Anal. Chem. 1990, 62, 1325-1331
1325
Square-Wave Anodic Stripping Voltammetry at Glassy-Carbon-Based Thin Mercury Film Electrodes in Solutions Containing Dissolved Oxygen Marek Wojciechowski* a n d John Balcerzak Department of Chemistry and Biochemistry, University of Maryland, Baltimore County Campus, Baltimore, Maryland 21228
Square-wave anodlc strlpplng voltammetry (SWASV) of nanomolar concentration levels of lead and calcium at glassycarbon-based thin mercury fHm electrodes was Investigated In the presence of dissolved oxygen. Anodlc strlpplng measurements In nondeaerated solutions were posslble because of the depletion of oxygen at the electrode surface prior to the strlpplng step and because of the fast-scanning ablllty of square-wave voltammetry, which allows for the completlon of the strlpplng step before any slgnlflcant oxldatlon of the amalgam by dlffuslng oxygen can occur. Slmllar measurements by dlfferentlal pulse anodlc strlpplng voltammetry are not posdble unless oxygen Is removed from solutlon. Thin mercury fllms showed good stability In the presence of dlssolved oxygen. Linear callbratlon curves were obtained for lead and cadmium In acldlc medla (HCI and acetate buffer, pH 4.8), wlth no slgnkant loge In strlpplng current sensHhrlty due to dlswlved oxygen. The SWASV method was successfully used for the determlnatlon of lead In untreated, alr-saturated tap water samples contalnkrg added 0.01 M HCI. The determlnatlons by SWASV agreed very well wlth the results obtained by electrothermal atomlc absorptlon spectrophotometry. Wlth the tlme-consumlng solution purglng step absent and the very fast strlpplng step, the deposltlon step Is the only tlme-llmltlng factor of the method.
INTRODUCTION Anodic stripping voltammetry is well-known for the rapid and sensitive measurement of trace metals in solution (I), especially when performed in conjunction with pulse voltammetric techniques which discriminate against charging currents and yield improved current sensitivities (2, 3). Among the electrodes used in anodic stripping voltammetry, thin mercury film (TMF) electrodes offer the highest sensitivity and resolution. Another attribute of TMF electrodes is their mechanical stability and the convenience of preparation and use. However, TMF electrodes, as well as some metals deposited into them during a stripping experiment, are known to possess a relatively poor chemical resistance to oxygen. Therefore, in order to eliminate the possibility of amalgam oxidation by dissolved oxygen, extensive purging of the analyzed solution is required to remove the oxygen. Because solution purging is inconvenient and consumes a majority of the total analysis time, a complete elimination of purging would be desirable. An earlier study proved that anodic stripping analysis employing the static mercury drop electrode (SMDE) can be performed in the presence of dissolved oxygen if square-wave voltammetry is used as the stripping technique (4). It was shown that, due to the fast scanning ability of square-wave voltammetry, the anodic stripping step can be completed before any significant amount of oxygen could reach the SMDE surface and oxidize the amalgam. The goal of the 0003-2700/90/0382-1325$02.50/0
study presented in this paper was to examine whether TMF electrodes can be used in conjunction with square-wave anodic stripping voltammetry (SWASV), for conducting trace metal determinations in the presence of oxygen. EXPERIMENTAL SECTION Solutions. Deionized, doubly distilled water was used to prepare all solutions. The 0.1 M solutions of HCl were prepared from concentrated hydrochloric acid (Ultrex, J. T. Baker). The 0.1 M solutions of acetate buffer (pH 4.8) were made from glacial acetic acid (Ultrex, J. T. Baker) and sodium acetate (Suprapur, Merck). Stock solutions of 0.05 M PMII),0.05 M Cd(II),and 0.015 M Hg(I1) were prepared from correspondingnitrates (Puratronic, Johnson Matthey) and also contained 0.01 M nitric acid (Instra-Analyzed, Baker), which was added to prevent formation of hydroxide complexes. The stock solutions of Pb(I1) and Cd(I1) were standardized by EDTA titration. Solutions of lower concentrations were prepared by subsequent dilutions of stock solutions. For experiments conducted in the absence of oxygen, solutions were purged with purified argon for 20 min and were kept under argon blanket. Apparatus. SWASV experiments were conducted with a 384B polarographic analyzer (EG&G PARC). Measurements of square-wave forward and reverse currents were performed on a BAS-100 electrochemical analyzer (Bioanalytical Systems). A DMP-40 series digital plotter (Bausch and Lomb) was used to plot voltammograms recorded on the PARC 384B. A 20-mL capacity electrochemicalcell (IBM Instruments) was used. The working electrode was a glassy carbon disk (0.22 cm2area) press fitted into a Teflon sleeve. The rotation of this electrode was controlled by an EC/219 RDE assembly (IBM Instruments). Potentials were applied and measured with respect to a saturated (KCl) calomel electrode. A platinum wire was used as the auxiliary electrode. Procedures. TMF electrodes were prepared by plating a mercury f i i on a glassy carbon rotating disk electrode (GCRDE) at -1.0 V for 5 min, while the electrode was rotated at 2000 rpm, from a deaerated solution containing 0.1 M HC1 and 1mM Hg(I1). The film was used for an entire day of work. Based on the steady-state current measured during the plating, and the geometric surfacearea of the glassy carbon disk, and assuming a 100% current efficiency of the mercury deposition, and a uniform distribution of the film, the calculated film thickness was 0.33 Irm. Unless otherwise stated, the SWASV experiments were conducted as follows. A conditioning potential of -0.4 V was applied to the electrode for 30 s to assure a complete oxidation of the analyte metal deposited during the previous experiment. Then a deposition potential, Ekp, of -1.1 V and a deposition time, tdep, of 120 s were used to deposit the analyte metal into the film. During both the f i i conditioning and the analyte deposition steps, the electrode was rotated at 1000 rpm. After the rotation was stopped, an equilibration period of 10 s was then observed to allow the solution to become quiescent. The square-wavestripping was then performed by scanning the potential from -1.1 to 4 . 4 V using a square-wavefrequency,f, of 100 Hz, a square-waveamplitude, E,,, of 24 mV, and a potential step, E,, of 2 mV. Tap water samples for the SWASV determination of lead were collected into polyethylene containers and were used untreated and unfiltered. The water samples were made 0.01 M in HCl,
'
0 1990 American Chemical Society
1326
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
30-
0.8
O2+2H++2e.-*H2Op
0.6 H202
0
'
I
I 0.0
- 0.2
+
i-
k
Oxygen present
J
Oxygen removed
2H'+2e-+Hz0
I
-0.4
-0.6
-0.8
-1.0
E,V
I -0.8 -0.6
-0.8 -0.6 Potential, V
Figure 2. Differential pulse anodic stripping voltammograms of 2 ppb lead in deaerated and nondeaerated solutions of 0.1 M HCI: E, = -0.9 v, td, = 120 s (at 1000 rpm); scan rate = 5 mV s-I.
t
Oxygen present
Figure 1. Square-wave voltammograms of oxygen in nondeaerated solution of 0.1 M acetate buffer (pH 4.8): E,, = 25 mV; E, = 2 mV; f = 50 Hz (scan rate = 100 mV s-'). The lower curve was recorded after a 10-s delay at the inltiai potential E,, = -1.1 V.
Oxygen removed
and the stripping voltammograms were recorded by using the procedure described above, except the electrode was rotated at 500 rpm and t&p was 60 8. The determination was conducted by the standard additions method using microliter aliquots of a standard Pb(1I) solution which were added directly to the electrochemical cell. -1.0
RESULTS AND DISCUSSION Reduction of Oxygen at TMF Electrodes. In order to examine the cathodic reduction of dissolved oxygen at TMF electrodes during a square-waveanodic stripping experiment, square-wave voltammograms were recorded in a nondeaerated solution containing only the supporting electrolyte (0.1 M acetate buffer, pH 4.8). Figure 1shows the voltammograms obtained at a stationary TMF electrode, by scanning the potential from +0.1 to -1.1 V (the upper curve) and from -1.1 to +0.1V (the lower curve). On the cathodic curve, the peak at -0.15V corresponds to the reduction of oxygen to hydrogen peroxide, and the increase in the current around -1.0 V is due to the reduction of hydrogen peroxide to water (5). The anodic voltammogram, which was obtained after a 10-sdelay at -1.1 V, shows a very low current a t -1.1 V and no peak at -0.15 V. This indicates that the delay at the initial potential was sufficiently long to deplete the oxygen from the surface of the electrode and that the scan rate used (100 mV s-l) was fast enough to complete the scan before any oxygen was able to diffuse to the electrode surface. For most of the square-wave stripping work a 200 mV s-l scan rate was used. Comparison of the Effect of Oxygen in DPASV and SWASV. The effect of chemical stripping of depwited metals by dissolved oxygen on the performance of differential pulse anodic stripping voltammetry (DPASV) and of SWASV was investigated. SWASV and DPASV experiments at TMF electrodes were performed in deaerated and nondeaerated solutions of 0.1 M HCl, containing ppb concentrations of added Pb(I1). The scan rate for differential pulse stripping was 5 mV s-l, which is very slow but is required in DPASV in order to achieve good peak resolution. The square-wave stripping scans were conducted with a scan rate of 200 mV s-l. The resulting stripping voltammograms are shown in Figures 2 and 3.
-0.8 -0.6 Potential
-1.0
-0.8 -0.6
,V
Figure 3. Square-wave anodic Stripping voltammograms of 1, 2, and 3 ppb lead in deaerated and nondeaerated solutions of 0.1 M HCI: E,, = -1.1 V, td, = 120 s (at 1000 rpm); E,, = 24 mV, f = 100 Hz, E, = 2 mV (scan rate = 200 mV SI). The square-wave stripping voltammograms obtained in the presence of oxygen show no loss of the stripping peak current compared to the voltammograms in the absence of oxygen. However, the differential pulse stripping voltammograms showed a significant loss of the stripping peak current of lead, as well as a large increase in the background current. These observations indicate that the scan rates commonly used in DPASV are slow enough to see the effects of the chemical oxidation of the amalgam by oxygen, and, therefore, DPASV at TMF electrodes can not be used effectively for trace metal determinations in solutions containing dissolved oxygen. Effect of Film Plating Conditions on Performance of TMF Electrodes in SWASV. A series of experiments was conducted to study any effects resulting from variations in the mercury film plating conditions on the performance of TMF electrodes in SWASV. The mercury films used for the majority of the work described in this paper were plated from deaerated aqueous solutions containing 0.1 M HCl and 1mM Hg(I1). The intention of the experiments presented below was to examine if improved mercury films could be obtained in different plating solutions, which would result in a more sensitive as well as more stable SWASV stripping response. Also, it was interesting to see if TMF electrodes suitable for SWASV could be plated from solutions containing dissolved oxygen. Film plating in the presence of dissolved oxygen would eliminate the oxygen removal step for the plating solution, which could result in a simpler and shorter preparation procedure for TMF electrodes.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
TMF electrodes with films plated in the presence of dissolved oxygen, using the same conditions as those used for plating in the absence of oxygen, were examined by running SWASV voltammograms in nondeaerated solutions. The background voltammograms recorded at these electrodes were very similar to those obtained with the films plated in deaerated solutions. The stripping peaks of lead and cadmium had the usual appearance and increased linearly with the analyte concentration. The peak currents, however, were significantly smaller than those recorded by using TMF electrodes prepared in the absence of oxygen. The experiments described above employed electrodes with preplated TMFs. The goal of the experimentsdiscussed below was to examine if in situ plated mercury films are suitable for SWASV measurementsin the presence of diasolved oxygen. SWASV experiments with in situ plated TMFs on GCRDE were carried out in nondeaerated solutions containing 0.1 M HCl and 1 mM Hg(I1). An electrode conditioning step was used, during which a potential of +0.4 V was applied to the electrode to anodically strip the previous mercury film. Stripping voltammograms recorded after this step were distorted by frequent and irreproducible current spikes, which made the performance of such electrodes unsuitable for anodic stripping measurements. This behavior can be explained by the presence of calomel on the electrode surface, which was formed during the conditioning step, and was not completely reduced to mercury during the next plating/deposition step. Stripping voltammograms of the typical appearance, i.e. without similar distortions, were obtained after the previous film was removed by wiping off the GCRDE surface with a damp tissue. However, since the main attribute of in situ plated TMF electrodes is the convenience of electrochemical cleaning of the electrode surface between runs, the necessity of mechanical removal of the film after each run makes the use of these electrodes in the proposed application highly impractical. Similar experiments with in situ plated TMF electrodes were performed in acetate buffer solutions in place of 0.1 M HCl. And again, poorly reproducible and distorted stripping curves were obtained, unless the glassy carbon surface was wiped off prior to running a new voltammogram. This was caused by traces of chloride ions present as a contaminant in the stripping solution. Similar problems were also encountered during our study of SWASV at mercury-plated graphite fiber microelectrodes (6). In another experiment, a TMF was plated from an acetonitrile solution of l mM Hg(I1) also containing 0.1 M LiC104 as the supporting electrolyte. SWASV voltammograms obtained at this TMF electrode in oxygen-containing 0.1 M acetate buffer yielded higher background currents (e.g. 40 PA, measured at -0.9 V) than those recorded at TMF electrodes prepared in deaerated and nondeaerated aqueous 0.1 M HCl solutions (10 and 20 PA, respectively). In addition, irreproducible stripping peaks were obtained. This study proves that the most reliable TMF electrodes for SWASV work in the presence of dissolved oxygen are those with the film plated in deaerated aqueous HCl solutions. Considering that a TMF electrode can be used for an entire day’s work, the 20-min period required for the purging of the film plating solution can be considered negligible. The stability of TMF electrodes during storage in solutions containing dissolved oxygen was also evaluated. A stationary TMF electrode was allowed to stand for a 14-h period in a nondeaerated 0.1 M HC1 solution, while a potential of -1.1 V was applied to the electrode. As expected, no damage occurred to the film because the oxygen was reduced to water at the applied potential. A stripping peak of 4.3 ppb lead, recorded after this period, was only slightly smaller than the
1327
I2
10
a
a -
-
8
Y
k
6
0
$
2
4
2
0
0
20
10
30
ppb of lead
Flgure 4. Deposition potential dependence of square-wave anodic
stripping peak current of lead in deaerated and nohaerated solutions of 0.1 M HCI. Curve B for E, = -0.9 V was obtained in deaerated
solution. Other curves were obtained in nondeaerated solutions. td, = 120 s (at 1000 rpm): E,w = 24 mV, f = 100 Hz, E, = 2 mV.
corresponding peak obtained immediately after the film was plated. However, when another, identically prepared TMF electrode was allowed to sit for the same length of time without any applied potential, an approximately 3 times lower peak was obtained. On the basis of these observations, one can conclude that if a TMF electrode is stored in air-saturated solutions for an extended period of time, the dissolved oxygen can seriously affect the electrode performance, unless a potential, sufficiently negative to electrochemically reduce the oxygen, is applied to the electrode during storage. Moreover, if a preplated TMF electrode is to be used continuously for stripping measurements in oxygen-containing samples, the standard additions method should be used for quantitations. The calibration curve method should be avoided because it can yield inaccurate results due to gradual changes that occur to the mercury film during its long exposure to dissolved oxygen. SWASV of Lead at TMF Electrodes in Solutions Containing Dissolved Oxygen. Deposition Potential Dependence. The effect of the deposition potential on the square-wave stripping peak current of lead in the presence of dissolved oxygen is illustrated in Figure 4. Standard addition curves based on SWASV measurements in the presence of oxygen show a significant increase in slope when the deposition potential is changed from -0.9 to -1.0 V. The slope increases only slightly when the deposition potential is changed from -1.0 to -1.1 V. The lower slope of the curve for -0.9 V deposition potential (curve A) is due to chemical stripping of the deposited lead by hydrogen peroxide that is formed when oxygen is reduced at potentials more positive than -1.0 V. The standard addition curve obtained from a deaerated solution using a deposition potential of 4 . 9 V (curve B) confirms the effect of hydrogen peroxide. The slope of this curve is similar to the slopes obtained in the presence of oxygen using the deposition potentials at which oxygen is reduced to water, e.g. -1.0 and -1.1 V. On the basis of these results, a deposition potential of -1.1 V was used in this work for all stripping experiments in the presence of dissolved oxygen. Scan Rate Dependence. The effect of dissolved oxygen on the amalgam during the square-wave stripping was evaluated for scan rates in the 10-200 mV s-l range. The scan rate, which in square-wave voltammetry is defined as a product of the square-wave frequency and the potential step, was varied by changing the square-wave frequency while keeping the potential step at a constant value of 2 mV. Stripping voltammograms of 4.1 ppb Pb(I1) were recorded in deaerated and nondeaerated solutions. The data in Table I demonstrates that at scan rates lower than 40 mV s-l, the peak currents measured in the presence of oxygen were significantly smaller
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
Table I. Dependence of Stripping Peak Current on Scan Rate in SWASV of 4.1 ppb Pb(I1) in 0.1 M Acetate Buffer, pH 4.8, with and without Dissolved Oxygen' frequency, Hz
scan rate, mV s-l
5 10 20 40 60 100
10 20 40 80 120 200
'Edep
= -1.1 V,
tdep
peak current, p A oxygen oxygen removed present 0.39 0.89 2.42 4.83 8.13 17.0
0.53 1.08 2.45 4.57 7.67 16.6
Table 111. Effect of Solution Stirring during Deposition, on the Slope of Stripping Peak Current vs Concentration Plot for SWASV of Lead at TMF Electrode in 0.1 M HCl Solutions with and without Oxygen
difference, 90 -26 -17 -1.2 5.7 6.0 2.4
slope, p A ppb-' std dev of slope corr coeff. re1 std dev of slope, %
oxygen removed first second run run
oxygen present first second run run
3.97 0.10 0.999 2.5
3.77 0.12 0.999 3.1
3.75 0.07 1.000 1.7
3.63 0.06 1.000 1.6
= 120 s (at IO00 rpm); E, = 2 mV.
Table 11. Effect of 1000 rpm Electrode Rotation during Stripping Scan, on the Slope of Stripping Peak Current vs Concentration Plot for SWASV of Lead at TMF Electrode in 0.1 M HCl Solutions with and without Oxygen
Table IV. SWASV Peak Current Dependence on Square-Wave Frequency for 4.3 ppb Lead in 0.1 M Acetate Buffer in the Presence of Dissolved Oxygen'
f, Hz
oxygen removed oxygen present without with without with rotation rotation rotation rotation slope, pA ppb-' std dev of slope corr coeff re1 std dev of sloDe, %
4.87 0.15 0.999 3.1
4.71 0.16 0.999 3.4
4.35 0.17 0.998 3.9
5 10 20 40 60 80 100 120
1.93 0.61 0.954 31.6
than in the absence of oxygen. This was expected because during slow scanning, more oxygen is allowed to diffuse to the electrode surface and the amalgam is exposed to dissolved oxygen for a longer period of time. As a result, the oxidation by oxygen reduces the amount of the deposited metal available for anodic stripping, which results in lower stripping peaks. Effect of Electrode Rotation during Stripping. The experiments discussed so far have shown that the oxidation of the amalgam by dissolved oxygen during square-wave stripping is not significant if the stripping step is conducted in quiescent solutions. The goal of the experiment presented here was to observe if the enhanced flux of oxygen to the electrode surface during the stripping step, generated by forced convection created by electrode rotation, can cause a significant oxidation of the amalgam. The effect of electrode rotation on the SWASV voltammograms of lead at a TMF electrode was examined in 0.1 M HCl solutions with and without oxygen present. The electrode was rotated at a rate of lo00 rpm during both the deposition step and the stripping step. The results obtained are summarized in Table 11, in terms of the peak current vs concentration dependence. In deaerated solutions, the slope of the standard addition curve decreased by 3% when the electrode was rotated during the stripping step. In the presence of oxygen, however, a 57% decrease in the slope resulted from the electrode rotation. Moreover, no stripping peak was detected for l ppb of lead if the electrode was rotated during the stripping. These results indicate that the high rate of convective mass transport of oxygen to the surface of a rotating TMF electrode causes a significant oxidation of the amalgam during the stripping step, despite the very short time of the square-wave scan. As a result, the stripping peaks decrease, the detection limit increases, and the stripping peak vs concentration dependence becomes less linear. The data obtained by using a stationary TMF electrode proves that in the absence of forced convection during the stripping step, the SWASV peaks are not affected by dissolved oxygen. Effect of Solution Stirring during Deposition. The effect of solution stirring by a magnetic stirring bar during the deposition step was also examined. An arbitrary stirring rate was chosen by setting the control dial on the Fisher Versamix
'Data Edep
stripping peak current, p A film 1 film 2 film 3 0.23 0.63 2.01 4.02 7.19 10.8 15.3 21.2
0.39 0.88 1.75 4.29 7.75 12.6 19.2 28.2
0.39 0.89 2.42 4.83 8.13 12.1 16.9 22.4
for three identically prepared mercury film electrodes.
= -1.1 V, tdep = 120 s (at lo00 rpm); E,, = 24 mV, E, = 2 mV.
stirrer to position 1 and by positioning the TMF electrode approximately 1cm from the stirring bar placed on the bottom of the electrochemical cell. After the deposition step, the stirring bar was interrupted and a 10-s equilibration period was observed prior to recording the stripping voltammogram. Peak current vs concentration plots were obtained for lead in the presence and in the absence of dissolved oxygen, which are summarized in Table 111. The slopes of the standard addition curves for nondeaerated solutions are only slightly smaller than for deaerated solutions and the correlation coefficients are excellent. This proves that dissolved oxygen has no significant effect on the stability of the amalgam in SWASV experiments which employ solution stirring during the deposition step. Square- Wave Frequency Dependence. The dependence of the stripping peak current of lead on the square-wave frequency was examined both in the absence and in the presence of dissolved oxygen. Data obtained by using three identically plated mercury film electrodes in a nondeaerated solution are listed in Table IV. The theory for SWASV a t mercury film electrodes (7), which was confirmed experimentally using silver- and iridium-based mercury film electrodes (8, 9),predicts that the square-wave frequency dependence of the stripping peak depends on a value of the film thickness parameter, A, which is defined as A = l(f/D)'f2 where 1 is the film thickness, f is the square-wave frequency, and D is the diffusion coefficient of the analyte metal in mercury. For A values lower than 0.1, which represent the thin film region, the peak current should be directly proportional to the frequency. For the mercury films (1 = 0.33 pm) and the frequencies used in our experiment, and assuming D (for Pb) of 1.25 X cm2s-l (IO), the calculated A varied between 0.021, for f = 5 Hz, and 0.102, for f = 120 Hz. With the data disregarded, for the frequencies 520 Hz, a t which, as was shown earlier, the amalgam is partially oxidized by dissolved oxygen during the stripping scan, the measured peak currents showed nonlinear dependence on the frequency. This
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
Table V. Dependence of SWASV Peak Current on Deposition Time and Potential Step, for 1 ppb Lead in Nondearated Solution of 0.1 M HCl
peak current, p A
Table VII. Reproducibility of SWASV Peak Current of Lead in Nondeaerated Tap Water Samples'
peak current, WA
E, = 2 mV
E. = 6 mV
E, = 10 mV
120 240 360
5.0 (2.5) 10.4 (2.6) 15.2 (2.5)
7.3 (3.7) 15.2 (3.8) 22.9 (3.8)
8.1 (4.0) 16.4 (4.1) 24.3 (4.1)
1
2
OEde = -1.1 v; E,, = 24 mv, f = 100 Hz. Peak currents normalizes to a 60-5deposition are given in parentheses.
a
&p
Table VI. Effect of Supporting Electrolyte Concentration on the Slope of Stripping Peak Current vs Concentration Plot for SWASV of Lead
HCl concn, M 0.1 0.01 0.001
slope, p A ppb-'
E,, V
3.50 -0.54 1.86 -0.48 no peaks detected
mean std dev
sample no.
tdep, s
bkg curr,O p A
3 3 14
Background current measured at -0.80 V. indicates that our measurements were conducted under the conditions correspondingto a transition between the thin and the thick film regions, which is contrary to what one would expect based on the calculated values of A. However, considering that mercury films on glassy carbon are known to have a nonuniform character, in terms of thickness, the observed nonlinearity is understandable. The frequency dependence experiments conducted by using the same mercury film electrodes, but in deaerated solutions, gave a similar, nonlinear dependence of the peak current on the frequency. This proves that the observed deviation from the expected linear dependence is not related to the presence of dissolved oxygen. Deposition Time and Potential Step Dependencies. The dependence of the square-wavestripping peak current of lead on both the deposition time and the potential step, E,, was examined in the presence of dissolved oxygen. The results of this experiment are given in Table V. When the voltammograms were recorded at different potential steps, the runtime data smoothing function on the 384-B polarographic analyzer had to be disengaged in order to obtain meaningful results. If left engaged, stripping peaks at higher potential steps were very broad and the peak current decreased as the potential step was increased. As expected, the peak current was observed to be a linear function of the deposition time. For all of the deposition times used, the peak current increased by a factor of 1.6 for a 5-fold increase in the E, (2 to 10 mV), and by a factor of 1.5 for a 3-fold increase in the E, (2 to 6 mV). The theory for SWASV at TMF electrodes (7) predicts that for mercury films in the thin film region, a %fold increase in the nE, (10 to 30 mV) should increase the peak current by a factor of 1.2. This is in a relatively close agreement with the experimental value, although the larger increase in the peak current observed in the experiment may suggest that the mercury film used was not completely in the thin film region. This conclusion is in agreement with the results of the frequency dependence experiment. Supporting Electrolyte Concentration Dependence. The effect of supporting electrolyte concentration on the SWASV peak of lead was investigated in solutions containing dissolved oxygen. The aim of this study was to determine if it would be possible to use SWASV at TMF electrodes for anodic stripping analysis of samples containing only small concentrations of electrolytes, such as tap water. Table VI contains the results of SWASV of lead in the 1-3 ppb range, obtained
1329
11.9 11.5 10.1 10.6 10.4
12.1 11.8 10.4 10.5 10.5
11.8 11.3 9.7 10.2 11.1
12.0 10.9 10.7 10.9
re1 std dev, %
11.7
0.40
3.4
10.5
0.38
3.6
Solutions contained 9 mL of tap water and 1 mL of 0.1 M HCl. = -1.1 V, tdep = 60 s (at 500 rpm); E , = 24 mV, f = 100 Hz, E,
= 2 mV.
in solutions with various concentrations of HC1. The decrease in the slope of the peak current vs concentration plot and the positive shift of the peak potential, observed as the concentration of HC1 was decreased to 0.01 M, indicate an increased effect of ohmic drop. However, considering that the background current and the half-peak width did not increase, it should be possible to obtain meaningful stripping results in this medium, despite the reduced current sensitivity. When the HCl concentration was lowered to 1 mM, severe changes in the stripping voltammograms were observed resulting from ohmic drop effects. One of the most noticeable differences was a huge rise in the background current. This increase was caused by a larger separation between the forward and reverse curves, which is indicative of a large increase in the charging current contribution to the measured currents on both the forward and reverse pulses. No stripping peaks were detected even for 3 ppb Pb(I1). The absence of stripping peaks may be due to the shift of the stripping signal to potentials more positive than -0.4 V which was the potential where the stripping scan was discontinued. These observations, prove that 0.001 M HCl is not a suitable medium for conducting SWASV measurements at TMF electrodes having a large surface area. Our recent study showed, however, that anodic stripping measurements in solutions with supporting electrolytes at concentrations below 0.01 M are possible with the use of microelectrodes (6). Peak Current us Concentration Dependence. Concentration dependence of the SWASV peak of lead was examined in deaerated and nondeaerated 0.1 M HC1 solutions, for Pb(I1) concentrationsranging from 0.5 to 10 ppb. Calibration curves, obtained by a least-squares fitting of the data points, were linear and had slopes of 3.68 and 3.55 PA ppb-' for deaerated and nondeaerated solutions, respectively. Detection limits were determined by using the formula dl = 3S,/,/b, where S,,, is the standard deviation, along the y axis, of the data points with respect to the fitted line, and b is the slope of the calibration curve (10). A detection limit of 0.03 ppb (1.5 X M) was obtained for Pb(I1) in both deaerated and nondeaerated solutions. The very similar slopes of the calibration curves and the same detection limits obtained in solutions with and without oxygen provide yet another proof of the great potential of square-wave voltammetry for anodic stripping measurements at TMF electrodes in the presence of dissolved oxygen. SWASV Determination of Lead in T a p Water Samples without Deaeration. The goal of the experiments that are discussed below was to give a practical proof of the potential and advantages, especially in terms of simplified sample preparation, of SWASV at TMF electrodes for trace metal analysis. The SWASV determination of lead in tap water samples was selected for this purpose. The sample^ contained 0.01 M HC1 added as supporting electrolyte. As shown in the previous section, this concentration of HC1 was the lowest at which meaningful measurements can be made using SWASV at large TMF electrodes, in solutions containing oxygen.
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
Table VIII. Results of SWASV Determination of Lead by Standard Additions Method"
concentration of lead, ppb individual results mean
sample no. 19.3 9.3 22.1 10.2
1 2 3 4
24.7 9.9 26.4 10.3b
22.8 11.5 24.5
19.3
21.5 10.2 24.3
std dev 2.7 1.1
2.1
'Experimental conditions were the same as in Table VII. Determined bv AAS. Table IX. Results of the SWASV Determination of Lead by the Standard Additions Method in Tap and 'Pure" Water Samplesa
sampleb tap I
slope,' fiA ppb-'
std dev of slope
0.435 0.436 0.456 0.518 0.599 0.736 0.692 0.689 0.713 0.732 0.758 0.685 0.472 0.538 0.662 0.752 0.724 0.801 0.855 0.624 0.698 0.764 0.814
0.014
1 2 3 4 1 2 1 2 3 4 5 6d
tap I1 pure I
pure 11
~d 2d
3d 4d
pure 111
ld 2d
pure IV
3d Id 2d
3d 4d
0.006
0.013 0.009 0.003 0.017 0.021 0.010 0.020 0.008 0.010 0.014 0.012 0.008 0.004 0.136 0.004 0.017 0.018
corr coeff 0.999 1.OO0 0.999 1.000 LOO0 0.999 LOO0 1.OOO LOO0 1.OO0 LOO0 LOO0 0.999 LOO0 LOO0 0.969 1.OO0 1.OO0 1.000
determined lead, ppb 21.8 27.2 24.9 21.5 14.8 14.7 0.0 0.0
0.0 0.0 0.0
25.0 24.8 24.8 22.2 24.0 26.5 27.2 23.1 24.6 24.4 23.0 23.4
'Experimental conditions were the same as in Table VII. 'Pure" water samples contained 9 mL of deionized distilled water and 1 mL of 0.1 M HCl. *Eachroman number representa a sample analyzed on a different day. Each arabic number represents a different aliquot of the same sample. 'Slope of the standard addition curve. dSamplecontained 23 ppb of added Pb(I1). The reproducibility data for the SWASV peak current of lead measured in two tap water samples is given in Table VII. A very low value of the relative standard deviation proves excellent precision of the measurement, despite that untreated tap water samples were used. The precision of lead determinations by the standard additions method in different aliquots of the same tap water sample was also evaluated. The results for four different tap water samples, which are listed in Table VIII, demonstrate
very good precision of the lead determination. Lead in sample 4 was also determined by atomic absorption spectroscopy (AAS) with electrothermal atomization, using a Varian 1475 atomic absorption spectrophotometerequipped with a GTA-95 graphite furnace. The AAS result agrees very well with the result of the SWASV determination. Comparison measurements were performed with tap water samples and "pure" water samples. The latter were prepared by using doubly distilled and deionized water instead of tap water. HCl(O.01 M) was added to both tap and "pure" water samples. Some samples of "pure" water were spiked with lead to give a concentrationof 23 ppb, which was the concentration of lead found in tap water samples analyzed earlier. For each sample, a series of determinations by the standard additions method was conducted,using the same, freshly prepared TMF electrode. Data for the standard addition plots are given in Table IX, together with results of the lead determination. A gradual increase of the slope of the standard addition curve can be seen for each series of the standard additions. This was observed for both "pure" and tap water samples, and, therefore, unknown components of the tap water must be ruled out as a possible cause of the increasing slopes. Changes in the mercury film, most probably caused by exposure of the film to air,can be responsible for the observed increase of the slopes. These changes, however, did not affect the linearity of standard addition curves and had no significant effect on the precision and accuracy of the lead determination. The results of water analysis prove that SWASV at TMF electrodes is a fast and convenient method of trace metal determination, capable of producing accurate results without a time-consuming deaeration of samples. SWASV of Lead in Acetate Buffer Solutions. The majority of SWASV experiments at TMF electrodes conducted in this study involved using 0.1 M HC1 as the supporting electrolyte. This highly acidic medium (pH 1)was chosen to assure that the electrochemical reduction of oxygen a t the deposition potential, which occurs concurrently with the reduction of analyte metal ions, proceeds to water and not to hydroxide ions ( 4 ) . It is commonly known that, in the absence of more effective ligand, hydroxide ions can indulge the stripping process. The aim of the experiments presented below was to determine if it is possible to use a less acidic medium, such as acetate buffer, for conducting SWASV measurements in the presence of dissolved oxygen. SWASV voltammograms for various concentrations of lead were recorded in nondeaerated solutions of 0.1 M acetate buffer (pH 4.8). Several calibration curves obtained were all linear and had an average slope of 3.5 MA ppb-', which is in the same range as the slopes obtained in 0.1 M HCl solutions. This proves that a successful SWASV measurement in the presence of dissolved oxygen can also be made in slightly acidic media. SWASV of Cadmium at TMF Electrodes in the Presence of Dissolved Oxygen. SWASV experiments were also conducted to investigate the effect of dissolved oxygen on the
Table X. Slopes of Stripping Peak Current vs Concentration Plots for Cadmium in SWASV'
solution no.
oxygen present
Edep,
tdap,
S
slope, PA ppb-'
std dev of slope
corr coeff
intercept,
V -1.0 -1.0 -1.1 -1.1
240 240 240 240 120 120 120 120
3.37 3.38 7.21 7.44 2.03 2.76 3.62 4.11
0.52 0.09 0.17 0.17 0.09 0.04 0.10 0.29
0.999 0.997 0.998 0.998 0.996
1.76 0.00 2.10 2.13 1.22 0.09 4.07 0.36
-1.1 -1.1 -1.1 -1.1
1,OoO
0.999 0.995
PA
'Solutions 1 through 6 contained 0.1 M HC1. Solutions 7 and 8 contained 0.1 M acetate buffer. E., = 24 mV. f = 100 Hz, E. = 2 mV.
Anal. Chem. 1990, 62, 1331-1338
SWASV peak of cadmium. The deposition time dependence of the square-wave stripping peak of 1 ppb cadmium in a nondeaerated solution of 0.1 M HC1 was linear, with a slope of 1.22 r A min-' and a correlation coefficient of 1.OOO. Concentration dependence experiments were performed initially in 0.1 M HC1 solutions, using a Cmin deposition time. Additions of microliter volumes of a standard Cd(I1) solution were made to a 10-mL volume of the supporting electrolyte solution in the cell, and a SWASV voltammogram was recorded after each addition of the standard. Slopes of the obtained peak current vs concentration curves are listed in Table X. Correlation coefficients of all of the obtained calibration curves were excellent. However, similarly to what was observed for lead, the slopes of the calibration curves for cadmium varied from film to f i i and increased for each series of calibrations on the same TMF electrode. For example, the slopes of three consecutive calibration curves obtained with the same TMF electrode were 2.76, 3.45, and 3.62 rA ppb-'. Similar instability of the slope was also observed in the 0.1 M acetate buffer medium. There is no evidence that dissolved
1331
oxygen has any significant effect on the stripping peaks of cadmium in both media.
LITERATURE CITED (1) Vydra, F.; Stulik, K.; Julakova, E. €lectrmhem/ca/ StrtpPing Ana&&; Wilev: Chichester. Sussex. 1976. (2) Osteryoung, R. A.;Osteryoung. J. Rtbs. Trans. R . Soc. London, A 1981, A302. 315-326. (3) Osteryoung, J.; O'Dea, J. J. In Electroanalytical Chemistry; A series of Advances; Bard, A. J., Ed.; Marcel Dekker: New York, 1986 Vol. 14, pp 209-308. (4) Wojciechowski. M.;Go, W.; Osteryoung, J. Anal. Chem. 1985, 57, 155-158. ( 5 ) Hoare, J. P. In Encycbpdla of EktrochernWy of the Ekmenfs; Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Voi. 11, Chapter 5. (6) Wojciechowski, M.; Balcerzak, J. J. Anal. Chem., in press. (7) Kounaves, S. P.; ODea, J. J.; Chandresekhar, P.; Osteryoung, J. Ana/. Chem. 1907. 59, 386-389. (6) Wikiei, K.; Osteryoung. J. Anal. Chem. lS89, 67, 2086-2092. (9) Wechter, C.; Osteryoung, J. Anal. Chem. 1989, 61, 2092-2097. (10) Miller, J. C.; Miller, J. N. Statlsffcsof Analj*lca/ Chemistry; Ellls !-lorwood Limited: Chichester. England, 1984; pp 96-100.
RECEIVED for review October 16, 1989. Accepted March 15, 1990.
Computer-Based Instrument for Alternating Current Impedance and High-Frequency Alternating Current Electrochemical Measurements Peixin He* and Xiaoming Chen Department of Chemistry, Fudan University, Shanghai 200433, People's Republic of China
The hardware and software deslgn of an inexpenslve computer-based Instrument for ac impedance and hlgh-frequency ac electrochemicalmeasurements Is described. The upper llmlt of the frequency range Is 100 kHz. Look-up tables are used to speed up the digital function generatlon. Two analog multlpllers have proved extremely helpful in data acqulsitlon and slgnal procersing. For ac impedance measurement, the system Is able to sweep the frequency, to select the sensltlvlty scale of the current-to-voltage converter, and to callbrate the current response. Data can be reported numerically and graphically. The sotutlon reststance and the double layer capacltance can be subtracted from the total cell Impedance. Varkus data pbts are available. These pkts greatly ease the interpretatlon of Impedance data. As the ac perturbation slgnai and the reference channel slgnals can be programmed Independently, other electrochemlcai measurements have been accompilshed readily, Including hlgh-frequency phaseselective ac voltammetry, high-frequency phase-selective second harmonic ac vottammetry, and hlgh-frequency square wave voltammetry. The performances of the instrument are also demonstrated.
INTRODUCTION Computer supervision of measurements provides many advantages, such as flexibility, automation, and intelligence. Various computer-based electrochemical instruments have been developed in the past 2 decades (1-11). On the other hand, the shortest experimental time scale available for most 0003-2700/90/0382-1331$02.50/0
computerized systems is about a millisecond or longer; faster experiments are often limited by the rate of function generation and data acquisition. The situation is even worse if a complicated waveform is required to perturb the electrochemical cell. The time needed to calculate the excitation signal could be unreasonably long. In this paper, we report the design of an inexpensive computer-based instrument for ac impedance and high-frequency ac electrochemical measurements. Look-up tables are used to speed up the function generation. Two analog multipliers have proved extremely helpful in data acquisition and signal processing. The idea of using look-up tables is similar to that proposed by Bond (7) and Smith (4, 5 ) , but much higher frequency (100 kHz) and better phase resolution are achieved in phase-selective ac voltammetry and the need of an expensive high-speed data acquisition system is eliminated in ac impedance measurements. The instrument offers great flexibility. High-frequency phase-selective second harmonic ac voltammetry, high-frequency square wave voltammetry, and cyclic voltammetry have been accomplished successfully with the same hardware configuration. High-frequency ac experiments are useful in ultramicroelectrode applications (12-14). By use of the concept of this design, development of other type high-frequency ac measurements, especially those involving complicated ac waveforms, can also be expected. ac impedance measurement is based on the correlation technique; the system is able to sweep the frequency, to select the sensitivity scale of the current-to-voltage ( i / V) converter, and to calibrate the current response. The solution resistance and double layer capacitance can be subtracted from the total impedance according to the equivalent circuit model of the 0 1990 American Chemical Society
