Anodic Stripping Voltammetry - detection and quantitation of metal contaminants

Anodic Stripping voltammetry (ASV), particularly with the use of pulsed potential waveforms, is one of the most sensitive, convenient, and cost effective analytical method for detection and quantitation of metal contaminants in rivers, lakes, process streams and drinking water. Another advantage is that several metals such as Cu, Pb and Cd can be analyzed simultaneously.

ASV has been used for several years to analyze heavy metals such as Cu, Pb, Cd and Zn. ASV can be thought of as a small scale electroplating experiment. The metals, as ions in solution, are plated onto an electrode by applying a negative potential (deposition potential) for a specific period of time. The deposition serves to concentrate the metal ions from the solution onto the electrode in the metallic form. If the electrode is Hg, the metals often form an amalgam. After deposition, the potential is scanned toward positive potentials. Current peaks appear at potentials corresponding to the oxidation of metals as they are oxidized (stripped) from the electrode back into the solution. The peak height or area can be correlated with the concentration of the metal ions in the solution. It is necessary to calibrate the procedure with standard solutions containing known quantities of the metal ions.

The technique was originally developed with a hanging Hg drop electrode. However, to limit the quantity of Hg needed, thin Hg films can be pre-deposited onto an electrode such as glassy carbon or co-deposited with the analyte metal ions,. Use of films circumvents the need for much Hg which is toxic. With such films, sensitivities in the low part-per-billion (ppb) level can be achieved.

Experimental Protocol:

Lightly polish a 1 mm or 3 mm diameter glassy carbon electrode on an alumina slurry which is on a flat glass plate or commercial polishing pad. Polish for 1-2 minutes using a circular figure 8 motion and then rinse the end of the electrode thoroughly with doubly distilled water. You can remove adhering water on the tip by touching Kimwipe to the outer edge.

Prepare a solution containing 1 ppm of Cu(II), Pb(II) and Cd(II) in 0.1 M KNO₃, 5% HNO₃ and 30 ppm Hg(II). All the metals are prepared from nitrate salts. Fill the electrochemical cell with the solution and deposit the metals at an applied potential of -1.0 V vs Ag/AgCl reference electrode for 2 minutes. During deposition, the solution is stirred with a magnetic stirrer. After 2 minutes, stop the stirrer and wait for 10 seconds before scanning the potential from -1.0 V to +0.3 V at a scan rate of 1 or 2 V/sec. A fast scan minimizes interference from dissolved oxygen, which can oxidize the metals from the electrode. To completely eliminate interference, it is best to purge the solution with nitrogen to remove oxygen prior to the deposition step.

The analog ,66-Omni90, or microprocessor controlled, 66-Omni101, or computer-controlled potentiostats 66-CS1090 and 66-CS1200, can be used for ASV. The 66-CS1090 and 66-CS1200 contain ASV software to simplify running and analyzing the data from the experiment. With the 66-Omni90 or 66-Omni101 potentiostats, the current can be recorded with a strip chart recorder. Acquisition of data with a computer is also possible with the 66-OMNI101 when used with the 66-ACQUIRE101 software program.

Typical results are shown in Figures 1 and 2.

Figure 1

Figure 2

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