elecfroanal. Chem. 1070, 2 7 , 273. (11) Wang, J.; Dewald, H. D. Anal. Chem. 1083, 55, 933. (12) Stojek, 2.; Kubllk, 2. J . €lecfroanal. Chem. 1070, 105, 247. (13) Kemula, W.; Kubllk, 2. Nature (London) 1058, 182, 1228. (14) Wlse, J. A.; Roston, D. A.; Heineman, W. R. Anal. Chlm. Acta 1083, 154, 95. (15) Shuman, M. S.; Woodward, 0. P. Anal. Chem. 1078, 48, 1979. (16) Ostapczuk, P.; Kubllk, 2. J . Elecfroanal. Chem. 1077, 83, 1. (17) Roston, D. A.; Brooks, E. E.; Heineman, W. R. Anal. Chem. 1070, 51,
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(18) Nleman, E. Y.; Petrova, L. G.; Ignatov, V. I.; Dolgoplova, 0. M. Anal. Chlm. Acfa 1080, 113, 277. (19) Abdullah, M. I.; Reusch Berg, B.; Kllmek, R. Anal. Chlm. Acta 1078, 8 4 , 307.
RECEIVED for review May 3, 1984. Accepted July 16, 1984. P. A. M. Farias acknowledges the financial support of the National Council for Scientific Development (CNPq) of the Brazilian government. D.-B. Luo acknowledges the financial support of the United Nations Educational, Scientific and Cultural Organization (UNESCO). This work was supported in part by the National Institute of Health, Grant No. GM30913-01A1.
