Determination of Trace Metals in Aqueous Environments by Anodic Stripping Voltammetry with a Vitreous Carbon Rotating Electrode A. H. Miguel' and C. M. Jankowski Chemistry Department, Northeastern University, Boston, Mass. 02 1 15
Effective performance in the monitoring and control of trace metal pollutants requires the existence of reliable and reproducible methods of analysis a t the parts-per-billion level and below. A t present, atomic absorption and fluorescence spectrometry, neutron activation analysis, electroanalytical methods of analysis, and others appear to offer the best promise for the determination of trace metals in natural and waste waters. The applicability of these techniques has been described by Mancy ( I ) and they have been used with varying degrees of convenience, success, and cost. Among the electroanalytical techniques, differential pulse anodic stripping (DPAS) (2) and anodic stripping voltammetry (ASV) with thin film or solid electrodes appear to be most suitable. The advantages over other techniques are that relatively fewer errors are introduced through contamination from glassware and chemical reagents. Of the two techniques ASV requires less expensive and simpler instrumentation and both permit determination of trace metals a t the sub-part-per-billion level without preconcentration which, a t times, can also be a source of error. This paper describes the determination of copper and lead in aqueous solution by the simultaneous deposition of Hg and the trace metals of interest directly onto a vitreous carbon rotating electrode. Using this modified version of the Hg film method, we were able to speed up the analytical procedure appreciably and improve the reproducibility to approximately 2%. The method of simultaneous plating was first reported by Florence ( 3 ) using a glassy carbon electrode. The calculated relative standard deviation for several separate runs on aliquots of the same sample was &2%. which is about the same precision obtained by Florence. The useful concentration range for the metals studied is M. from about to
EXPERIMENTAL Reagents. All chemicals were reagent grade, with no further purification attempted. Solutions were prepared with water purified hy passage through a mixed bed cation-anion exchange resin, then distilled once. Hizh purity nitrogen gas was used for deaeration of the samples. .4 mercuric nitrate solution was prepared by dissolving triply distilled mercury in reagent grade nitric acid and diluting to give a 1 X 10-2Af solution. Standard solutions of copper and lead were prepared by dissolving high purity metals in reagent grade nitric acid. E S C P ~acid S from both solutions was eliminated by evapora-
'
Present address, School of Chemical Sciences, Department of Chemistry, University of Illinois, Urbana, Ill. T o whom correspondence should he addressed. Mancy in "Analytical Chemistry: Key to Progress on National Problems,'' NBS Special Publication 351. W. W. Meinke and J. K . Taylor, Ed.,
(1) K . H.
1972, p p 343-53. (2) Application Note AN-107 Princeton Applied Research, Princeton, N . J.
08540. (3) T. M. Florence. J. Electronanal. Chem.,27, 273 (1970). 1832
tion. These solutions were used as standards for known increment measurements and were transferred with disposable micropipets (Micropet). Apparatus. All measurements were done using the Beckman Electroscan Model 30P Electroanalytical System in the potentiostat mode of operation. Cell and Electrodes. The cell (Figure 1) used in this study was made from quartz tubing 50 m m in diameter by 50 mm long. By using the quartz cell, absorption of trace metals on the cell walls, usually found with glass and polyethylene containers (4,.5) was minimized. A silver/silver chloride reference electrode was prepared as described by Matson ( 4 ) .The working electrode was a Beckman Rotating Electrode with a vitreous carbon electrode tip (Catalog No. 39084). T h e counter electrode was made from 0.3-mm high purity platinum wire. Both the reference and the counter electrodes were contained in 3-mm diameter open quartz tubing directly in the solution. These electrodes, together with a 1.5-mm diameter Teflon bubbling tube were held in a machined Teflon head, as shown in Figure 1. This arrangement facilitates the ease of transfer between samples, and maintains a stable configuration of the electrodes and the bubbling tube. In a separate study, we found it to be of paramount importance that the position of the bubbling tube be left unchanged; otherwise irreproducible peak currents are obtained. Procedures. Pretreatment of the Working Electrode. T h e vitreous carbon rotating electrode (working electrode) surface was cleaned before each set of analyses. This was done simply by polishing the electrode with a fine textured cloth, followed by wiping with wet tissue paper, then giving a final polish with dry tissue paper. These cleaning operations were done while the electrode was rotated. Sample solutions were deaerated by bubbling nitrogen gas for about 10 minutes. Prior to entering the cell, the nitrogen is saturated, a t room temperature, with sample solution or supporting electrolyte contained in a gas saturator to prevent evaporation of the sample in the ASV cell. A sample of dionized water made 0 . 3 4 in NaCl (sea water -0.54M NaC1) was selected as a test solution. .4 30-ml aliquot of this solution was placed in the quartz cell, followed by the addition of 200 pl of 1 X lO-?M Hg(NOs)e, making the final concentration of Hg 6.6 X 10-5MM. The machined Teflon head containing the reference and counter electrodes and the Teflon bubbling tube was then lowered into the cell. Nitrogen gas was bubbled through the solution for about 10 minutes, a t a rate of 70 ml/min. T h e pre-cleaned working electrode was then inserted into the cell and rotated a t the desired speed. The electrodeposition potential ( E d )was set at -1.0 volt for copper, lead, and cadmium and the simultaneous electrodeposition of the mercury film and trace metals was carried out for exactly 5 minutes. At the end of the electrodeposition period, both the rotating working electrode and the nitrogen bubbling were turned off. A rest period of 5 seconds was allowed before stripping ire-oxidation) of the trace metals from the Hg film was conducted. Stripping was accomplished by scanning the potential from -1.0 to -0.1 volt. Continuous tracing of stripping potentials and stripping currents were monitored. Potential scanning rates as fast as 100 mV/sec were possible using the Beckman instrument without pen response becoming a problem. Care should be taken not to scan t,he potential beyond -0.1 volt, as the Hg film starts to strip off. Matson, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1968. (5) D. E. Robertson, Anal Chim. Acta. 42, 533 (1968). (4) W. R.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
Tefion bubbler
T e f l o n head
I
1
,
1
I
1
I
1
I
- 0 1 - 0 2 - 0 3 - C 4 - 0 5 - 0 6 -07 - 0 8 - 9 9 Potcn+)a' (Volts v s Ag/AgClI
'
-
3
Figure 2. Anodic stripping voltammogram of deionized water 0.5M in \\quartz (open)
tublng
NaCl
Figure 1. The ASV cell consisting of the rotating electrode, the
Conditions: 5-min deposition at -1.0 volt, 30 rps, 7.5 ppb Cu2*, 2.6 ppb Pb2+. 6.6 X 10-5MHg2+, 100 mV/sec, pH 4.1, 26 OC
counter and the reference electrodes, and the bubbling tube Standard addition aliquots were added via a 1.5-mm diameter hole bored through the machined Teflon head.
Table 11. Variation of Peak Current with Scan Number for Leada
Table I. Variation of Peak Current with Scan Number for CoppeP Scan
so.
1 2 3 4 5
i,,
@A
2.95 3.11 2.95 2.95 2.95 Av 2.98, i- 0.072
Scan
YO.
1 2 3 4 5
- E p , volts
0.264 0.260 0.260 0.260 0.255
a0.5M NaC1, 100 mV/sec, 30 rps, 100-sec deposition time at -1.0 volt, 6.6 X 10-5M Hg2+, 7 . 5 ppb C U ~ +p, H 4.1, 26 "C. * Successive deposition and scans on the same test solution.
b
- E p , \olts
ip, u A
0.98 0.98 0.98 0.98 0.97 Av 0.97, i 0.0045
0.505 0.500 0.495 0.490 0.48i
a0.5M NaCI, 100 mV/sec, 30 rps, 100-sec deposition time at -1.0 volt, 6.6 X 10-5M HgzL, 2.6 ppb Pbz', 26 "C. Successive deposition and scans on the same test solution.
A typical anodic stripping voltammogram is depicted in Figure 2.
To establish the peak current amplitude for the trace metals by standard addition, an aliquot of the trace metal of interest is added to the sample solution contained in the ASV cell and the experimental procedure is repeated. Usually between 3 and 4 consecutive additions of each trace metal was found to be sufficient to define a peak current calibration curve. Peak current measurements from the base-line offset were measured for each standard addition of t h e trace metal. T h e x intercept obtained by extrapolation of the straight line defines the original concentration of the metal of interest in solution.
RESULTS AND DISCUSSION The concentration of copper and lead in the test solution was found to be 7.5 and 2.6 ppb, respectively. Values obtained for cadmium were less than 0.1 ppb. The experimental conditions are discribed in the legend of Figure 2. Reproducibility of peak current for Cu2+ and Pb*+ are shown in Tables I and 11. Conditions are defined in the legends. Relative standard deviation for peak currents were f2.40% for copper and f0.46% for lead. This compares favorably with the f4.98% obtained by Florence (3) for a solution containing 41 ppb Pb2+ ( 2 X 10-;M). We attribute this enhancement to the more reproducible mass transfer conditions inherent of rotating electrodes and also to the minimization of wall effects by the elimination of glass from the system. Standard deviation for concentration between several runs on separate aliquots of the same sample was found to be f2%. Slight reduction in the value of the peak potentials with scan number was noted; however, reasons for this are not known. A similar observation was made by Florence (3).
Table 111. Variation of Peak Current with Rotational Speeda io, u A
Rotational speed, rev s e c - ' b
5 10
20 30 40 50
CU2+
Pb!*
2.36 3.52 5.32 7.48 10.1 11.8
0.91 1.38 2.07 2.66 3.15 3.56
0.5M NaCI, 100 mV/sec, 5-min deposition time at -1.0 volt, 6.6 X lO-5M HgZL, 2.6 ppb P b Z + , 7 . 5 ppb C u z - , p H 4.1, 26 "C?' Measurement made on the first scan.
Presumably this is an electrode aging phenomenon. An interesting observation is that our results differ from those of Florence ( 3 )in that his results showed a marked increase in the peak current value between the first and second scans. Deviations in our results appear to be random for all scans. The influence of rotational speed of the rotating electrode is illustrated in Table I11 for the peak current of copper and lead. High rotational rates give higher peak currents but the relationship is not linear. Also, a t high rotational rates, the electrode response deteriorates. Optimum rates were around 30 revolutions per second (rps).
ANALYTICAL CHEMISTRY, VOL. 46. NO. 12, OCTOBER 1974
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Table IV. Variation of Peak Current with Scan Ratea ip, U A
Scan rate,
mvisec b
C "27
P b2+
10 20 50 100
0.98 2.03 5.22 10.0
0.39 0.61 1.58 2.74
0 0.5M NaC1, 30 rps, 5-min deposition time at -1.0 volt, 6.6 x l O - 5 M HgZ+, 2.6 ppb Pb2-, 7 . 5 ppb C u z + , 26 "C. * Measurements made on the first scan.
Table V. Variation of Peak Current with Deposition Timea ipr u A
Deposition time, sec b
C"2+
Pb2+
100 200 3 00
2.98 6.02 9.00
0.98 1.87 2.81
0.5M NaC1, 30 rms, 6.6 X 10-5M Hgz-, 2.6 ppb Pb2+, 7 . 5 ppb Cu*+, deposition at -1.0 volt, p H 4.1,26 " C . Measurements made on the first scan. (I
Table IV shows the variation of peak current with scan rate for copper and lead. Peak current amplitudes increased linearly with scan rate as would be expected with this film electrodes (6). We found that high potential scanning rates gave results of the same reproducibility as the low rates. For rapidity of the analyses, fast rates are to be preferred. Variations of peak current with deposition time are illustrated in Table V. Peak currents were extremely linear with deposition time. For the part-per-billion range, deposition times of approximately 5 minutes were suitable.
CONCLUSION The method described is simple and very fast for the determination of trace metals a t the part-per-billion level and below. ACKNOWLEDGMENT The authors express sincere appreciation to D. H. Howling of New College, for his constructive criticism and to E. N. Pollock, L. P. Zopatti, and J. C. Cornwell of Ledgemont Laboratory for furnishing the quartz cells and the high purity reagents used in this study. RECEIVEDfor review January 16, 1974. Accepted June 27, 1974. This work was partially supported by the National Science Foundation under Grant No. G.1.-31605. (6) E. Barendrecht, in "Electroanalytical Chemistry, Vol. 2," A. J. Bard, Ed., Marcel Dekker, New York. N.Y., 1967, p 69.
Semiquantitative Potentiometric Method for Direct Measurement of Nitrogen Dioxide in Air G. G. Barna and R. J. Jasinski Texas lnstrurnents Incorporated, P. 0. Box 5936, Dallas, Texas 75222
The potentiometric measurement of electroactive gases, in contrast with amperometric measurements such as using a Field Lab 0 2 Analyzer (Beckman Instruments, Part No. 100800),a Faristor NO2 Sensor (Environmetrics, Marina del Rey, Calif.), or the method of Bay et al. ( I ) ,has the inherent advantage of simplicity and low cost, in that the only components required are the electrochemical transducer for the particular species, a reference electrode, and a high impedance voltmeter. There exist, however, only a very limited number of cases where such measurements are perfor:ned, and except for 0 2 ( Z ) , those involve the indirect measurement of the aqueous reaction product of the particular gas, via an ion-selective electrode. The determination of SO2 using an SO2 electrode (Model 95064, Orion Research Inc., Cambridge, Mass.) or the method of Young et al. ( 3 ) ,NH3 with an NH3 electrode (Model 95-10, Orion), and NO and NO2 ( 4 ) has been achieved by these means. All these techniques involve a preconcentration step to achieve the necessary sensitivity, obtained by purging the (1) H. W. Bay, K. F. Blurton, H. C. Lieb, and H. G. Oswin, Amer. Lab., No. 7, p 57 (1972). (2) P. B. Hanh, M. A. Wechter, D. C. Johnson, and A. F. Voigt, Anal. Chem., 45, 1016 (1973). (3) M.Young, J. N. Driscoll, and K. Mahoney, Anal. Chem., 45, 2283 (1973). (4) E. M. Kneebone and M. Freiser, Anal. Chem., 45, 449 (1973).
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air sample through a scrubbing solution; consequently, they do not yield real-time analyses. They are also, in the case of the p H measuring devices, amenable t o interference by a range of p H altering gases. This paper describes the direct potentiometric measurement of NO2 in air by means of a properly activated Fechalcogenide glass electrode. This system avoids the complication of a preconcentration step while still maintaining high sensitivity; achieves selectivity directly via the nature of the chemical interaction between the gas and the sensor material; while it maintains the advantage of simplicity in measurement.
EXPERIMENTAL T h e active element of the electrochemical 9 0 2 sensor is a nonoxide chalcogenide glass with the approximate composition of Sefi&e&blz doped with 1.7-2% Feo (the subscripts refer to the mole percentages of the elements). This material is prepared by the direct fusion of the elements, as described in a previous publication ( 5 ) ,and will subsequently be designated as "Fe 1173." T h e electrode was constructed by attaching a glass disk (7 mm in diameter and 1 mm thick) cut from the melt to a brass screw fitted inside a cylindrical rod of paper-based phenolic resin (A-1 Plastic Supply, Dallas, Texas). This material was chosen for its compatability with the necessary high-temperature air-oxidation (5) R. Jasinski and I. Trachtenberg, J. Hectrochem. SOC.,120, 1169 (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
