Trace Zinc Determination in Acid Media by Differential Pulse Anodic Stripping Voltammetry at a Hanging Drop Mercury EIect rode Harry Blutstein and Alan M. Bond” Department of Inorganic Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia
On collection, natural water samples are frequently acidified to minimize interactions of the sample and container. With dc anodic stripping voltammetry, the direct determination of zinc in acidic media is not possible because of the presence of the hydrogen ion wave, and the solution has to be buffered to higher pH. In the present work, it is demonstrated that, for a wide range of natural water systems, trace amounts of zinc can, in fact, be determined directly in acidic media by differential pulse anodic stripping voltammetry at a hanging drop mercury electrode. Avoidance of the buffer addition stage eliminates one of the greatest potential sources of contamination and permits Zn, Cd, Pb, and Cu to be determined directly and simultaneously on the acidified sample.
Anodic stripping voltammetry (ASV) has been used extensively for trace determination of heavy metals ( 1 , 2 ) . In many environments, for example aquatic media, the natural ionic strength arising from dissolved salts is sufficient to act as the supporting electrolyte. T h e need t o add chemicals to the solution in anodic stripping voltammetry for any direct reason is therefore eliminated, and a possible source of contamination of the sample is avoided. However, chemicals may still have t o be added on some occasions to minimize undesirable interactions between the sample and container if samples have to be transported to the laboratory and stored prior to analysis. Sorption of heavy metals both from the container to the solution and vice versa is known to occur with a wide range of metals and container materials (3-5). T h e first problem, t h a t of desorption of heavy metals into solution from the container walls, may be avoided by careful acid washing and aging of the containers ( 4 ) . With many metals, the adsorption of metal ions in solution onto the container generally presents a more difficult problem to overcome although in some cases it may be minimized or prevented by immediate acidification of the sample to below p H 2 on collection (3-7). Fortunately, the amount of acid required to do this is small, and with the ready availability of highly purified acids, the need to acidify is not a significant source of contamination. Direct determination of copper(II), lead(II), and cadmium(I1) by ASV has been reported for acidified solutions of natural waters (6, 8-10) and the addition of acid in principle does not preclude direct determinations being undertaken. However, one possible difficulty is t h a t the potential of the hydrogen reduction becomes more positive under dc polarographic or voltammetric conditions in highly acidic media, and the reduction wave for zinc(I1) may be masked ( 1 1 ) .Presumably on this basis, the convention has become established t h a t the direct determination of zinc(I1) by ASV in acidic media should not be undertaken, and zinc(I1) has been determined either after adjusting the pH to between 3.5 and 4.6 with an acetate buffer (6, 9, 10) or a t natural p H (12-25).
From available evidence, it would seem t h a t the determination of zinc(I1) a t natural p H (Le. >6) is a suspect procedure because significant adsorption of this metal onto the container surface occurs (3-5). Consequently, acidification would appear t o be a useful precaution for storage of solutions for zinc(I1) determinations. However, the next stage in the determination, the addition of buffer, presumably to prevent the hydrogen wave interfering with the zinc wave, may introduce contamination. This step needs to be considered carefully because, with acetate buffers, it is particularly difficult to obtain a low zinc(I1) blank (16). A second reason buffers have been used in ASV may be attributed to the frequently reported unsatisfactory behavior of thin film electrodes in highly acidic media (17-21). With such electrodes, it has been common practice to use buffered solutions. Despite the widespread availability of a range of electrode materials and modern electroanalytical methods and the new possibilities these methods offer, addition of buffers t o the acidified solution prior to the zinc determination still seems prevalent. The reduction of hydrogen a t mercury electrodes is very irreversible, and under short time scaie ac polarographic or voltammetric conditions, Beyer and Bond ( 2 2 )recently demonstrated t h a t excellent zinc waves could be obtained in 2 M HC1 despite the fact that the zinc(I1) wave was masked by the hydrogen wave under dc polarographic conditions. The same conclusions could be valid for anodic stripping voltammetry a t a hanging drop mercury electrode and, provided the evolution of hydrogen occurring a t the deposition potential does not cause difficulties, ac and differential pulse methods for determining trace levels of zinc(I1) could be feasible. In the present work, we have chosen to demonstrate t h a t with the method of differential pulse ASV, the addition of a buffer to acidified acid aquatic samples is frequently unnecessary for determination of zinc.
EXPERIMENTAL Apparatus, Samples were determined by differential pulse anodic stripping voltammetry with a PAR, Polarographic Analyzer Model 174 interfaced to a Model 315 Automated Electroanalysis Controller. A three-electrode system was employed. The working electrode was a hanging drop mercury electrode (Metrohm E-410), potentials were measured with reference to a saturated calomel electrode (SCE), and the auxiliary electrode was a platinum wire (0.03-mm diameter). Procedure. Samples were acidified immediately on collection with “Aristar” nitric acid. Samples were determined for zinc(II), cadmium(II), copper(II), and lead(I1) within a week of collection. In the present communication, only the zinc data are reported. T h e polarographic cell, reference, and auxiliary electrodes were initially washed with 2 M distilled nitric acid, and then with the sample prior to the commencement of the actual determinations. After this procedure, a 25-ml aliquot of the solution for determination was added to the polarographic cell and deoxygenated by passing high purity argon through the sample for 15 min. During the potential scan, argon was passed over the solution. A plating potential of -1.20 V (vs. SCE) was applied for 200 s ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
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\b
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-120 -1.10 -100-090
-120 -110 -100-090-080
-110 -1.00 -0.90 - 0 8 0
VOLTS vs.SCE
VOLTS vs SCE Figure 1. Differential pulse anodic stripping voltammogram of distilled water. [ Z n Z f ] = 12 Wg/l., pH 0.97
with stirring and then 30 s without stirring. The voltammogram was then recorded using a scan rate of 5 mV/s, a pulse modulation amplitude of -25 mV, and a duration between pulses of 0.5 s. Quantitative results were obtained by the method of standard additions using a freshly prepared stock solution containing 250 f i g / ] . zinc(I1). All results were obtained in triplicate. The pH of the deoxygenated samples was measured with a Radiometer PHM62 Standard pH Meter.
RESULTS AND DISCUSSION Four different types of sample matrices have been selected t o demonstrate the direct determination of zinc(I1) by differential pulse ASV a t various p H values below 2. T h e determination of zinc in a n acidified sample of distilled water from our laboratory is shown in Figure 1. T h e sample was acidified by 1%nitric acid t o give a final p H of 0.97. Despite the low pH, the zinc(I1) wave can be readily distinguished from the hydrogen wave and the zinc concentration subsequently determined using the method of standard additions as plots of peak height vs. concentration in acid media have been found to be linear. In this sample, the zinc was shown to be present in the water and not introduced by the acidification step. Batley and Florence ( 2 3 ) have commented on the large currents per unit concentration found for differential pulse anodic stripping voltammetry of zinc in' dilute hydrochloric acid. The same observation is valid for dilute nitric acid. Figure 2 shows two representative samples (fresh and salt water) taken near the mouth of the Yarra River (Victoria, Australia). Both voltammograms in Figure 2 show that there is no difficulty in determining zinc(I1) at p H 1.3 in estuary water. The peak current increase on addition of known amounts of zinc is a linear function of concentration and the method of standard addition was used for the determination of zinc. T h e last sample presented is contaminated ocean water collected from waters south of Tasmania (Australia). T h e well defined wave obtained in Figure 3 shows that zinc(I1) can be determined in acidified seawater ( p H 1.9) without difficulty. Finally, it should be noted that data in our laboratories for the direct determination of zinc(I1) in acidified solutions have been confirmed by the solvent extraction-atomic absorption method (16,24). I t was quite apparent from visual observation of the mercury electrode that hydrogen gas is being evolved a t the electrodeposition potential of -1.2 V vs. SCE in the above experiments. Because hydrogen ion is being reduced con760
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Figure 2. Differential pulse anodic stripping voltammograms of Yarra River water.
( a ) Sample taken at a depth of 1 meter. [Zn2+] = 29 pg/l. Salinity 1.3 %o, pH 1.3. ( b )Sample taken at a depth of 12 meters. [Zn2+] = 41 wg/l. Salinity 34.6 %o, pH 1.3
4 T
,OonA
1
VOLTS vs SCE Figure 3. Differential pulse anodic stripping voltammogram of seawater taken off the coast of Tasmania, Australia (latitude 45' S, longitude 147' 44' E). [Zn*+] = 10 pg/l., pH 1.9
58.5'
comitantly with zinc and other ions from acid solution, the current efficiency for the deposition of zinc is far below 100%. However, 100% current efficiency is not necessary in the preliminary deposition step; it is necessary only that it be constant under a given set of conditions. This latter requirement was readily met in the sense that the reproducibility of the stripping curve was found to be identical in both acidic and neutral media. In view of the above findings, the use of differential pulse ASV can be used to simultaneously determine copper(IIj, cadmium(II), lead(II), and zinc(I1j in most aquatic samples after acidification directly and without the need for further treatment of the sample for the determination of zinc(I1). Several hundred samples have been successfully determined for these four elements in our laboratory using this direct method. From our experience, addition of buffers provides one of the greatest potential sources of contamination in trace analytical work, and avoidance of this step wherever possible would appear to be very sound analytical practice.
LITERATURE CITED (1) W. R. Matson and D.K. Roe, "Trace Metal Analysis in Natural Media by Anodic Stripping Voltammetry" in "Analysis Instrumentation", Vol. 7, Plenum Press, New York, N.Y., 1967. (2) W. Kemula, Pure Appl. Chem., 21, 449 (1970). (3) D.E. Robinson, Anal. Chim. Acta, 42, 533 (1968). (4) G. Tolg, Talanta, 19, 1489 (1972). (5) A. W. Struempler. Anal. Chem., 45, 2251 (1973). (6) T. M. Florence, J. Electroanal. Chem., 35, 237 (1972). (7) American Public Health Association, American Water Works Association and Water Pollution Control Federation in "Standard Methods for the Examination of Water and Waste Water", 13th ed., American Public Health Association, Washington, D.C., 1971, p 417. (8) H. E. Allen, W. R. Matson, and K. H. Mancy, J. Water Pollut. Control Fed., 42, 573 (1970). (9) T. Rojahn, Anal. Chim. Acta, 62, 438 (1972). (10) J. Gardiner and M. J. Stiff, WaterRes., 9, 517 (1975). (11) I. M. Kolthoff and J. J. Lingane in "Polarography", Vol. 2, Interscience, New York, N.Y., 1952, p 503.
(12) M. Ariel and V. Eisner, J. Electroanal. Chem., 5, 362 (1963). (13) G. Macchi. J. €lectroanal. Chem., 9, 290 (1965). (14) M. Branica, M. Petek, A. Baric, and L. Jeftic, Rapp. Comm. In?. Mer. Medit., 19, 929 (1969). (15) J. D. Smith and J. D. Redmond, J. Electroanal. Chem., 33, 169 (1971). (16) H. Blutstein. P. L. Boar, A. M. Bond, and K. M. Bone, unpublished results, 1974-1975. (17) R. G. Clem, G. Litton. and L. D. Ornelas, Anal. Chem., 45, 1306 (1973). (18) G. W. Harrington, W. Miles, and S. Vohra in 'Interface", Vol. 13, No. 5, Michigan State University, East Lansing, Mich., 1974. (19) P. J. Elving and A. F. Krivis. Anal. Chem., 30, 1645 (1958). (20) R. G. Clem and A. F. Sciamanna, Anal. Chem., 47, 276 (1975). (21) R. G. Clem, Anal. Chem., 47, 1778 (1975). (22) M. E. Beyer and A. M. Bond, Anal. Chim. Acta. 75, 409 (1975). (23) G. E. Batley and T. M. Florence, J. Hectroanal. Chem., 55, 23 (1974). (24) A. T. Phillips, R. W. Penis. J. E. Harris, A. J. Fabris, P. L. Boar, and K. M. Bone, Proc. Royal Aust. Chem. lnst.. 42, 209 (1975).
RECEIVEDfor review October 15, 1975. Accepted December 23, 1975.
Computer-Assisted Optimization of Anodic Stripping Voltammetry 0. V. Thomas, Lars Kryger,' and S. P. Perone* Purdue University, Department of Chemistry, West Lafayette, Ind. 47907
Computer-assisted optimlzatlon of anodic stripping voltammetry (ASV) measurements has been investigated. The approach taken has been to specify desired performance criteria; then, using a suitable theoretical model, the on-line computer makes initial measurements, imposing subsequently the values of the various experimental parameters required to achieve the desired response. The overall result is to provide the desired quality of analytical data wlth a minimum analysis time. In this study, we have chosen to speclfy signal-to-noise and signal-to-background ratios for linear sweep ASV, studying metal ion soiutlons in the range 1X to 1 X lo-* M. An interactive approach to peak location and current measurement has been used, allowing the analysis of multicomponent solutions to be performed with a minimum of sophisticated programming algorlthms. Careful selectlon of values for the performance criteria will allow a satisfactory automated analysis to be performed over a wide range of concentratlons, while minimizing analysis time for each sample.
Optimization of analytical instrumentation can be described as a process whereby response is made as nearly perfect, effective, or functional as possible. T h e scientist usually determines which measurable features should be optimized. When the explicit relationships between controlled parameters and measured response are not clear, experimental optimization attempts often follow a "trial and error" method. T h a t is, individual experimental parameters are varied and resulting signal changes are observed. If the variation enhances the signal measurement, the variation is retained. If not, the parameter is returned to its previous value and another parameter is varied. T h e Simplex ( 1 ) method of optimization offers a more systematic version of the trial-and-error method. Several paramePermanent address, D e p a r t m e n t of Chemistry, Aarhus U n i v e r sity, Aarhus, D e n m a r k .
ters are varied simultaneously until the optimum response is found, or oscillation is observed. For analytical systems where response can be related mathematically to various experimental variables, a more direct optimization approach may be applied. The procedure would involve, first, establishing performance criteria for system response; then, from first principles and preliminary measurements, the values for various experimental parameters could be chosen to achieve the desired response. If any values are clearly beyond the capabilities of the instrument, new performance criteria may be tried, or the best available settings may be used with the other parameters adjusted accordingly. This process is ideally suited to the computational and control abilities of the laboratory computer. Typical examples of performance criteria which could be considered are the signal-to-noise ratio, signal-to-background ratio, resolution, analysis time, cost, etc. T o demonstrate this approach, we have chosen the technique of Anodic Stripping Voltammetry (ASV) with a linear potential sweep (2). This is a technique which lends itself well to computerized optimization because of the wide dynamic range of the control parameters, which include plating time and scan rate. (Other stripping functions, like a square-wave or staircase sweep, could be used and suggest other considerations for optimizing experimental parameters.) For this study, the signal-to-noise and signalto-(scan-rate-dependent)-background were selected as the performance criteria. As described below, there is no "optimum" response in ASV when plating time is a controlled variable, as the measured signal improves linearly with time. However, for routine handling of multiple samples, a procedure which accomplishes the minimization of analysis time, while simultaneously meeting desired signal quality specifications, can be considered an optimization procedure. In practice, the procedure requires the operator to specify the desired performance criteria. On the basis of a suitable theoretical model, and preliminary experimental measurements, the laboratory computer subsequently calculates and imposes ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
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