Effect of Supporting Electrolyte Concentration in Pulsed Stripping Voltammetry at the Thin Film Mercury Electrode Sir: We have noted with interest recent reports of differential pulse anodic stripping analysis (1, 2 ) , particularly a recent Instrumentation column in this journal ( 3 ) . These papers contain the first reports of pulsed stripping at the thin-film mercury electrode, although application to the hanging mercury drop electrode has been known ( 4 , 5 ) . In all of these papers, it is a t least implied (and explicitly stated in Refs. 1 and 2) that t h e addition of supporting electrolyte is both unnecessary and undesirable in the analysis of trace contaminants in aqueous media. While such an idea is superficially attractive, eliminating contamination from added chemical reagents, our experience indicates that pulsed stripping analysis in the absence of any supporting electrolyte is impractical. We find that both the plating and the stripping processes a t a thin-film mercury electrode are critically dependent on the solution resistance, which is determined by the supporting electrolyte concentration. Figure 1 shows differential pulse stripping voltammograms for a solution of -4 ppb in both Cd and P b for varying supporting electrolyte concentrations. Figure 2 shows the variation of peak current and peak potential with supporting electrolyte concentration. Note that the peaks are greatly diminished a t lower supporting electrolyte concentrations; one expects any voltammetric technique to show decreased response in the presence of large uncompensated resistance (6). The ratio of the peak heights for Cd and P b is not constant because of the anodic shift of the true plating potential from the apparent value of -800 mV by the large iR drop in the more highly resistive solutions. At more anodic true potentials, the plating of Cd is less complete and the peak on subsequent stripping is smaller. Note that the peak potentials for both peaks shift toward more cathodic values as the solution resistance decreases. An analogous effect is seen in Figures 8 and 9 of Ref 3, which show an anodic shift in peak potentials as the stripping current increases (higher metal ion concentrations). It is also apparent from these figures that the peak currents, even for Pb, for which there should be no complication with intermetallic compound formation (7), are not directly proportional to concentration. As a confirmation that the effects were due to uncompensated resistance, we conducted a set of experiments in which an external series resistance was inserted between the indicator electrode and the input of the current follower. Figure 3 shows a plot of peak stripping current observed for 8 ppb P b as a function of the reciprocal of the added resistance. (This presentation is chosen to be analogous to Figure 2a.) As additional uncompensated resistance was added, the peak current decreased and the wave shifted in the anodic direction. These observations are in complete accord with our explanation presented above.
(1) H. Siegerman and G.O'Dom,Amer. Lab., 4, 59 (1972). (2) H. Siegerman, G . O'Dom, and J. Flato, "Electrochemical Contributions to Environmental Protection," T. R . Beck, et a / . . Ed.. The Electrochemical Society, Inc., Princeton, N.J., 1972, pp 76-87. (3) J. E. Flato,Ana/. Chem., 44(11), 75A (1972). (4) G. D. Christian,./. ElectroanaL Chem., 23, 1 (1969). (5) E. P. Parry and D. H. Hern, AlAA Paper No. 71-1119, presented at the Joint Conference on Sensing Environmental Pollutants, Palo Alto, Calif., Nov8-10, 1971. (6) K. B. Oldham, ./. Electroanal. Chem., 11, 71 (1966). (7) M .Kozlovskyand A. Zebreva, Progr. Polarogr., 3, 157 (1972).
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Figure 1. Differential pulsed stripping of Cd and Pb from the mercury thin-film electrode Plating conditions: 2 min at -0.8 V vs. SCE; electrode rotated at 60 Hz. Stripping conditions: 100 mV pulse amplitude: 8.6 msec pulse width; 1 pulse/0.5 sec; 5 mV/sec scan rate. Solution: 1.35/1.OOF acetic acid/ potassium acetate buffer. The number on each curve is the ionic strength of the test solution
IONIC STlCNOTW I L!(.103 I
Figure 2. Dependence of ( a ) differential pulsed stripping peak current and ( b ) peak potential on ionic strength of test solution 4 ppb Pb in 1.35/1 .OO acetic acid/potassium acetate buffer. Instrumen-
tal parameters: same as for Figure 1 ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, M A Y 1973
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Figure 3. Dependence of differential pulsed stripping peak current on the reciprocal of the added uncompensated resistance
Solution: 8 ppb Pb in 1.70 acetic acid/1.25F potassium acetate. Instrumental conditions: same as for Figure 1
Similar experiments of O'Dom (8) using the hanging mercury drop electrode indicate that a similar effect of uncompensated resistance obtains a t this electrode, but that an added uncompensated resistance of -50 kl2 is (8) G O'Dom. Princeton Applied Research Corporation, Princeton N J , private communication, 1973
necessary for the effect to show clearly. That a higher resistance is necessary to demonstrate the effect is clearly due to the smaller area of the drop as compared with the thin film; the smaller area manifests itself in a smaller interfacial capacitance and a smaller stripping current, both of which require a larger uncompensated resistance to exhibit the effect seen a t lower resistance values with the thin-film electrode. A complete theoretical and experimental study of pulse anodic stripping voltammetry is under way in our laboratory. Until results of this complete parametric study are available, we suggest that pulsed stripping voltammetry and other voltammetric techniques be used with caution in high resistance solutions. The response depends strongly on the resistance of the test solution which may not be under the analyst's control unless he adds additional supporting electrolyte to swamp the effects of the variable amount of electrolyte naturally present in his sample. Thomas R. Copeland Joseph H. Christie Rodney K. Skogerboe Robert A. Osteryoung Department of Chemistry Colorado State University Fort Collins, Colorado 80521
Received for review November 30, 1972. Accepted February 8, 1973. This work was supported. in part, by the National Science Foundation under Grants GP-31491X and GI-34813X.
I AIDS FOR ANALYTICAL CHEMISTS Comparative Study of Different Methods of Packaging Liquid Reagents G . J. Curtis, J. E. Rein,l and S. S. Yamamura
Analytical Chemistry Branch, Allied Chemical Corporation, P. 0. Box 2204, ldaho Chemical Programs-Operations Office, ldaho falls, ldaho 83401 Liquid reagents of many types are an integral part of the operation of any laboratory performing chemical studies. The reagents, stored in a variety of containers, are used for periods ranging from a day to several years and it is not uncommon to have one question the integrity of a given reagent that has been stored for any length of time. In addition to precipitation or chemical change, evaporation and/or transpiration is the major cause for observed change of reagent concentration with time. To determine the magnitude of evaporation-transpiration change with time, a one-year study of the containment of different liquids in various glass and plastic containers was conducted. Included in the study were 9 common aqueous solutions and 5 organic liquids and 8 different container-closure combinations. In all, more than two thousand observations were made among the 135 specimens involved in the study. The significance of those observations is presented in this paper. I J E Rein I S presently employed at Los Alarnos New Mexico
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, M A Y 1973
EXPERIMENTAL The investigation involved measurement of the weight of various bottles containing different liquids on periodic intervals (weekly for the first month, then monthly thereafter) for one year. The bottles were not opened until the weighings were concluded. The container-closure-liquid combinations covered in the study and the details of certain special experiments are summarized in Table I. The weighings were made on a single-pan 200-gram capacity balance; hence, the quantity of liquid originally packaged was maintained at about 100 ml with the plastic bottles and about 50 ml with the heavier glass bottles. There was one exception t o this. In the case of the polyethylene bottle, a 50-ml water sample also was included to determine whether the amount of vapor space in a container affects the loss. Selected container-reagent combinations were stored in a vapor atmosphere similar or closely similar to that within the container. Cerium(II1) at about a 0.075M level was included in all nitric acid solution and water tests to permit correlation of any observed evaporation losses with corresponding concentration gains.. The cerium(II1) was determined by titration with EDTA to a visual xylenol orange end point. A straightforward base titration to a phenolphthalein end point also was performed on the 1M nitric acid specimens.
