Anal. Chem. 1991, 63, 953-957 (5) Partheserathy, N.; Buffle, J.; Haerdi, W. Anal. Chim. Acta 1977, 93,
121-128.
(6) Shatkay, A. Anal. Chem. 1976, 48, 1039-1050. (7) Mod, W. E.; Simon, W. Ion-Selectlve Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum': New York, 1978; Vol. 1, Chapter 3. (8) Mort W.: Lindner, E.;Simon, W. Anal. Chem. 1975, 47, 1596-1601. (9) Markovic, P. L.; Osburn, J. 0. AIChE J. 1973, 19, 504-510. (10) Lindner, E.; TBth. K.; Pungor, E. Anal. Chem. 1978, 48, 1071-1078. (11) L$dner, E.; TBth, K.; Pungor, E. Anal. Chem. 1982, 54. 72-76. (12) Toth, K.: Pungor, E. Anal. Chim. Acta 1973, 64, 417-421. (13) Tbth, K. Ion-Selectlve Electrodes; Pungor, E.; BuzBs, I., Eds.; Akad6miai Klado: Budapest, 1973;pp 145-164. (14) Davies, C. W.; Jones, A. L. Trans. Faraday SOC. 1955, 51. 812-817. (15) Lindner, E.; Tbth, K.: Pungor, E.; Berube, T. R.; Buck, R. P. Anal. Chem. 1967, 59, 2213-2216. (16) Nash, J. C. Numerlcal Methods for Computers: Linear Algebra and Function Minimization; Halsted Press: New York, 1979;pp 170-178.
853
(17) E. Lindner, Technical University of Budapest, unpublished results, 1989. (18)Berube, T. R.; Buck, R. P.; Lindner, E.; Gratzl, M.; Pungor, E. Anal. Chem. 1989, 61, 453. (19)Mackor, E. L. Red. Trav. Chim. Pays-Bas 1951, 763-783. (20) Reinmuth, W. H. J. Phys. Chem. 1961, 6 5 , 473. (21) Bard, A. J.; Faulkner, L. R. Ektrochemlcal Methods, Fundamentals and Applications; John Wiley and Sons: New York, 1980; p 517. (22)Dencks, A.; Neeb, R. Fresenlus' 2. Anal. Chem. 1979, 297, 121-125. (23)Muller, R. H. Anal. Chem. 1969, 41(12),113A-116A.
RECEIVED for review January 24,1990. Acceptld February 22,1991. Support from the NSF and Hungarian Academy of Sciences is gratefully acknowledged.
Differential Pulse Voltammetric Determination of Cobalt with a Perfluorinated Sulfonated Polymer-2,2-Bipyridyl Modified Carbon Paste Electrode Zhiqiang Gao, Guangqing Wang, Peibiao Li,* a n d Zhofan Zhao
Department of Chemistry, Wuhan University, Hubei 430072, China
A highly selectlve method for the determlnathm of cobalt with a chemkally modifled carbon paste electrode Is proposed. It is based on the chemical reactlvlty of an knmoblllred modlfler, 2,2-blpyrklyl, toward cobalt( I I ) Ion to ylekl the corresponding cation complex ((Bpy)sCo)2+, which Is taken up by another modlfler, perfluorlnated sulfonated polymer (Naflon). Dlfferentlal pulse voltammetry Is employed, and the oxidation of the complex, at 4-0.1 V vs SCE, Is observed. For a 5-mln preconcentration perlod, a llnear callbratlon curve Is obtained for cobalt concentratlons ranglng from 7 X to 1 X mol/L, and the detection llmlt Is 3 X mol/L. A lower detection llmlt can be obtalned for longer preconcentratlon times. For elght preconcentratlon/determlnatlon/renewal cycles, the differentlal pulse voltammetric response could be reproduced wlth 4.3% relatlve standard deviation. Rapld and convenlent acld renewal allows the use of an lndlvldual modifled electrode surface In multiple analytical quantitatlons. Many coexisting metal Ions have llttle or no effect on the determination of cobalt. The procedure was applled to the determination of cobalt for four standard reference materials with relatlve standard devlatlon of 4.0-5.2%.
INTRODUCTION Chemically modified electrodes (CMEs) have received a great deal of attention (1, 2 ) , particularly to enhance the sensitivity and selectivity of electrochemical techniques (3). Generally, such procedures involve the immobilization of the chemically active species on the electrode surface. The target analyte is preconcentrated to the modifier, which has a particular affinity for the analyte. Subsequently, a measurement of the electrochemical response such as the current or charge is made. The magnitude of the analytical signal obtained by electrochemical oxidation or reduction of the accumlated analyte is correlated with the concentration of the analyte in
solution. Conceptually, this approach is analogous to conventional voltammetric stripping method in that a preconcentration step is used to enhance the sensitivity and selectivity. The difference is that the sensitivity and selectivity of the CME are determined by the chemical reactivity of the modifier rather than the oxidation/reduction potential of the analyte. The CME approach is considered more selective than conventional stripping analysis. Reports on the analytical utilization of chemically modified electrodes have included complexation (4),precipitation (5), electrostatic accumulation (6),bioaccumulation (7), potentiometric response (8), covalent attachment (9),and others (10-13). Chemically modified electrodes have been employed to enhance the selectivity or/and prevent electrode fouling in analtyical measurements of organic and biorganic substances such as dopamine (14),ascorbic acid (15),amine (16), and others (17,18). In addition, many modification methods have been used to introduce modifiers onto the electrode surface. Of these, the use of carbon paste electrodes appears especially advantageous, because only the addition of a sparingly soluble modifier to an otherwise conventional graphite powder/Nujol oil mixture is involved. This offers several attractive features to the prospective electrochemical analyst. The chemically modified carbon paste electrodes are exceedingly easy to fabricate and can be readily prepared with a varity of modifier loading levels. Furthermore, the surface of the freshly modified electrode can be generated rapidly and reproducibly in quantity. The low background current is a further advantage for practical analytical applications. To date, a number of studies on the use of chemically modified carbon paste electrodes for analytical determinations have been reported. One of the earliest examples was that of Cheek and Nelson (19), who employed carbon paste electrodes modified with aminosilanes for the determination of silver. Recently, Baldwin and co-workers employed modified carbon paste electrodes for the determination of nickel (4) and copper ( 5 ) with excellent sensitivity and reproducibility. A mi-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
croorganism for bioaccumulation and subsequent voltammetric quantitation of copper (7) was reported by Wang et al. Other studies on the determination of metal ions have also appeared (3). In this paper, we describe a new concept for the preparation of chemically modified carbon paste electrodes. A watersoluble modifier such as 2,2-bipyridyl is immobilized by introducing another modifier, Nafion. 2,2-Bipyridyl is a selective complexing reagent for cobalt(I1) (20). Because the complex formed between cobalt(I1) and 2,2-bipyridyl is a cation, for which Nafion possesses high selectivity (21),the selectivity of the proposed method is shown to be fairly good. EXPERIMENTAL SECTION mol/L) was preReagents. A cobalt(I1) stock solution pared by dissolving a required amount of cobalt(I1) nitrate (analytical grade, Shanghai Chemicals Factory, China) in a 0.01 mol/L sulfuric acid solution. Working solutions were prepared by diluting the stock solution with water just before use. 2,2-Bipyridyl (Bpy) was of certified analytical grade (Beijing Chemicals Factory, China). Nafion solution (5%) was obtained from Du Pont (Wilmington, DE). Graphite powder and Nujol oil used were of reagent grade (Beijing Chemicals Factory, China). Other chemicals were of certified analtyical grade and used without further purification. Double-distilled water from a quartz still was used to prepare all solutions. Electrodes and Apparatus. Unmodified carbon paste was made by carefully mixing 1.0 g of graphite powder and 0.4 g of Nujol oil in a mortar. The modified carbon paste was prepared in the same way except that the graphite was first mixed with the desired weights of 2,2-bipyridyl and Nafion solution. An alcohol slurry of the graphite/modifiers mixture was placed in an ultrasonicator for about 10 min, and uniform disposition of the modifiers was obtained when the alcohol was evaporated away under an IR lamp. Subsequently, the carbon paste was packed into the end of a glass tube (4-mm i.d., 6-mm, o.d.), and a chrome-nickel steel wire was inserted through the opposite end to make electrical contact. The carbon paste surface was polished with a piece of tracing paper until the surface was shiny. The freshly modified electrode surface was subsequently conditioned by exposure to a B-R buffer solution (pH = 5.5) containing 2 X lo4 mol/L cobalt(I1) (60 s), then to a 1mol/L hydrochloric acid solution (5 min), and finally to blank B-R buffer solution (pH = 5.5, 2 min). The conditioning was repeated 3-5 times (see below). A PAR 174A polarographic analyzer with an EG&G PAR Model 0089 X-Y recorder (PARC) was used for cyclic and differential pulse voltammetric measurements. Unless otherwise indicated, the pulse amplitude in differential pulse voltammetric experiments was 50 mV and the potential scan rate was 10 mV/s. A threeelectrode system consisting of a chemically modified carbon paste electrode, a platinum wire auxiliary electrode, and a saturated calomel reference electrode (SCE) was used for all voltammetric measurements. All potentials given in this paper are referred to the SCE. A magnetic stirrer with a 1.5-cm magnet (Taixian Electronic Factory, China) was used for preconcentrations and electrode renewals. Procedure. The preconcentration/medium exchange/voltammetric determination/renewal scheme ( 4 ) was used in all experiments. For the preconcentration step, the modified electrode was immersed in 20 mL of B-R buffer solution containing cobalt(I1) ion (pH = 5.5), which was stirred by a magnetic stirrer (500 rpm) 1.0 cm below the electrode surface. The preconcentration was carried out at open circuit. After a given period of time, the electrode was taken out and rinsed with water thoroughly and was transferred to another cell containing blank B-R buffer solution (pH = 5.5). The initial potential (-0.3 V) was applied for 60 s, the electrode was scanned anodically from -0.3 to +0.7 v, and the peak current was measured in the usual way. After each determination, the electrode was transferred into a 1.0 mol/L hydrochloric acid solution stirred by a magnetic stirrer for several minutes, and then rinsed thoroughly with water and B-R buffer solution (pH = 5.5). The removal of cobalt(I1) from the electrode surface was monitored by a differential pulse voltammeter following the renewal step. There should be no waves in the potential
L
I
I
-0.3 -0.1
0.1
0.3
0.5
I
0.7
E.V(vs.SCE I Flgure 1. Differential pulse vottammograms for cobalt(I I) on different electrodes. (1) Blank carbon paste electrode: folbwing 5 min of stirring in 5 X lo-’ mol/L cobalt(I1)containing 6-R buffer solution (pH = 5.5) and electrolyzed 60 s at -0.3 V (vs SCE) in blank B-R buffer solution (pH = 5.5). (2) Nafion-2,2-bipyridyl modified carbon paste electrode in blank 8-R buffer solution (pH = 5.5). (3) 2,2-Bipyridyl modified carbon paste electrode: conditions as for (1). (4) Nafion-2,P-bipyrklyl modified carbon paste electrode: conditions as for (1). Potentlal scan rate 10 mV s-’, pulse amplitude 50 mV, repetition time 1 s.
scan range: otherwise, the renewal step should be repeated. The electrode can thus be used again.
RESULTS AND DISCUSSION Electrochemical Behavior of Cobalt(I1) on the Modified Electrode. Figure 1 shows differential pulse voltammograms obtained after immersing a blank carbon paste electrode (l),2,2-bipyridyl modified carbon paste electrode (3), and Nafion-2,2-bipyridyl modified carbon paste electrode (4)in a stirred B-R buffer solution (pH = 5.5) containing 5 X mol/L cobalt(I1) for 5 min. These solutions are essentially the same as those recommended for optimum formation of the 2,a-bipyridyl complex with cobalt(I1) in solution (20). Open circuit conditions were employed during the preconcentration step; then the electrodes were rinsed with water and placed in a B-R buffer solution (pH = 5.5). The cobalt(I1) ion taken up from such dilute solutions is clearly shown by the exchange of solutions between the preconcentration and determination steps. The oxidation of the surface-bound cobalt(II)-2,2-bipyridyl complex yields a welldefined differential pulse peak current with a peak potential of +0.1 V (Figure l(4)). This cobalt wave was not observed with the blank (Figure 1(1)),and a very small peak current appeared at +0.1 V with the 2,2-bipyridyl modified electrode (Figure l(3)). Further experimental results show that the carbon paste electrode containing 2,2-bipyridyl is not stable when immersed in an aqueous solution. Upon a few minutes of exposure to water, the initially polished electrode surface becomes noticeably roughened and subsequently exhibits little or no complexing capability. Furthermore, in a relatively high concentration of cobalt(I1) solution mol/L), when the 2,2-bipyridyl modified electrode was immersed, a yellowish color gradually developed; this is undoubtedly due to the loss of these modifiers to the aqueous medium. On the contrary, the Nafion-2,Bbipyridyl modifiers are, however, retained in the carbon paste and remained chemically active in aqueous solution. The cobalt complex on the electrode surface is a cation that can be accumulated by Nafion to form an insoluble complex with a (-SO3)- site in the Nafion-modified electrode surface.
ANALYTICAL CHEMISTRY, VOL. 63, NO.
0.1
0.3
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E,Vlvs SCE I
Flgue 2. Voltammetric response of cobatt(I1) on different electrodes. Scans are run in both anodlc and cathodlc directlons. (1) Cyclic voltammogram of blank carbon paste electrode: conditions as for Flgure l(1). Potential scan rate 100 mV s-l. (2) Cyclic vdtammogram of Nafion-2,2-bipyrldyl modified carbon paste electrode In blank B-R buffer solution. Potential scan rate 100 mV s-'. (3)Cyclic voltammogram of Naflon-2,2-bipyridyl modified carbon paste electrode: condltlons as for Figure l(1). Potential scan rate 100 mV s-'. (4) Dlfferentlal pulse voltammogram of Nafkm-2,2-biwridylmodlfid carbon paste electrode: conditions as for Flgure l(1).
Figure 2 shows cyclic and differential pulse voltammograms for the modified electrode in B-R buffer solution (pH = 5.5) with and without cobalt(I1). As is shown in Figure 2(1), a stable and low residual current is obtained over a wide potential range (-0.3 to +0.7 V) in B-R buffer solutions by using a blank carbon paste electrode. In this potential scan range, both for 2,2-bipyridyl and Nafion, no distinct redox waves are observed (Figures 1(2), 2(2)). These voltammograms are not significantly different from that obtained with a blank electrode in the same solution. The only difference is that the residual current observed with the former is higher. Figure 2(3,4) shows the cyclic and differential pulse voltammetric responses with the modified electrode after preconcentration in cobalt(I1) containing B-R buffer solution (pH = 5.5). A couple of redox waves are obtained with one El/zof ca. 0.05 V and E,, of ca. 90 mV. These waves occur in the same potential region as those observed at the glassy carbon electrode for a Ndi~n-(Bpy),Co)~+ film in weakly acidic medium (24). They are attributed to the oxidation of the accumulated ((B~y),co)~+ film and reduction of the corresponding oxidized form of the complex, ( ( B ~ y ) , c o ) ~ +The . electrode process should be
CO'+
+ 3Bpy
+
+
((BP~),CO)'+
-
( ( B p y ) , c ~ ) ~ + 2(Nafion-S03)2 (Nafion-SO,)-( (Bpy),Co)'+ 2 (Nafion-S03)-( (Bpy),Co) 2+
*e +e-
B(Nafion-SO,)-( (Bpy),Co),+ A particularly attractive advantage of chemically modified electrodes is the convenient renewal of the electrode surface. The effective renewal is coupled with the reproducible preconcentration and the robust electrode surface. Various cleaning solutions for effective renewal of the modified elec-
955
trode surface were evaluated. It was found that renewal of the surface is easily accomplished by immersing the modified electrode in 1.0 M hydrochloric acid solution for a period of time, usually 5-10 min, and the subsequent differential pulse voltammogram should show a very small or no peak current. Otherwise, the renewal step must be repeated. The effective renewal and reproducible preconcentration have been studied in eight experiments-preconcentration/voltammetric determination/renewal cycles during a continuous 120-min period of operation. An average peak current of 5.2 MAwith a relative standard deviation of 4.3% was obtained (conditions as in Figure 1). These experimental results indicate that the method results in a stable modified surface with very low leaching of modifiers to the aqueous medium. The ability to perform multiple analytical measurements with high reproducible results with an individual surface is an important feature of the complexing reagent modified carbon paste electrodes. In fact, a single modified electrode was used for several days, performing several tens of preconcentration/ measurement/renewal cycles without noticeable loss of sensitivity and stability. It was observed that virgin modified electrode surfaces were somewhat less effective for cobalt(I1) accumulation than those that had been previously exposed to cobalt(I1) sample solutions, but after several cycles, highly reproducible differential pulse voltammograms could be obtained. This phenomenon has also been found by other authors (4,5,25). The reason for the sensitivity enhancement during the initial cycles might be that the surface complexing groups of the modifier attains a certain degree of ordering, orienting, and most efficient arranging, which facilitates complexation with the specific analyte of interest. In order to avoid a time-dependent sensitivity in subsequent measurements, virgin modified electrodes require initial conditioning as described in the Experimental Section. Similar procedure were found to be essential to the operation of other complexing reagent modified carbon paste electrode (4,5, 25). Optimum Conditions for Electrochemical Determinations. The effect of the carbon paste composition on the determination of cobalt was tested. The response increases with an increase in the amount of Nafion in the modified electrode. A trend was observed in the differential pulse voltammetric measurement of 5 X 10s mol/L cobalt(I1). (The amount of 2,2-bipyridyl in the carbon paste was lo%.) The nonlinear relationship between Nafion content and voltammetric response may be ascribed to mass-transfer and charge-transfer limitations within Nafion. Because there was considerable change in the background current with higher Nafion loading, it was very difficult to determine trace amounts of cobalt in the sample solutions. Carbon paste with 8-10% Nafion loading was employed in all subsequent experiments. The effect of 2,2-bipyridyl loading in carbon paste of 8% Nafion was also tested. It was found that the largest cobalt(I1)-Bpy complex peak current occurred when "-10% 2,2-bipyridyl (by weight) was contained. The carbon paste electrodes containing either lower or higher 2,2-bipyridyl were generally much less effective in cobalt(I1) accumulation. Solutions employed in every step but especially in the accumulation step, have a profound effect on the electrochemical response. Among various buffer solutions, the B-R buffer solutions were found to be the best. .The optimum pH range is from 5.2 to 6.0. Attempts to preconcentrate in more acidic medium caused a drastic decrease in the response. The use of preconcentration solution more alkaline than pH 9 often resulted in the precipitation of cobalt(I1) hydroxide, and when relatively high cobalt(I1) concentrations were encountered, the preconcentration efficiency was also decreased drastically.
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I -0.5 0
2
4
6
8
IO
12
14nin
Figure 3. Dependence of differential pulse voltammetric response of modified carbon paste electrode on the preconcentration time. (1) 1 X 10" mol/L cobaR(I1); (2)5 X 10-6mol/L cobalt(I1); (3) 2 X mol/L cobalt(I1). Other conditions as for Figure l(1).
Hydrogen ion was not involved directly in either the complex or the ion association. However, owing to the alkalinity of 2,Zbipyridyl competing with cobalt(II), the formation of the complex and its stability decrease and therefore the differential pulse voltammetric peak current decreases. In alkaline medium, because of the hydrolysis of cobalt(I1) and cobalt(11)-Bpy complex, the preconcentration of cobalt(I1) onto the modified electrode surface is thus less effective. The differential pulse voltammetric waveform offered improved characteristics over linear scan voltammograms and was thus used in practical analysis (asshown in Figure 2). The sensitivity was also greatly improved. The peak current was found to increase quasi-linearly with the pulse amplitude over the 10-100 mV/s range. The best signal to background current characteristic can be obtained by using a 50 mV/s pulse amplitude. The dependence of the peak current on the preconcentration time for different cobalt(I1) concentrations is shown in Figure 3. The rate of cobalt(I1) uptake is dependent on concentration. The peak current increases rapidly at first and finally a steady-state current was reached. For each of the four different concentrations, the steady-state current of cobalt(I1) accumulated on the modified electrode surface was different; a larger current was obtained for a higher concentration of cobalt(II), though the response was not linear. In all these cases, the preconcentration process was relatively rapid; more than 70% of the final response was generated within the first several minutes of preconcentration period. Such profiles were observed a t the different concentrations employed and appeared to represent the kinetics of the cation uptake. This behavior of the ion-exchanger modified electrode is in agreement with common models for the rates of ion accumulation in ion exchangers (26). Under the conditions used, the differential pulse voltammetric peak current increases linearly with cobalt(I1) concentration from 7 x lo-' to 1 x mol/L and then starts to level off (5-min preconcentration period). The detection limit was found to be 3 X lo-' mol/L ( S I N = 3). A lower detection limits can be reached by making the preconcentration period longer. For the same preconcentration time (5 min), the detection limit of this method is better than that of anodic stripping voltammetry a t a unmodified graphitic electrode (27). Effect of Common Ions. As mentioned above, the most significant potential advantage of using chemically modified electrodes in analytical chemistry is that the modifier($-an-
1
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E,V(vs.SC E I Figure 4. Differential pulse voltammogram of modified carbon paste mollL copper(I1) electrode in 5 X lo-' mol/L cobaR(I1)-3 X containing BR buffer solution (pH = 5.5). Conditions as for Figure l(1).
alyte interaction delivers greatly enhanced sensitivity and selectivity. By chosing a highly selective or specific modifier, the chemically modified electrode should allow interferences from coexisting species possessing electrochemical behavior similar to the analyte to be bypassed in a straightforward fashion. It is precisely this kind of interference that can pose a nearly insurmountable problem for the usual stripping analysis. In addition, because of its high specificity or selectivity toward an analyte, a number of possible interfering species have little or no effect on the determination. Metal ions can interfere with the determination of cobalt(I1) if they compete for complexation with the modifier and binding sites on the modified electrode surface. When 3 X loa mol/L Cu2+ was added to 5 x lo4 mol/L Co2+sample solution, a new peak current-as shown in Figure 4-was observed a t -0.15 V, nearly the peak potential of cobalt(I1). So copper seriously interferes with the determination. Additions of 3 X lob mol/L Fe2+,Ru3+,Ni2+to this solution resulted in 23,20,16, and 14% depression of the peak currents of cobalt(II), respectively. Furthermore, a well-defined differential pulse peak current of iron(I1) could be observed at a potential of about +0.88 V. Anions such as CN- and SCN- can form a stable complex with cobalt(I1); they also seriously interfere with the determination of cobalt. Nonaccumulated electroactive constituents do not interfere because of the use of the medium exchange procedure. Metal ions (5 x low6mol/L) such as Cd2+,Pb2+,Zn2+, Pd2+,Mn2+,In3+,Sn2+,and Ag+ were found not to interfere in the determination of 5 x lo4 mol/L cobalt(I1). In practical use, large excesses of these ions can depress the signal. This implies that, in an unknown quantitation, where a very large excess of such indirectly interacting ions might be expected, cobalt quantitation by using a calibration curve approach might produce erroneously low results. In such an instance, more accurate measurements are likely to be achieved by a standard addition method or a prior separating step. Furthermore, the selectivity of the method can also be improved. Except for copper, the most common ions have little effect on the determination. Thus, for the determination of cobalt, the selectivity of this method is better than that of anodic stripping voltammetry (27) in neutral solutions. Practical Application. The proposed procedure was used for determining cobalt in certified standard reference materials using the standard addition method. The samples were pretreated by using the method proposed by He (28) and separated by using the procedure recommended by Young (29). The results are listed in Table I.
Anal. Chem. 1991, 63,957-963
Table I. Determination of Cobalt in Standard Reference Materials (n = 6) sample
std value
found"
RSD, %
BCS 254 steel 0.0027% BHOlOl vanadium-iron ore 0.0145%
0.0029% 0.0140%
BH1016 alloy steel peach leap
0.083%
5.0
0.250 pg/g
4.2
0.085% 0.260 p g / g
5.0 5.2
" The average value of six determinations. *From Beijing Environmental Chemistry Institute. ACKNOWLEDGMENT This work was supported by the National Natural Science of Foundation of China, the aid from which is gratefully acknowledged.
LITERATURE CITED Murray, R. W. In EEectroanalyNcel Cbemlstry; Bard, A. J., Ed.; Marcel Dekker: New York. 1984; Vol. 13, pp 191-368. Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Cbem. 1987, 59, 379A-390A. Dong, S.; Wang, Y. Electroanalysis 1989, 7 , 99-106. Baldwln, R. P.; Christensen, J. K.; Kryger, L. Anal. Cbem. 1988, 58, 1790- 1798. Prabhu, S. V.; Baldwln, R. P.; Kryger, L. Anal. Cbem. 1987, 59, 1074-1078. Oyama, N.; Anson, F. C. J . Am. Chem. Soc.1979, 107, 3450-3456. Oardea-Torresdey, J.; Darnali. D.; Wang, J. Anal. Cbem. 1988, 60, 72-76. Heineman, W. R.; Wleck, H. J.; Yacynych. A. M. Anal. Chem. 1980, 52, 345-346.
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)
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Price, J. F.; Baldwin, R. P. Anal. Cbem. 1980, 52. 1940-1944. Umana, M.; Waller, J. Anal. Chem. 1988. 58, 2979-2983. Ye, J.; Baldwin, R. P. Anal. Chem. 1988, 60. 1979-1982. Coury, L. A.; Birch, E. M., Jr.; Heineman, W. R. Anal. Cbem. 1988, 60, 553-560. Thomsen, K. N.; Baldwin. R. P. Anal. Cbem. 1989, 67, 2594-2598. Gerhart, G. A.; Oke, A. F.; Nagy, 0.; Moghaddam, B.; Adams, R. N. Brain Res. 1984, 290, 390. Wang, J.; Golden, T.; Tuzhi. P. Anal. Cbem. 1987. 59, 740. Ohnuhi, Y.; Matsuda, H.; Ohasha, T.; Oyama, N. J . Electroanal. Chem. 1983, 758, 55. Wang, J.; Ll, R. Anal. Cbem. 1989, 67, 2809-2811. Wang, J.; Lu, Z. J . Electroanal. Cbem. 1989. 266, 287. Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, A l l , 393-402. Cheng, K. L.; Ueno, K.; Imamura, T. CRC Handbook of Organic Analytcal Reagents; CRC Press: Boca Raton, FL, 1982; Chapter 4. Martin, C. R.; Frelser. H. Anal. Chem. 1981, 53, 902-904. Rubinstein, I.; Bard, A. J. J . Am. Cbem. SOC. 1980, 702, 5007-50 13. White, H. S.; Leddy, J.; Bard, A. J. J . Am. Cbem. SOC.1982, 104, 4811-4816. Martin, C. R.; Dollard, K. A. J . Electfoanal. C h m . 1983, 757, 127- 135. Wang. J.; Tuzhi, P.; Ll, R.; Zadeii, J. Anal. Lett. 1989, 22, 719-727. Qian, T. Ion-Exchenger and Its AppllcaNons; Tianjlng Sclnce and Technology Press: Tiangjing, 1984; pp 84-95. Bralnina, Kh. 2.; Roizenblat, E. M.; Belyavskaya, V. B.; Klva, N. K.; Fomicheya, Zav. Lab. 1967, 33. 274. He, R. Applications of Polarograpby; Indrustrial Press: Beljlng, 1963; p 27. Young, R. S. Separation Procedures in Inorganic Analysis; Zhang. G., Transl.; Shangahi Science and Technology Press: Shanghai, 1984; pp 192-21 1.
RECEIVED for review September 26,1990. Revised manuscript received February 4, 1991. Accepted February 7, 1991.
Potentiometric Stripping Analysis at Microelectrodes in Various Solvents and Some Comparisons with Voltammetric Stripping Analysis J. F. Coetzee* and M. J. Ecoff Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Potentlometric stripping analysls (PSA) dlffers from the betler known voltammetric strlpping anaiysls (VSA) In that the electrochemkally preconcentrated analyte Is stripped chemically, rather than electrochemically. We present here comparlsons of lower detectlon limits and other features of PSA and VSA at both macro- and microelectrodes conslstlng of thin films of mercury on glassy carbon, carbon fiber, and gold substrates. The posslblilty that certain amalgams and/or metals that undergo sluggish electrochemlcal oxidation would exhlblt more facile chemlcal oxidation was Investigated. I t was found that In fact the PSA of manganese and nickel Is analytically more favorable than the VSA of these metals. The appllcablllty of PSA to such electroposltlve elements as sodium and potassium, described by us before, was further Improved. Falr resolution of sodlum from potassium was obtained by solvent opthnlzatlon and the use of microelectrodes on gold substrates. The PSA of llthlum Is much less favorable than that of sodium or potasslum.
INTRODUCTION Stripping analysis (SA) involves preconcentration of an analyte by electrodeposition on an electrode, followed by 0003-2700/9 110363-0957$02.50/0
stripping of the analyte from the electrode either voltammetrically or chemically. As compared to direct voltammetry or several optical methods, this technique has somewhat restricted scope, but for those analytes to which it is applicable, its lower detection limits (LDLs) can be exceptionally favorable, owing, in part, to the preconcentration step. In addition, SA has multielement capability, allowing the determination of several elements in the same experiment. These and other advantages of SA, as well as its limitations, have been discussed (1).Voltammetric stripping analysis (VSA), in which the stripping current is monitored as a function of applied potential, is usually carried out by fast linear sweep voltammetry or differential pulse voltammetry and has been extensively applied (1).More recently, J. and R. A. Osteryoung (2,3)and Wojciechowski (4)have applied fast square wave voltammetry to SA, thereby greatly reducing the time required for the analysis as well as obviating the need for deaeration of the solution. In the second variant of SA, chemical or potentiometric stripping analysis (PSA),in which the potential of the working electrode is monitored as a function of time, dates back to 1961 when Bruckenstein and Nagai stripped thallium and lead amalgams with mercury(I1) ion (5). This approach has been greatly extended by Jagner to the determination of a number of metal ions in a variety of matrices, including beverages, blood, urine, and seawater 0 1991 American Chemical Society
