Anal. Chem. 1989, 6 7 ,
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except for modifiers which have a negligible response in the flame (i.e. formic acid, formamide, or water). Consequently, the UV or mass spectrometric detectors are used in SFC with modified mobile phases. However, the use of the thermionic detector in microbore SFC could potentially allow highly sensitive detection as well as the use of a wide range of organic modifiers with the exception of those containing nitrogen or phosphorus. Additionally, the decreased solute capacity factors resulting from the use of modified supercritical mobile phases can be important to SFC in terms of decreasing time of analysis for certain compounds. LITERATURE CITED (1) Karmen, A,; Giuffrida, L. Nature 1984, 210, 1204-1205. (2) Lakota, S.; Aue. W. A. J. Chromatogr. 1989, 4 4 , 472-480. (3) Dressier, M. Selective Gas Chromatographic Defectors ; Elsevier Sci-
ence Publishers: Amsterdam, 1986. (4) Kolb, 9.: Bischoff. J. J. Chromafogr. Sci. 1974, 12, 625-629. (5) Kolb, 9.; Aver, M.; Paspisil, P. J . Chromatogr. Sci. 1977, 15, 53-63. (6) Fjeldsted, J. C.; Kong, R. C.; Lee, M. L. J. Chromatogr. 1983, 279, 449-455. (7) West, W. R.; Lee, M. L. HRC CC, J. High Resoluf. Chromafogr. Chromatogr. Commun. 1986, 9 , 161-167. (8) Patterson, P. L.; Gatten, R. A.; Ontiveros, C. J. Chromatogr. Sci. 1982, 2 0 , 97-102. (9) Patterson, P. L. J . Chromatogr. Sci. 1988, 2 4 , 41-52.
2086-2092
(IO) David. P. A.; Novotny, M. J. Chromafogr., 1988, 452. 623-629. (11) David, P. A.; Novotny, M. J. Chromatogr. 1989. 461, 111-120. (12) Novotny. M.; David, P. HRC CC, J. High Resoluf. Chromatogr. Chromafogr. Commun. 1988, 9 , 647-651. (13) Lee, 9. I.; Kesier, M. G. A I C M J . 1975, 21, 510-527. (14) Re& R. C.; Prausnitz, J. M.; Poling, 6. E. The Properties of Gases and Liquids, 4th ed.;McGraw-Hill: New York, 1987; Chapter 5. (15) Wooley, C. L.; Kong, R. C.;Richter, 6. E.; Lee, M. L. I.IRC CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1984, 7 , 329. (16) Wooley, C. L.; Tarbet, 9. J.; Markdes, K. E.; Bradshaw, J. S.;Lee, M. L.; Battle, K. D. HRC CC, J . High Resoluf Chromafogr . Chromafogr . Commun. 1988, 1 1 , 113-118. (17) Wright, 6. W.; Peaden, P. A.; Lee, M. L.; Stark, T. J. J. Chromatogr. 1982, 248, 17-24. (18) Guthrie, E. J.; Schwartr, H. E. J. Chromafogr. Sci. 1986, 24, 236-24 1. (19) Lubkowtiz, J. A.; Semonian, 9. P.; Galobardes, J.; Rogers, L. 9. Anal. Chem. 1978, 50, 672-676. (20) van de Weijer, P.; Zwerver, 9. H.; Lynch, R. J. Anal. Chem. 1988, 6 0 , 1380-1387. (21) Yonker, C. R.; Smith, R. D. Anal. Chem. 1987, 5 9 , 727-731. (22) Fields, S. M.; Markides, K. E.; Lee, M. L. J . Chromafogr. 1987, 406, 223-235.
.
RECEIVED for review January 9,1989. Accepted June 27,1989. This work was supported by Grant CHE 8605935 from the National Science Foundation and a grant-in-aid from Dow Chemical Co.
Square Wave Voltammetry at a Mercury Experimental Results
Electrode:
Kazimierz Wikiel’ and Janet Osteryoung*
Department o f Chemistry, State University of New York at Buffalo, Buffalo, New York 14214
Experimental results for dlrect and anodlc stripping square wave voltammetry at a sliver-based mercury flim electrode (SBMFE) are presented for -2 < log ( / ( t / D ) ” 2 )< 1, where I is film thickness, f frequency, and D dlffuslon coefflclent. Theoretical predictions are firmly confirmed by the experimental results for Pb( II)/Pb(Hg) for the magnltude, shape, and positlon on the potential scale of the voitammograms. The usefulness of the SBMFE under square wave condltlons is established even at hlgh frequency. However, some deviation for square wave experiments at silver-based microelectrodes is observed at hlgh frequency.
The mercury film electrode (MFE) is presently a wellrecognized tool for electroanalytical practice. Anodic stripping voltammetry a t the MFE by means of pulse techniques is applied frequently in trace analysis (1). Although the theoretical calculations for direct ( 2 ) as well as anodic stripping ( 3 , 4 )square wave voltammetry have been published, only a few experimental results following these theoretical treatments have been reported. Some results obtained a t the glassycarbon-based MFE ( 5 )and iridium-based MFE (6-8) deviate from theoretical predictions. Markedly smaller than expected reverse current has been observed for both direct and anodic stripping square wave voltammograms when high square wave frequency has been applied. In some cases, however, ideal behavior was observed.
The mercury-plated glassy carbon electrode exhibits behavior predicted for a film electrode quantitatively over a wide range of conditions for linear scan stripping. Mercury on glassy carbon substrates exists as a collection of individual droplets dispersed across the surface. The connection between the droplets and carbon is sufficiently weak that for deposition at a rotating disk electrode even at modest rotation rates (e.g., 2000 rpm) mercury droplets slide across the carbon surface and on to the surrounding insulator ( 5 ) . Thus differences between experimental results and those of a film model should not be surprising. On iridium substrates coherent films of mercury are formed (6-9). However in both direct and stripping square wave voltammetry at such electrodes anomalies occur a t higher square wave frequencies (6, 8). In this report we present some experimental results obtained at mercury film electrodes under square wave conditions over a wide range of mercury film thickness and square wave frequency using a silver-based mercury film electrode. It has been reported (10, 11) that this type of MFE follows theoretical predictions for the Pb(II)/Pb(Hg) system under cyclic and anodic stripping linear scan conditions. Considering the above discussion, it can be seen that the objective was twofold. One goal was to confirm experimentally the theoretical predictions based on the thin layer model for SWV (2, 3). A related but quite different goal was to demonstrate that the simple model of a film electrode has a practical experimental realization which permits the general extension of square wave voltammetry to mercury film electrodes. EXPERIMENTAL SECTION
Present address: Institute of Precision Mechanics, Duchnicka 3, 00-967 Warsaw, Poland.
All chemicals used in this study (Pb(N0J2, NaN03, CH,COOH, CH3COONa,EDTA, HC104,Hgz(C104)2, NaOH) were analytical
0003-2700/89/0361-2086$01.50/00 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 18, SEPTEMBER 15, 1989
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Flgure 1. Net (-), forward (A), and reverse (V)current function of DSWV for 2 X 4.2 pm. A€s = 5 mV; E,, = 25 mV; f = (1) 6, (2) 30, (3) 120, and (4) 600 Hz.
grade and used as received. An EG&G PARC 273 potentiostat interfaced to a PDP 8/e computer system was employed for voltammetry. A PAR 173 potentiostat equipped with a PAR 179 coulometer was employed for deposition and stripping of mercury. A three-electrode conventional jacketed cell was used with temperature maintained at 25 i 0.2 "C. An SCE was used as the reference electrode and a platinum wire as the counter electrode. Solutions were deaerated by sparging with high-purity argon. Silver-based mercury film electrodes (SBMFE) were prepared according to the procedure presented by Stojek and Kublik (10). The electrode was a cylinder formed by sealing 3-4 mm of 0.5 mm diameter silver wire into soft-glass tubing. Final electrodeposition of mercury was preceded by the formation on the silver surface of solid silver amalgam. This was formed by depositing a thick layer, approximately 10 pm, of mercury directly onto the silver electrode and then storing in deionized water for about 24 h. After that the excess of liquid mercury was stripped electrochemically. In all experiments this solid silver amalgam was in fact the substratefor the mercury fii. The quantity nACo*Do112 was determined for the SBMFE electrochemically by comparison of linear scan results for cathodic reduction of Pb(I1) at the static mercury drop electrode (SMDE PARC 303A, medium size drop, A = 0.017 cm2)with results obtained at the SBMFE under exactly the same conditions. The geometric area of the silver substrate was used to calculate the thickness of mercury films from the deposition charge. Mercury was deposited electrochemically from a solution of 0.05 M Hg(1) in 0.1 M HCIOI. The charge passing during the deposition was monitored coulometrically. The thickness of the mercury film was determined from the charge passed during the oxidation of mercury (n = 2) from the substrate in a solution of 0.05 M EDTA in acetate buffer, pH = 4.8. The film thickness is less than 1% of the diameter of the wire. Under microscopic examination it is uniform. The concentration of lead in the mercury phase, necessary to calculate the current function in anodic stripping experiments, was determined from the charge of the oxidation of lead under linear scan conditions. As suggested by Stojek and Kublik (IO),in order to refresh the surface of mercury, the SBMFE was negatively polarized at ap-
4
-1
M Pb(I1) in 1 M NaNO,. SBMFE: A = 4.4 mm2; I =
proximately -6 V in 2 M NaOH for approximately 2 s before each set of experiments. This treatment is accompanied by copious evolution of hydrogen and results in a perfectly adherent and reflecting film. The proper preparation of the SBMFE was verified experimentally by comparison of results obtained under linear scan conditions with theoretical predictions presented by De Vries and van Dalen (11) and Donten et al. (12). The agreement was very good. For the time scales employed here cylindrical diffusion should be unimportant. A silver-based mercury microelectrode was prepared by sealing a silver wire 25 pm in diameter into a 2-pL micropipet. After fine polishing with 0.05-pm alumina suspension, the silver microdisk electrode was dipped into bulk mercury. Microscopic examination of the resulting electrode showed that a hemisphere of mercury with a diameter of approximately 25 pm had been attached to the silver substrate. RESULTS AND DISCUSSION Direct Square Wave Voltammetry (DSWV). According to the theoretical predictions the shape and position of square wave voltammograms obtained for the system M"+/M(Hg) at a MFE depends on the value of mercury film thickness and square wave frequency as combined in the dimensionless parameter A which is defined as A = l(f/DR)ll2 where 1 is the mercury film thickness, f is the square wave frequency, and DR is the diffusion coefficient of the species in the mercury phase. Figure 1 presents typical DSWV voltammograms for the system Pb(II)/Pb(Hg) obtained at an SBMFE with the same mercury film thickness and various frequencies. The values of log A given in the figure captions were calculated by using the value D(Hg) = 1.25 X cm2/s (13). The current is displayed as a dimensionless current function defined as
# ( t ) = i(t)d/2/nFADo'/2f1/2Co* where $ ( t ) is the dimensionless current function at the time
ANALYTICAL CHEMISTRY, VOL. 61, NO. 18, SEPTEMBER 15, 1989
2088
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t , i(t) is the current at the same time, n is number of electrons involved in the electrode reaction, F is Faraday's constant, A is the surface area of the MFE, DOis the diffusion coefficient of Mn+ions in solution, f is square wave frequency, and Co* is the bulk concentration of Mn+ions. The factor nACo*Do112 required to obtain the dimensionless current, $ ( t ) ,from the experimental current, i ( t ) ,was determined as described in the Experimental Section. The curves presented in Figure 1 are very well shaped in teims of net peak as well as both forward and reverse peaks, without any anomalies even for square wave frequency as high as 600 Hz. (It should be emphasized that in the case of glassy carbon and iridium-based electrodes properly shaped square wave voltammograms cannot be obtained reliably at such high frequencies (5-8)). As predicted by theoretical calculations ( 2 ) the net current voltammograms retain their symmetrical shape, whereas the individual forward and reverse peaks change shape significantly with changes in dimensionless parameter A. As under linear scan conditions, the position of these voltammograms shifts toward more negative potential as the value of A is decreased. Quantitative results for the net peak position obtained a t the SBMFE over a wide range of parameter (mercury film thickness 0.25-8 pm; frequencies 6-600 Hz) are presented in r'igure 2. The solid line represents calculated results for two-electron reversible charge transfer at the MFE. The agreement between experimental and theoretical results is very good over the entire range of li values. It has been shown (2) that for the thin layer region ( A .C: 0.1) the peak position can be described by = -34.1
0 -4
0
-2.0
+ 59.2 log A, mV
0.0
2 0
4.
L O G LRMBDA
Figure 2. Net DSWV peak position as a function of log A. Other conditions are given in Figure 1. Solid line denotes theoretical results.
n(E, -
- 1
LnmBoa
(3)
where E , is the potential of the net peak and is the reversible half-wave potential. The least-squares linear regression of E, - Ellz on log A for the range of log A from -1.76 to -0.71 yields the experimental value of 29.8 mV/log A unit with correlation coefficient of 0.992. This value agrees very well with the theoretically predicted value of 29.6 mV/log A unit for a two-electron reversible process. Figure 1 also shows that the dimensionless peak current depends on the value of the dimensionless parameter A. The dimensionless net peak current is larger for li values around 1 (Figure 1,curves 2 and 3) and voltammograms obtained with either smaller (curve 1) or larger (curve 4) values of A have smaller dimensionless net peak currents. Figure 3 presents some quantitative results for the net, forward, and reverse peak currents as a function of log A. Generally, the shapes of all of these plots agree reasonably well with the theoretical calculations represented by solid lines. The net and reverse peak current functions are peak shaped with the maximum and minimum, respectively, occurring around the value of log
Figure 3. Net (O), forward (A),and reverse (V)DSWV peak current function versus log A. Conditions are given in Figure 1. Solid lines denote theoretical results.
1
0
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-2.0
0.0 L O G LRtlBR
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1.0
2.0
Half-width of the DSWV net peak as a function of log A. Condiions are given in Figure 1. Solid line denotes theoretical results. Figure 4.
.i = 0. Also experimental plots for forward peak current function agree quite well with theoretical predictions. Some spread of experimental points around theoretical lines is caused by inaccuracy in determination of mercury film thickness. It was calculated theoretically (2) that the net peak halfwidth is not affected by changes in A, remaining relatively constant (nWlj2= 126 f 2 mV) except around log A = 0 (nW112= 130 mV). The values of 120-130 mV obtained experimentally as shown in Figure 4 are in good agreement with theoretical predictions. Square Wave Anodic Stripping Voltammetry (SWASV). As in the case of DSWV experiments the shape and position of anodic stripping square wave voltammograms are affected by changes in the value of the dimensionless parameter A. Figures 5 and 6 show two sets of anodic stripping experiments for the system Pb(II)/Pb(Hg). Curves presented in Figure 5 were obtained a t the SBMFE with the same thickness and various frequencies whereas Figure 6 presents results for constant frequency and various thicknesses. As above for DSWV experiments the net peak retains its symmetrical shape whereas both forward and reverse peaks are affected markedly as the value of A is changed. Notice that the relative positions of the forward and reverse peaks change with changing value of A. For low values of A (log A < 0) the forward peak (oxidation peak) occurs at a potential more negative than the reverse one (Figure 5, curve 1,and Figure 6, curves 1-3). For large values of A (log A > 0.5) the forward peak is a t a more positive potential than the reverse one (Figure 5, curve 4)and the shape of voltammograms obtained for such values of A resembles the shape of the square wave voltammograms for the reversible couple with unrestricted diffusion control.
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Figure 5. Net (-), forward (A), and reverse (V)current of anodic stripping square wave voltammograms 5 X lo-’ M Pb(I1) in 0.1 M NaNO,. SBMFE: A = 6.5 mm2, I = 7.2 pm. A€, = 5 mV; E, = 25 mV; f = (1) 6, (2) 30, (3) 200, and (4)600 Hz. (A = (1)0.50,(2)1.12, (3) 2.88, (4)4.99). 60 s (stirring) -k 15 s (quiescent) deposition at -0.7 V.
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(A),and reverse (V)current of anodic stripping square wave voltammograms. Conditions as in Figure 5 except I and (4)4.9 pm; f = 30 Hz (A = (1) 0.043,(2) 0.070,(3)0.17,and (4)0.76).
where CR* is the concentration of metal deposited in the mercury film and other symbols have the same meaning as for eq 1 and 2. It has been shown also (3) that for the thin $( t ) = i ( t)T ~ / ~ ~ F A D ~ ~ / ~ ~ Cregion ~ * (log A C -1) the net peak current function is (4)~ ~ /layer
In the case of SWASV the dimensionless current function is defined by
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 18, SEPTEMBER 15, 1989
strictly proportional to the value of 1
A$, = 0.528A
(5)
Figure 7 shows quantitative results for the thin layer region. The plot of the net peak current function versus lambda is a straight line with a slope of O.525/A unit (correlation coefficient 0.995) which may be compared with the theoretical value of 0.528; Le., the agreement for this region is excellent. These results are significant from the analytical point of view, since the total current does not depend on mercury film thickness in this region. Replacing the A value in eq 5 by its definition (eq 1) and bearing in mind the definition of $ ( t ) (eq 41, one can obtain
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Figure 7. SWASV net peak current function versus dimensionless parameter A. Solution and conditions as in Figure 5. Soli line denotes the best linear fit.
Since qR, the total charge of deposition, is defined as q~ = nFAC,*l
0.04 0.06 LAPIBGA
0.02
(6)
(7)
the final expression for net peak current is
Aip = 0.298qRf
(8)
This means that the anodic stripping net peak current depends straightforwardly on the total amount of metal deposited in the mercury layer, provided that log A is smaller than -1. Thus for the same conditions of metal deposition (the same deposition time and potential, constant mass transport conditions) the peak current depends straightforwardly on the bulk concentration in solution. The results for peak current function over the entire range of log h under investigation are presented in Figure 8. Together with results for the net peak, experimental results for both forward and reverse peak current function are presented. Solid lines represent the theoretically calculated results for the same case. Generally the shape of all three plots agrees with the theoretical shape. There is some deviation of experimental points from the theoretical prediction. However, bearing in mind that the determination of CR*,the value of which is necessary to calculat,e the peak current function in this case, introduces some additional degree of inaccuracy, it seems that the agreement between the experiment and theory is quite good. Figure 9 shows the experimental results for anodic stripping net peak position as a function of the dimensionless parameter A. Again, the solid line represents the results of calculation. Theoretically ( 3 )this dependence was described as follows:
n(E, - El/z)= -46.7
+ 59.2 log A, mV
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i
-
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provided that is smaller than 0.1. Again, the slope of the plot of (EP- E l j z )vs log A for a two-electron reversible system should be 29.6 mV/log . I unit. The value obtained experimentally by least-squares linear fit (range of log A from -1.71 to -0.47, correlation coefficient 0.986) for the slope, 28 mV/log unit, is very close to the theoretically predicted value. The solid line presented in Figure 10 represents theoretical calculations for net peak half-width as a function of log A . This line was obtained on the basis of results reported by Kounaves et al. (3). The dashed line shows exactly the same dependence but calculated according to Penczek and Stojek ( 4 ) . Points represent experimental results for the net peak half-width obtained for SWASV. It follows from Figure 10 that the distribution of experimental points is not very much worse than the difference in the results of two different approaches to the calculation, and the experimental value of W,,, is in the range 50-60 mV for the entire range of log A. Note that the waveform of ref 4 is not exactly the same as that of ref 3. The dashed curve of Figure 10 is the result of ref 4
-"'=I -2
0
-2
0
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0
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modified to take this into account. Experimental results presented above show that the Pb(II)/Pb(Hg) system behaves as predicted theoretically at the silver-based mercury film electrode under direct and anodic stripping square wave conditions. The excellent behavior of the SBMFE under linear scan conditions has also been reported (10, 12). The present work confirms the usefulness of this type of mercury film electrode also under square wave conditions over a wide range of frequencies. Results presented above do not contradict the reports of anomalies resulting from application of SWV to other types of mercury film electrodes (5-8). They rather emphasize the
ANALYTICAL CHEMISTRY, VOL. 61. NO. 18, SEPTEMBER 15, 1989
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high frequencies, as there is no procedure for cleaning it. (The procedure for the SBMFE, when applied to an iridium-based MFE, blows the mercury off of the surface.) There is substantial convenience in carrying out anodic stripping voltammetry at a microelectrode, because the deposition step can be carried out efficiently by diffusion in quiet solution. Figure 11 presents results obtained a t the silverbased mercury microelectrode described in the experimental section for 10 mM Pb(I1) solution under direct square wave and staircase conditions. For staircase conditions curves obtained both a t low and high frequencies are reasonably well shaped. The voltammetric deposition of Pb(I1) a t low frequency (Figure 11-3) has the characteristic S-shape which is observed when the diffusion layer is comparable in thickness to the small dimension of the electrode. The pronounced enhancement of the anodic current at low frequency is due to accumulation of P b in the film. At higher frequencies (Figure 11-4) the dimensionless size of the electrode is greater (r(flD)1/2 = 7 ) and thus the response resembles more closely that expected for unrestricted planar diffusion. The square wave voltammogram a t low frequency (Figure 11-1)displays nearly steady state forward current but a reverse peak because the product is concentrated in the film. Note that the forward limiting current is the same as in the staircase mode (Figure 11-3). The reverse peak is much less pronounced than the staircase reverse peak because the elapsed time at the peak, and hence the extent of electrolysis, is less ( t = (Ei - E ) / f A E for square wave, t = (Ei- 2Ef E ) / f A E for staircase). On the other hand, the square wave voltammogram obtained at 300 Hz displays no reverse current. This anomaly in the reverse current is consistent with the results of Wechter and Osteryoung (6) for the iridium-based MFE and definitely is not connected with the choice of silver as the substrate for mercury. Thus it may be that the anomalies observed at the iridium-based MFE, which is quite small (the iridium substrate is an embedded disk 127 pm in diameter), have more
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line according to (3);dashed line according to (4). differences in properties of these electrodes. Although no difference has been observed under linear scan conditions (9, 10,12,14), differences are observed under square wave conditions which, because of large amplitude potential pulses, is more demanding. The SBMFE has two unique characteristics. First, the solid substrate is a well-defined solid amalgam, which provides a chemically favorable environment for adhesion of mercury. Second, a procedure exists whereby the mercury film can be restored to its initial condition after each experiment. It should be emphasized that the performance of the SBMFE deteriorates quickly with use, and the polarization in 2 M NaOH is essential to maintain theoretical response over a working day. A hanging mercury drop also deteriorates in performance rapidly; its reliability is ensured only by using a fresh drop for each experiment. Thus it is perhaps not surprising that the iridium-based MFE displays anomalies at
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Direct square wave (1, 2) and staircase (3, 4) voltammograms obtained for 10 mM Pb(I1) in 1 M NaNO, at the silver-based mercury microelectrode ( f = 12.5 pm). A€* (mV) = (1, 2) 5 and (3, 4) 1.41. f ( H r ) (1) 3, (3) 6, and (2, 4) 300. E,, = 25 mV.
Flgure 11.
Anal. Chem. 1989,6 1 , 2092-2097
2092
to do with electrode size than with the nature of the substrate. The conclusions, then, are that the SBMFE, when macroscopic, behaves ideally when used for square wave voltammetry in either the direct or stripping modes, and that the experimental results agree with the theory for these cases over wide ranges of variation of both frequency and film thickness. ACKNOWLEDGMENT We thank C. Wechter and M. Seralathan for their valuable comments and helpful discussion. LITERATURE CITED Osteryoung, Janet G.; Schreiner, M. M. CRC Crit. Rev. Anal. Chem. 1988. 19. Suaal. 1. S1-S19. --rr Kounaves. S. P.; O b a , J ; Chandresekhar, P.: Osteryoung, J. Anal. Chem. 1986, 56, 3199. Kounaves. S.P.: O'Dea, J. J.; Chandresekhar, P.; Osteryoung, J. Anal. Chem. 1987, 5 9 , 386.
----.
-
(4) Penczek, M.; Stojek, 2. J. Electroanal. Chem. 1986, 213, 177. ( 5 ) Schreiner, M. Ph.D. Dissertation, SUNYAB, 1987. (6) Wechter, C.; Osteryoung, J. Anal. Chem., following paper in this issue. (7) Goias, J.; Galus, Z.; Osteryoung, J. G. Anal. Chem. 1987, 59, 389. ( 8 ) Kounaves, S.;Osteryoung, J. Pittsburgh Conference, Atlantic City, NJ, March 1987, Abstr 179. (9) Kounaves, S. P.; Buffle, J. J. Electroanal. Chem. 1988, 239, 113. (10) Stojek, Z.; Kublik, Z. J. Electroanal. Chem. 1975, 60, 349. (1 1) De Vries, W. T.; van Daien, E. J. Electroanal. Chem. 1967, 14, 315. (I2) Donten, M.; Stojek. Z.; Kublik, 2 . J. Electroanal. Chem. 1984, 163, 11.
(13) Galus, 2. CRC Crit. Rev. Anal. Chem. 1975, 370. l l A \ Stojek, Z.; Stepnik, B.; Kublik, 2. J. Electroanal. Chem. 1976, 7 4 , ,.-I
277.
7
RECEIVED for review March 8, 1989. Accepted June 14, 1989. This work was in part by the Science Foundation under Grant No. CHE8521200.
Square Wave and Linear Scan Anodic Stripping Voltammetry at Iridium-Based Mercury Film Electrodes Carolyn Wechter and Janet Osteryoung*
Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214
The voltammetric response of an iridium-based mercury film electrode is described. Linear scan and square wave voltammetry are used In the direct and anodic strlpplng modes for the determination of lead. Mercury film electrodes of this geometry (radius 63.5 pm, thickness 1-100 pm) evidence mixed dmusion regimes in solution and within the mercury layer. Experimental results are compared with existing theoretical models.
Mercury film electrodes (MFE) have long been employed for the determination of metals using anodic stripping voltammetry (ASV). They are prepared by depositing a thin layer of mercury (0.001-10 gm) onto a solid substrate (Pt, Ag, Au, carbon). The MFE retains the advantages of the mercury drop, for example the favorable overpotential for hydrogen, yet avoids the hazardous and problematic use of bulk mercury. In addition species soluble in mercury typically display restricted diffusion within the mercury layer. Resolution is increased therefore with respect to the mercury drop, as voltammetric peaks become more narrow. Excellent reproducibility and sensitivity are also achieved, and MFEs are easily adapted to fit a variety of cell geometries, including placement in flow systems. A variety of potential waveforms have been used with anodic stripping voltammetry. Linear scan ASV (LSASV) at the MFE has a well-developed theory and ample experimental results in the literature. The voltammetric stripping peaks are characterized by peak current (or 4, the dimensionless peak current), peak position (or n ( E , - E l j z ) ,the normalized peak position), and peak half-width (or nWllz,the normalized peak half-width). Pulse voltammetric techniques have also been combined with ASV. These methods are noted for their discrimination against charging currents. Differential pulse, in particular, has often been used for routine analytical investigations. Square wave voltammetry (SWV) appears to be
a favorable technique due to its fast scan capabilities, peakshaped response, and increased sensitivity compared with differential pulse voltammetry (I).The recent introduction of commercial instruments that perform SWV should increase its use with ASV. The main purpose of this work is to test the useful range of practical application of the iridium-based mercury film electrode both with respect to the parameters of the electrode itself and those of the voltammetric techniques employed, staircase and square wave voltammetry. Despite the complexity of the resulting diffusional problem as described below, the work is restricted to electrodes employing a 127-pm-diameter substrate, because this small size facilitates the plating process and is readily available. Iridium-Based Mercury Film Electrodes. Some materials commonly used as substrates present problems with dissolution of the base metal (Pt, Ag, Au) in mercury, or nonwetting of the substrate (carbon) by mercury. Kounaves and Buffle ( 2 )have investigated the properties that make a material suitable for use as a substrate for mercury deposition and concluded that iridium is best because it is wetted by but is not soluble in mercury. They have deposited mercury on an embedded circular iridium electrode of radius 2 mm and determined that no intermetallic Ir/Hg species are formed (3). For films less than 1 pm thick, experimental results confirmed theoretical predictions for LSASV (4).However, for thicker films, whereas the current-scan rate behavior agreed with theory for thin films, the experimental values of peak potential and half-width corresponded to those of a thicker film probably because mercury forms a spherical segment, rather than a flat film. Golas et al. (5,6)reported the use of iridium-based mercury film electrodes (IrMFE) for chronoamperometry and linear scan voltammetry. Cohesive films of mercury were achieved, the most stable corresponding to a hemisphere. These electrodes were prepared from 127 pm (0.025 in.) diameter iridium wire, the smallest size commercially available. The use of this
0003-2700/89/0361-2092$01.50/0@ 1989 American Chemical Society
