ES&T
FEATURES Anodic stripping voltammetry as an analytical tool Here is how it works and may be applied properly to environmental surveillance, especially where heavy metal contaminants are concerned
Joseph Wang Department of Chemistry New Mexico State University Las Cruces, N.M. 88003
Heavy metals are one class of contaminants that can produce undesirable effects even if they are present in extremely minute quantities. For instance, unlike many other pollutants, they are not biodegradable, and may persist for long time periods. Therefore, techniques are being devised to measure them at very low concentration levels. Anodic stripping voltammetry (ASV) is a sensitive, precise, and economical electroanalytical technique for detecting trace metals. Consequently, ASV has become one of the popular approaches to environmental analysis. Electrochemical methods of analysis are generally applied to the qualitative and quantitative determination of electroactive species in solution. Anodic stripping voltammetry belongs to the voltammetric branch of electroanalytical techniques. Voltammetric techniques are those in which a current response is measured as a function of a potential waveform increasing in amplitude. In ASV, metals are concentrated by reduction into or onto a microelectrode, followed by anodically reoxidizing (stripping) them to produce a peak-shaped plot of
current as a function of potential. Although ASV cannot be regarded as a new analytical technique, new approaches and instruments have been introduced recently that have enhanced the technique's capabilities. The increasing use of ASV is attributable to its ability to measure simultaneously several elements at concentration levels down to the fractional parts-per-billion (ppb) at a relatively modest cost. (A complete ASV system would cost $4000-5000.) It is fair to say that there is no technique for trace metal analysis that can compete with ASV on the basis of sensitivity per dollar investment. Little expertise is required, and analysis time is on the order of a few minutes. The inherently high degrees of accuracy and precision of ASV stem from its application of Faraday's law. Comparative studies with other analytical techniques, such as various atomic absorption procedures, have emphasized the particular reliability of ASV in trace metal analysis for all types of environmental matrices, especially in various types of natural
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Environ. Sci. Technol., Vol. 16, No. 2, 1982
Why anodic stripping voltammetry is successful • Sensitivity: detection limits at the sub parts-per-billion level • Multi-ion analysis per sample • Precision and accuracy • Powerful tool for determining the chemical nature of trace metal ions (speciation) • Ability to perform on-line analysis • Modest cost
water. The technique has been applied to solving numerous trace metal analysis problems in a variety of other matrices such as food, blood, or fingernail, and has proved to be very sensitive. Moreover, the special property of ASV to be species-sensitive makes this technique an efficient tool in speciation studies of toxic trace metals dissolved in natural waters. Like atomic absorption spectroscopy, ASV is subject to various interferences when it is applied to real life samples; most of these interferences can be eliminated by proper selection of experimental conditions. Water analysis Anodic stripping voltammetry has been used extensively to analyze natural water samples. Ariel and Eisner used ASV, in one of its early (1963) environmental applications, to analyze zinc, cadmium, and copper in Dead Sea brine (7). The major ion content in the Dead Sea water did not interfere with the trace metal determination. Much of the success in applying ASV for analyzing natural waters is attributed to Florence, who used ASV about 10 years ago to determine trace levels of lead, cadmium, zinc, copper, thallium, bismuth, indium, and antimony in seawater (Pacific Ocean), as well as in fish, seaweed, abalone, and oysters inhabiting the water (2, 3). Since then, and through the 1970s, ASV has been widely used for analyzing about 20 trace metals in different types of natural waters such as oceans, lakes, rivers, as well as in sewage or industrial effluents and tap waters. Waters analyzed came from Lake Superior (4), Oslofjord (5), the
0013-936X/82/0916-104A$01.25/0 © 1982 American Chemical Society
ASV: an actual working system in the laboratory English Channel (6), the Arctic Ocean (7), Madison (Wis.) tap water (#), and sewage effluents from plants around San Francisco (9). The more sensitive version of ASV, involving the sensitive differential pulse excitation ramp during the stripping step, is usually employed for measuring the ultratrace concentration level (0.001-1 ppb) of some toxic metals such as thallium, lead, and cadmium in seawater and inland waters. The advantage of using ASV for measuring trace metals in natural water is the direct analysis capability, that is, elimination of concentration steps, and minimum use of reagents (sample pretreatment). This is an important key to minimizing contamination possibilities and changes in the physiochemical nature of the species being measured. Acidification (to pH 1 or 2) of natural water samples, upon their collection, is desirable to prevent loss of the
metals by adsorption on the container walls; an ultrapure acid should be used for this purpose. The added acid also serves as the supporting electrolyte in analyses of inland waters with low salt content. Depending upon the desired metal form, the sample may be filtered through an acid-washed filter. Under the myriad of variances, such as surfactants, metals, and ligands, that are present in natural waters, ASV is subject to many potential interferences. Anamolous ASV response may be observed in natural waters containing considerable quantities of dissolved organic matter. Sorption of surface-active agents on the electrode surface can affect both diagnostic parameters (i p and Er) used in ASV. The sorbed organic layer may slow the rate of metal deposition or change the reversibility of the metal oxidation reaction, resulting in lower i,,, broader peak, or shift of Er to more positive values.
These effects are greater in differential pulse ASV than in the linear scan data, because differential pulse peak currents are more sensitive to small changes in the rate of the electrode reaction. These sorption effects and their implications on the interpretation of ASV data have been discussed in detail by Brezonik et al. (10). For some types of organic rich waters (industrial effluents, marshes, and the like) destruction of the organic matter (by UV irradiation or ozone oxidation (9)) or its removal prior to ASV analysis is recommended. In addition to sorption effects, stripping analysis of natural water samples is subject to interferences caused by intermetallic compound formation and overlapping stripping peaks. Table 1 lists some of the most common interferences to which ASV measurements in natural water samples may be subject, together with successful approaches to avoid them. Environ. Sci. Technol., Vol. 16. No. 2, 1982
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Flow-through ASV systems have been introduced in connection with in situ (real time) analysis (21, 22, 23). Such systems can provide adequate warning in cases of sudden contami nation, and may be very useful for large-scale marine surveys. In addi tion, since sample handling is mini mized and rapid analyses are per formed (with no time delays as in lab oratory-based analyses), these systems offer the most obvious way to minimize errors brought about by contamination or adsorption losses. An automated ASV flow system, based on a mer cury-coated graphite tubular elec trode, has been applied by Zirino and his coworkers for the continuous monitoring of copper and zinc in San Diego Bay (24). New designs and more applications of these flowthrough A S V sensors should be forthcoming in the near future. Speciation An important feature of ASV is its capability to differentiate the various chemical forms of a given trace metal in solution in contrast to atomic ab sorption or neutron activation, which measure the total metal content. This feature makes ASV the technique of choice for speciation and physicochemical characterization of dissolved trace metals in natural water. Knowledge of the chemical state of trace metals in solution is important for understanding their transport, toxicity, and reactivity in natural wa ters. In this regard, emphasis has been placed on trace metal complexation because of the role it might play in
Diagnostic parameters used in anodic stripping voltammetry a Hanging mercury drop electrode: \p = k n 3 / 2 D 2 / 3 r v 1 / 2 C b f m
Thin mercury film electrode ( < 1 0 μ thick): \p = nf/Κνφ
e~^DCbtm
- c o , 2.3, Ep E + Ιθ9 c
~
Τ
δ\νφ
ΓτΓ
a k = Constant, η = number of electrons, D = diffusion coefficient, r = radius of the mercury drop, ν = potential scan rate, Cb = concentra tion of the ion in the bulk solution, f = deposition time, m = mass-transport coefficient, E 1 / 2 = polarographic half-wave potential, R = gas constant, Τ = absolute temperature, F = Fara day's constant, φ = nF/RT, e = base of Naperian logarithm, E° = formal (standard) redox potential,
