Staircase Voltammetric Stripping Analysis at Thin Film Mercury Electrodes Uri Eisner,’ John A. Turner, and R. A. Osteryoung* Department of Chemistry, Colorado State University, Fort Collins, Colo. 80523
Anodic stripping voltammetry from a rotating thln film mercury electrode has been carrled out employing a staircase waveform. Experimental results are shown to be in excellent agreement with previously developed theory. The method is of the same order of sensitivity as dlfferentlal pulse stripping, but is much faster to perform. Reproducibillty is better than 1% on the same film and 5 % on replated fllms at the 10-ppb level.
Anodic stripping voltammetry (ASV) is a powerful analytical tool for trace metal analysis. Two kinds of electrodes are commonly employed, the hanging mercury drop electrode (HMDE) and the thin film mercury electrode (TFME).The first stage in ASV a t mercury electrodes is a pre-concentration step in which the trace metal under consideration is reduced and is deposited in the electrode. This is carried out by controlled potential electrolysis for a certain time. The solution is usually stirred or the electrode rotated during this period. In the second step, the potential is scanned anodically, the trace metal is oxidized, and the resulting current is measured. A linear relationship between the peak current and the solution concentration of the deposited constituent is the basis for the analytical determination. Different wave forms have been applied for the second step. The most common wave form used is that of linear scan voltammetry. High sweep rates can be employed to increase sensitivity but as the charging current depends linearly on sweep rate, the result is a steep background current. Perone et al. ( I ) used derivative techniques to overcome this problem. More sophisticated wave forms used are square wave (2), phase sensitive ac ( 3 , 4 ) ,second harmonic ac ( 5 ) ,and differential pulse (6, 7). Recently Christie and Osteryoung (8) have presented a theoretical treatment for a staircase wave form employed in stripping from a thin film. In this paper, an experimental study of the staircase ASV will be presented. The various parameters governing the stripping current were studied and analytical applications for the determination of low level trace metals were demonstrated. The TFME which is used in this study is superior to the HMDE for trace analysis, allowing determination of lower levels of trace metals in a shorter period. The stripping current for the HMDE depends on the concentration of the metal in the amalgam and is linearly dependent on the radius of the drop. Higher stripping currents can be achieved by increasing the electrode area but one is limited by the instability of larger drops. For the TFME, the stripping current depends on the quantity (charge) of trace metal in the amalgam (under thin film conditions) independent of film thickness. The amount of metal reduced into the amalgam under constant conditions is dependent on the area of the electrode. Large TFME of 0.5 cm2 are commonly employed and a 106-fold increase in con-
1 Present address, Israel Chemicals Ltd., P.O. Box 7164, Tel Aviv, Israel.
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centration of trace metal in the amalgam relative to its concentration in solution can easily be obtained. The film is easily produced and is stable for a long period. The electrode is rotated by a synchronous motor a t high speeds which results in excellent reproducibility as mass transport to the electrode is unaffected by minor changes in cell geometry.
EXPERIMENTAL Instrumentation. The staircase wave form was applied and current-potential curves were evaluated by a Digital Equipment Corporation PDP-12 computer. An HP xy Recorder model 7009 B driven by the computer was used to plot i-E curves. PAR models 173 and 174 were used as potentiostats. The working electrode was a mercury thin film deposited on a glassy carbon disk (0.2 cm2).The disk was press fitted ints a Teflon sleeve and polished to a mirror finish before mercury plating. The electrode was rotated by a Pine Instrument Rotator a t 400-10 000 rpm. The reference electrode Ag/AgCl (satd KCl) was made from a silver wire coated with silver chloride immersed in KCl contained in a Teflon tubing and separated from the sample solution by a porous glass plug (Corningglass Vycor No. 7930). The counter electrode was a platinum spiral immersed in the sample solution. The cell which had a capacity of 50 ml was made from a quartz tube 50 mm-0.d. The nitrogen bubbler was made of Teflon. Reagents. Mercury stock solution, Hg(N03)~ 0.01 M was prepared by dissolving triply distilled mercury in nitric acid. Mercury plating of the electrodes was carried out in a solution 2 X 10-4-2 X M. Acetate buffer, 2M, was prepared from reagent grade acetic acid and ammonia. All solutions were diluted by distilled and deionized water. Procedure. Mercury Plating. After subtraction of the background current which was in the order of 5 PA, we found a 90%efficiency of mercury plating from 2 X M Hg(N03)~.As practical films were M Hg(N03)~in 0.08 M-0.04 M produced from a solution of 2 X acetate buffer where the plating current was about 500 PA, one can assume an efficiency of 100%. According to Stulikova (9),the mercury film is homogeneous for a plating potential of about -1.0 V (SCE). Plating potentials of -0.9 to -1.0 V were employed throughout our study. The electrode was rotated between 2500 and 6400 rpm and the solution purged by a nitrogen stream during the Hg-plating process. Incomplete coverage was easily observed by the shiny black appearance of the uncovered areas in contrast to the grayish color of a homogeneous film. At the end of the plating time, the potential was switched to 0.0 V in order to dissolve any trace metal co-deposited with the mercury. Film thicknesses were calculated from the coulombs passed (using the PAR Model 179 coulometer) during mercury plating, assuming 100%efficiency and homogeneous coverage and the electrode area. Preconcentration. The cell was then disconnected from the potentiostat, the bottom part removed, and the electrodes and bubblers were rinsed with distilled water. A second cell bottom holding the sample solution was then placed in the electrode assembly and the potential switched to 0.0 V. Five-minute purging of the solution by nitrogen preceded the determination. Electrolyses times of 2 to 10 min were employed and measured with a stop watch. The electrode was rotated and the solution purged by purified nitrogen during the plating stage. After the pre-selected plating time, the nitrogen stream was diverted above the solution, the rotation stopped, and the staircase scan applied following a rest period of 30 s.
RESULTS AND DISCUSSION The current potential dependence results in a peak (Figure 1)and a peak current i, is determined by i, = g,(nAE/T) q m (81, where g, is a normalized stripping factor (dependent on AE and the thickness parameter 1 2 / ( D 7 ) ) ;is the step width
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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Flgure 1. Characteristic peak for staircase ASV Lead 40 ppb, acetate buffer 4 X M,2-min plating time, A€ 16.7 ms, I = 15 000 A, instrument, PAR 174
Figure 2. Dependence of peak current on A€
= 5 mV, ‘T =
(ms); 1 is the thickness of film (cm); D is the diffusion coefficient (cm2/s); AE is the step height (mV); and qm is the charge (yC) equivalent to the amount of trace metal plated into the mercury. For very thin films, g, will be a function of AE only (Ref. 8 , Figure 6). This thickness independent function will be denoted as g,’ and thus
Lead 40 ppb, acetate buffer 4 X M, 2-min plating time, 7 = 20 ms, I = 15 000 A, instrument, PAR 174. ( 0 )Experimental: (+)Theoretical -0.6
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The dependence of i, on the various parameters was studied as well as current and potential dependence on the thickness factor 12/D7. qm.The quantity of trace metal deposited should depend on the effectiveness of mass transport for constant plating time. The dependence of stripping peak current on rotation speed a t fixed plating time was investigated. A linear relationship of i, vs. square root of rotation speed, w was expected by the Levich equation (10) and was observed from 400-8100 rpm with a slope of 0.59 yAlw1/2. A decrease of the peak current a t 10 000 rpm can be attributed to accumulation of bubbles on the electrode surface caused by the turbulence in solution. The high stability of the mercury film is manifested by its usefulness a t high rotation speed. Dependence on r . The dependence of i, on 7 was studied in the range of 6-50 ms. When the PAR 174 was used as a potentiostat, linearity of i, on 1/r broke down for 7 less than 11ms because of potentiostat response. Using the PAR 173, linearity was observed throughout the range studied. 7 values can be synchronized with the 60-cycle line frequency which results in lower noise levels. i, Dependence on AE. Theoretical dependence of peak current on AE is curved. The explicit dependence of i, on AE is linear (Equation l ) , but as g,’ depends on AE, one ends up with a curved relationship. It was found, experimentally, that i, followed the theoretical curve up to a A E of 6 mV (Figure 2) and then deviated positively following a straight line up to 9 mV. The higher AE values are employed to gain higher sensitivity. The upper practical limit for step height, A E , is 10 mV due to peak resolution problems. For most cases, AE of 5 mV is recommended to achieve good resolution with reasonable sensitivity. Dependence on the Thickness of the Film-1. Theoretically, one expects a broad range of film thickness in which i, is independent of 1 (8). However, this range also depends on
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Figure 3. Dependence of peak potential on step time Cadmium 50 ppb. acetate buffer 4 X instrument, PAR 173
M, plating time 2 min. I = 10 000 A,
AE and 7 . For A E = 10 mV and r = 20 ms, i should be independent of 1 in the range 500 < 1 < 3500 This independence of i, on 1 was verified; the peak current on stripping remained essentially constant for 2500 8, < 1 < 30 000 A. A drop in the peak current was observed for 1 = 60 000 A. For 1 values less than 1000 A, i, decreases because of incomplete coverage of the electrode. One should note the shift of E , with change in the thickness parameter, 12/D7 (8).In the thin film region, the peak potential should be close to a linear function of log (12/Dr).To verify this change, E , dependence on r a t fixed 1 was plotted and a shift of 77 mV was observed for cadmium (Figure 3). This is in fair agreement with the theoretical value of approximately n.30 mV shift in that region of 12/Dr as determined from previous calculation (Ref. 8, Figure 3). Analytical Aspects. Calibration Curves. Peak currents were linearly dependent on trace metal concentration when all other parameters were constant (Figure 4). One should note the drop in i, for concentrations greater than 30 ppb. When plating time was decreased to one half, and the value of i , observed was multiplied by a factor of two, the i, calculated brought up the point right on the line. In a second set of ex-
x.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976 * 1609
inside the staircase step has been studied by other workers (11, 12) but has not been applied to stripping. This case was not treated theoretically. Analytical Application. Cadmium and lead were determined in 1.6 X 10-3 acetate buffer by double standard additions. With a Hg film thickness of 10 000 A, an electrolysis time of 10 min, a AE value of 10 mV, and 7 of 20 ms, the concentration of Cd in the electrolyte was found to be 0.1 ppb (-1 X M) while the P b was found to be 0.06 ppb (-3 X MI.
CONCLUSIONS
Figure 4. Calibration curve for cadmium Acetate buffer 2 X M, 3-min plating time, A€ = 5 mV, 7 = 16.7 ms, l = 10 000 A, instrument, PAR 173
Staircase ASV is a sensitive and rapid method for determination of trace elements on a level of 0.1-10 ppb. This method compares with Differential Pulse Stripping but is much faster. Stripping times for DPS are several minutes compared to less than 2 s for staircase stripping. The experimental results agree well with the theory developed (8). I t is worthwhile to note that carrying out a successful analysis by staircase ASV a t a TFME does not require adherence to stringent conditions. One has a large leeway in choosing film thickness, electrolyte concentration, step height, and step width. Our recommended conditions are a film thickness of 2500-15 000 A, a plating solution of 2 X 10-4-2 X lom3M Hg(N03)2, supporting electrolyte M, AI3 in the range of 5-10 mV, r as short as possible compatible with the rise time of the system, and a rotation speed of 1600-8100 rpm.
ACKNOWLEDGMENT periments, once the curve became nonlinear, the thickness of the film was doubled, the experiment repeated to deposit the same qm;this caused the current to assume its correct value. The reason for this deviation from linearity is unknown. As this method is suitable for analytical determinations of 10 ppb and less, this deviation from linearity does not impose any difficulty. Plating Time. i , was linear with plating time q m (of trace metals) in the range of 2-10 min. Reproducibility. Repetitions of plating and dissolution using the same film resulted in a reproducibility of better than 1%a t the 10-ppb level. One can employ the same film for an entire day so long as the electrode is kept in solution. Reproducibility of i, for different films is less than for the same film but in our hands is better than 5% for a 10-ppb trace element level. Resolution of Adjacent Peaks. Better resolution of overlapping peaks can be achieved by increasing r to 50 ms and measuring the current inside the step allowing the tail of the preceding peak to decay to the background. Measurement
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The continued interest in this work shown by Joseph Christie is appreciated.
LITERATURE CITED (1) S.P. Perone and J. R. Birk, Anal. Chem., 37,9 (1965). (2)G. D. Christian, J. Electroanal. Chem., 23, 1 (1969). (3) N. Velghe and A. Claeys, J. Electroanal. Chem., 32,229 (1972). (4) W. L. Underkofler and I. Shain, Anal. Chem., 37,221 (1965). (5) M. Stullkova and F. Vydra, J. €lectroanal. Chem., 42, 127 (1973). (6) J. 9. Flato, Anal. Chem., 44, ( I I ) , 75A(1972). (7) T. R. Copeland, J. H. Christie, R. A. Osteryoung, and R. K. Skogerboe, Anal. Chem., 45, 2171 (1973). (8) J. H. Christie and R. A. Osteryoung. Anal. Chem., 48, 869 (1976). (9) M. Stulikova, J. Electroanal. Chem., 48, 33 (1973). (IO) V. E.Levich, “Physico-Chemical Hydrodynamics,” State Publishing House of Physico-MathematicalLlterature, Moscow, 1959. (11) D. R. Ferrier and R . R. Schroeder, J. Electroanal. Chem., 45, 343 (1973). (12) J. J. Zipper and S.P. Perone, Anal. Chem., 45, 452 (1973).
RECEIVEDfor review March 15,1976. Accepted May 17,1976. This work was supported in part by the National Science Foundation under Grant No. MPS575-0332 and by the Office of Naval Research under Contract N00014-76-C-0092.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
