ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
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Anodic Stripping Voltammetry with Collection at Tubular Electrodes for the Analysis of Tap Water G. W. Schieffer' and W. J. Blaedel" Deparlment of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Portable, battery operated equipment is described, tested, and characterized for the performance of anodic stripping voltammetry with collection (ASVWC) at two mercury-coated glassy carbon tubular electrodes in series. By operating the collection electrode at a constant cathodic potential, charging current background6 are reduced greatly, permitting better perceptlon of peak currents than with conventional ASV. The equipment is applied to the analysis for Cd, Pb, and Cu in tap water at subnanomolar levels.
Anodic stripping voltammetry with collection is a technique t h a t was invented for the rotated ring-disk electrode, but which may be performed a t two tubular electrodes in series on a flowing stream. Trace metal cations are deposited on the upstream electrode from the flowing sample solution, stripping from t h a t electrode with an anodic potential scan, and are collected by deposition on the downstream electrode, which is held at a constant cathodic deposition potential. The constant potential applied t o the collection (analytical) electrode eliminates the charging current normally encountered with conventional anodic stripping voltammetry (ASV). In a previous paper ( I ) we reported the theoretical and experimental dependences of stripping and collection peaks on flow rate, deposition time, scan rate, and concentration observed with a twin tubular electrode designed for ASVWC. T h e potential of the stripping electrode was controlled with a commercial two-electrode polarograph, while the potential of the collection electrode was held constant with a battery voltage source and a ten-turn precision potentiometer. This simple setup enabled d a t a of sufficient quality to be accumulated t o characterize the cell, support the theory, and to indicate the inherent sensitivity of the method. Based on the earlier theoretical treatment, and the need for on-site probes suitable for rapid analysis of environmental samples, we have designed and built battery operated dual potentiostat equipment and a flow-through electrolysis cell that have the capability for on-site analysis. The equipment is characterized and applied to the analysis of a t a p water sample. EXPERIMENTAL Apparatus. The cell design was similar to that described previously ( I ) except that a platinum wire counter electrode was inserted into the tubular channel downstream from the reference electrode bridge, which consisted of cation-exchange membrane washers. The stripping and collection electrodes were of the same area (0.067 cm2)and were separated by a 0.011-inch Teflon spacer. (A cell in which the stripping electrode area was three times that of the collection electrode was also constructed. However, exploratory results indicated only a slight increase in sensitivity over the previous version, but with a significant increase in peak broadening and loss of resolution. As a result, this cell was not studied further.) The dual potentiostat for simultaneously and independently controlling the potential of both working electrodes was similar Present address, Norwich Pharmacal Co., Norwich, N.Y. 13815. 0003-2700/78/0350-0099$01 .OO/O
in design to that described by Napp, Johnson, and Bruckenstein (2) with a few minor modifications. First, a low noise precision FET operational amplifier current follower (Model 52K, Analog Devices, Norwood, Mass.) was used to monitor the current of the collection electrode. The stripping electrode current was monitored with an FET differential instrumentation amplifier (Analog Devices, Model 603K) which measured the potential drop across a load resistor (R9, located between the output of A2 and the noninverting input of F2 in Figure 1 of Ref. 2). A current suppressor circuit was used t o shift the collection or stripping current baseline. (A circuit diagram of the dual potentiostat and current suppressor circuits will be sent upon request.) Two rechargeable 12-V batteries (Gel Cell, Globe-Union, Inc., Milwaukee, Wis.) were used as the power supply. An X-Y recorder was used for the scan rate study. A strip chart recorder was used for the remaining work. Reagents. All solutions were prepared from analytical reagent grade chemicals and tap distilled water that was redistilled from alkaline permanganate. Supporting electrolytes were 0.1 M HC1, 0.04 M NH4Ac-HAC(pH 4.8), and 0.01 M NH4Ac-HAC(pH 4.7). Trace metal cation solutions were prepared immediately before analysis from standard 1.000 mM Cu(N0J2,0.975 mM Cd(N0J2, and 1.002 mM Pb(NO3I2stock solutions using micropipets and Nalgene volumetric flasks. Mercury film deposition and analysis procedure were identical to that reported previously ( 1 ) . A volume flow rate of 3.42 mL/min was used for all experiments. R E S U L T S AND DISCUSSION Effect of Scan R a t e o n Sensitivity a n d Resolution. In the previous work ( I ) it was found that the collection peak current and peak width a t half height (b1I2)increased as the scan rate was increased from 1 t o 3 V/min. The effect of applying a wide range of scan rates when a single trace metal cation is present is shown in Figure 1. The magnitude of the peak current increases significantly with scan rate up to a scan rate of about 6 V/min without any increase in charging or background current. From 6 to 14 V/min, little increase in sensitivity is noted although the peak width increases appreciably. The corresponding stripping peaks showed a much greater increase in peak height (and a significant charging current) with a much smaller increase in peak width over this scan rate range. The fact that the collection peak width, rather than the peak height, increases as the scan rate is increased beyond 6 V/min indicates that the rate of mass transport of the stripped cations to the collection electrode limits the magnitude of the collection peak. Apparently for high scan rates (and short stripping times), the potential range of the stripping electrode is traversed before all of the stripped cations reach the collection electrode. This reasoning is supported by the large anodic potential shift for the high scan rates depicted in Figure 1. Figure 1 indicates that the optimum scan rate for a single peak might be 4 or 5 V/min. However, when two collection peaks occur relatively closely together as shown in Figure 2 for cadmium and lead, lower scan rates should be employed to completely resolve the two peaks. In this case, the optimum scan rate is around 2 V/min. Comparison of S t r i p p i n g and Collection Peaks at Low Trace Metal C a t i o n Concentrations. T h e advantages of ASVWC are demonstrated in Figure 3, which shows collection ?2 1977 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978 14 /--
STRIPPING
.IY II \
‘ION
-0.4
-0.6
-
-0.2
POTENTIAL O F STRIPPING ELECTRODE, VOLTS
Figure 1. Collection peaks for lead at various scan rates. 1 X lo-’
M lead in 0.1 M HCI. 5-min depositions. Collection electrode voltage, -0.7 V. Numbers on curves indicate scan rate in V/min
-0.4 0.o POTENTIAL O F STRIPPING ELECTRODE, VOLTS
-0.8
”\,
8
Figure 3. Stripping and collection peaks for 6 nM cadmium, 7 nM lead, and 7 nM copper in 0.04 M acetate buffer. 5-min deposition, 1 V/min. Collection electrode voltage, -0.80 V Table I. Summary of Analytical Results for Copper Ion in Tap Water Copper ion content, mM
Method of analysis I -0.8
I
-0.6
I
-0.4
I
-0.2
POTENTIAL OF STRIPPING ELECTRODE, VOLTS
Figure 2. Collection peaks for cadmium and lead at various scan rates. 1 X lo-’ M cadmium and lead in 0.04 M NH,Ac-HAC buffer. Conditions similar to those of Figure 1, except that collection electrode voltage is -0.80 V peaks for 6 n M cadmium, 7 n M lead, and 7 nM copper in an acetate buffer. Despite the smaller magnitude of the collection peak currents, the lower collection current background permits setting the recorder at a higher sensitivity to give much more perceptible peaks. The improvement in perceptibility is especially noticeable for copper, whose stripping peak is almost indistinguishable from the charging and mercury oxidation background currents. Even though charging current has been eliminated from the response of the collection electrode (which is held a t constant potential), i t is apparent from Figure 3 that the collection background current still increases as the stripping potential is increased anodically. The cause of the rising collection current background has been postulated to be due in part to the decreasing effectiveness of the upstream stripping electrode in shielding the downstream collection electrode from traces of oxygen or other electroactive species as the stripping electrode potential is scanned to more positive values. A test of this postulate was made by noting that the slope of this background is decreased, but not eliminated, when the stripping electrode is scanned at stopped flow. The remaining contribution to the background collection current depends linearly on the applied stripping potential, and may be a result of improper alignment of the electronic circuitry ( 3 ) . For the stripping step, the average peak currents and relative standard deviations for cadmium (seven consecutive determinations) and lead (nine determinations) were 8.3 nA
ASV WC
pH 7.1 pH 5.0 Copper ion-selective electrode
160
132
pH 7 . 1
pH 5.7 Ion-exchange enrichment pH 7.1 pH 5.7 Carbon furnace atomic absorption spectroscopy
2 (activity) 80 (activity) 2 (activity) 58 (activity)
240
f 5.5% and 10.7 nA f 3.6%, respectively. For the collection step, the respective values for cadmium and lead were 4.04 nA rt 2.0% and 4.44 nA f 0.7%. The higher precision of the collection values is probably a result of the better defined baseline current. No comparison is made for copper because of the poor quality of the stripping peak. An indication of the sensitivity of ASVWC a t tubular electrodes is demonstrated by the collection peaks for traces of cadmium and lead in a sample of 0.01 M acetate buffer shown in Figure 4. Six-point standard additions from 0.2 to 2.0 nM added cadmium and lead were linear for both metals. A least-squares analysis of the standard addition data yielded a concentration and standard deviation of 0.15 f 0.02 n M for cadmium and 0.48 f 0.03 n M for lead. Analysis of Tap Water. A t a p water sample (City of Madison) was adjusted to p H 5.0 with acetate buffer, and analyses were performed by multiple standard additions of cadmium, lead, and copper. The analyses gave concentrations and standard deviations of 0.5 0.1 n M for Cd, 0.5 & 0.1 n M for Pb, and 132 f 6 nM for Cu (1nM represents 0.11 ppb Cd, 0.21 ppb Pb, and 0.06 ppb Cu). The plots of collection current vs. added metal ion concentration were strictly linear in all cases. Such linear behavior was not noted for the determi-
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978 I
I
-0.4 POTENTIAL OF STRIPPING ELECTRODE, VOLTS
-0.8
Figure 4. Collection peaks for 0.15 n M cadmium and 0.48 nM lead
in 0.01 M acetate buffer. 12-min deposition. 2 V/min. Collection electrode voltage, -0.80 V
nation of copper in seawater by multiple standard addition ( 4 ) , the nonlinearity being ascribed to the presence of significant concentrations of organic and inorganic complexing materials (equivalent to copper concentrations around 10 nM). A considerable amount of further work was done on the tap water sample, t o ascertain the meaning and accuracy of the ASVWC result for Cu. Further multiple standard addition analyses were made for copper alone, by medium exchange, in which the deposition was performed with the unaltered tap water (pH 7.1) flowing through the cell, and in which the stripping-collection was done with 0.01 M acetate buffer (pH 5.0) flowing through. The copper content was 160 f 30 nM, in approximate agreement with the value of 132 obtained when both the deposition and stripping-collection were performed a t p H 5.0, indicating little or no dependence on p H in this range. T h e tap water sample was measured a t its native pH (7.1) with a copper ion-selective electrode, to give an estimate of t h e copper ion activity. T h e ion-selective electrode was calibrated against an Orion double junction reference electrode (No. 90-02-00) with an Orion cupric standard (No. 94-29-06), made up in Ca(N03)2solution, and usable as an activity standard as well as a concentration standard. A plot of potential against cupric ion activity on a log scale was linear down to an activity of 0.5 gM, with a slope of 30.6 mV per decade. T h e activity of the t a p water sample was around 2 nM, obtained by extrapolating the straight line calibration plot about two decades below the lowest calibration point (0.5 gM). The cupric ion activity in the tap water sample acidified to p H 5.7 was also measured in triplicate and more precisely as 80 f 3 n M by the same procedure. The tap water sample, acidified to p H 5.7 was also analyzed by a cation-exchange enrichment procedure ( 5 ) to give an estimate of the exchangeable copper ion activity. A large volume of the acidified tap water was passed through a small column containing 10 g of Zipax SCX (a pellicular cationexchange resin, Instrument Product Division, Du Pont & Co., Wilmington, Del.), until the resin became equilibrated to the sample, and the copper ion activity measured with a copper ion selective electrode was the same in the effluent solution as in the influent solution. The resin was then extracted by passing a 0.040 M Ca(N03)zsolution through the column, extracting the metal ions that were originally in the tap water sample according to t h e Donnan distribution ( 5 ) . Mea-
101
surement in the extractant of the copper ion activity (with a copper ion-selective electrode) and of a pilot ion activity (calcium, obtained with a calcium ion-selective electrode), together with measurement of the pilot ion activity in the tap water sample, permitted calculation of the copper ion activity in the tap water sample from the Donnan principle. Duplicate measurements gave an exchangeable copper ion concentration of 58 h 2 nM in the tap water adjusted to p H 5.7. A measurement in the original tap water sample (pH 7.1) gave 2 nM. The analytical results for copper in tapwater are summarized in Table I. Several conclusions may be drawn: (1) a t p H 7.1 and 5.0, ASVWC measures significantly less than (only about 60%) the total copper obtained by carbon furnace atomic absorption spectroscopy. (2) At p H 7.1, ASVWC measures about two orders of magnitude more than the free or exchangeable cupric ion activity obtained with the copper ion-selective electrode or the ion-exchange enrichment procedure. (3) At p H 5.0-5.7, ASVWC measures about twice as much as the free or exchangeable copper ion activity. This work indicates that in native (pH 7.1) or acidified (pH 5.0-5.7) tap water, a significant fraction of the cupric ion is complexed or otherwise bound, and that ASVWC measures not just the free copper ion but also at least a fraction of the bound. These conclusions are not a t variance with those of other workers who have investigated ASV (6). The tap water samples were also analyzed by carbon furnace AA, for copper (240 nM), cadmium (2 nM), and lead (15 nM), which values are much higher than the ASVWC values. Apparently, even a t p H 5.0 ASVWC does not give t h e total metal ion contents. Other workers have reported similarly low values by ASV (7). In general, ASV analysis on untreated water samples appears to give only a fraction of the total metal content, and pretreatment with strong acid, oxidizing agents, or digestion is needed to make the ASV result approach the total metal content (6).
CONCLUSIONS ASVWC with a twin tubular electrode using a dual potentiostat is a sensitive and precise technique which might be a useful alternative to differential pulse stripping voltammetry at mercury film electrodes. At thin film electrodes, the pulse technique does not yield a significant improvement in sensitivity over linear scans for many ASV determinations (8) and is generally restricted to low scan rates. ASVWC, on the other hand, gives a significant reduction in both the magnitude and noise of the background current, allowing more precise, sensitive analysis and the use of relatively fast scan rates. In addition, the battery operated dual potentiostat circuitry and siphon-flow system are small size and readily portable, indicating that the system might be useful for on-site analysis. Upon application of ASVWC to the analysis of t a p water for copper it was found that the ASVWC result was much higher than the cupric ion activity or the cation exchangeable copper ion activity. It was also found that the copper content by ASVWC was less than the total determined by carbon furnace atomic absorption spectroscopy. A similar conclusion applied to cadmium and lead. Under the assumption that Madison t a p water is not unique, a great deal of method development and selection of analysis conditions appears to be necessary before ASVWC results can be confidently related to either the total metal ion concentration or its activity. This conclusion is in line with a recent review and evaluation of ASV (6).
ACKNOWLEDGMENT The assistance of P. J. Kinlen in designing the dual potentiostat is highly appreciated.
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LITERATURE CITED (1) G. W. Schieffer and W. J. Blaedel, Anal. Chem., 49, 49 (1977). (2) D. T. Napp, 13.C. Johnson, and S. Bruckenstein, Anal. Chem., 39,481
(8) G. E. Batiey and T. M. Florence, J. Electroanal. Chem. Interfacial Nectrochem., 55, 23 (1974).
.- - . .
I,1 987),
(3) D. C. Johnson and R. E. Allen, Talanta, 20, 305 (1973). (4) T. M. Florence and G. E. Batley, J. Electroanal. Chem., 75, 791 (1977). (5) This method is presently being worked out by R. A. Niemann at the University of Wisconsin, with the support of the Environmental Protection Agency, Grant No. R-804179. (6) W. Davison and M. Whitfield, J. Electroanal. Chem., 75, 763 (1977). (7) M. Kopanica and V. Stara, J. Electroanal. Chem., 77, 57 (1977).
RECEIVED for review February 14, 1977. Accepted October 11,1977. This research has been supported in Part by a grant from the Environmental Protection Agency (No. 804179-02-1), and in part by a grant from the National Science Foundation (NO. CHE-7615128).
Ozone Oxidation of Organic Sequestering Agents in Water Prior to the Determination of Trace Metals by Anodic Stripping Voltammetry Ray G. Clem" and Alfred T. Hodgson Energy and Environment Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94 720
Ozone is investigated as a general oxidant for trace organic sequestering agents in water. The course of destruction of sequestering agents is monitored by observing the liberation of free-Ion Pb and Cd at the part-per-billion level. The model compounds used for developing the presented method are: EDTA, APDC, tannic, and humic acids. The method is applied to the determination of Pb and Cd in sewage effluent and in San Francisco Bay water. A method by which humic acid having an initial ash content of >11% is reduced to 0.1%. This procedure should be of general interest to anyone using instrumental techniques to develop methods for trace metals in water.
EXPERIMENTAL The instrumentation used consisted of an MPI Electroanalyzer Model 1502B operated in the ASV Mode in conjunction with an ElectRoCell-ASVModel A-2000. The Lucite MPI ASV cell was modified to accept a quartz cell, fabricated in this laboratory's glass shop, having the same internal dimensions as the plastic one. The preparation of the electrode used was described preC 1977 American Chemical Society
