Stripping Voltammetry
Stripping voltammetry is a very sensitive method for the analysis of trace concentrations of electroactive species in solution. Detection limits for metal ions at sub-ppb concentrations have been reported.
There are 3 important parts in a stripping experiment:
- Deposition
- Quiet time
- Stripping
These components can best be explained by discussing the stripping experiment for detection of lead. In this experiment, a mercury working electrode is used - either the Hanging Mercury Drop Electrode (HMDE) (using the BASi® CGME) or the Thin Mercury Film Electrode (TMFE) (using the BASi® RDE-2). The TMFE is made by a depositing a mercury film on the surface of a glassy carbon electrode, typically during the deposition step.
Figure 1. Parameters for deposition step when using the CGME.
Figure 2. Parameters for deposition stepwhen using the RDE-2.
The parameters for the deposition step are shown in Fig1 and Fig2. During the deposition step, the potential applied to the mercury electrode is held at a value (Deposition E) at which the lead ions are reduced to lead metal for a pre-determined time period (Deposition Time). If the Use Initial E as Deposition E is checked, the Initial E of the stripping step is used as the Deposition Potential. The metallic lead then amalgamates with the mercury electrode (when the TMFE is used, mercuric ions are generally added to the solution, and mercury metal is codeposited with the lead during the deposition step). The effect of this amalgamation is to concentrate the lead in the mercury electrode, and hence the concentration of lead in the electrode is much greater (typically 2 or 3 orders of magnitude) than the concentration of lead in the solution (consequently, the deposition step is often called the preconcentration or accumulation step). The efficiency of the deposition can be increased by stirring either the solution (when using the CGME) or rotating the electrode (when using the RDE-2). Stirring (for the CGME), rotating (for the RDE-2), and purging during this step can be controlled remotely from the software by checking the Stir/Rotate during Deposition and Purge during Deposition boxes, respectively. Cell Stand in Setup / Manual Settings (I/O) in the Experiment menu must be set to CGME SMDE Mode when using the CGME and to RDE-2 when using the RDE-2.
After the deposition step, the stirring is stopped, and the system is allowed to reach equilibrium. This is the Quiet Time, which is typically 10 - 15 s.
During the stripping step, the applied potential is scanned in a positive direction, and the lead in the mercury electrode is oxidized back to lead ions in solution; that is, the lead is "stripped" from the electrode. The potential at which the stripping occurs is related to the redox potential of the analyte, and hence the potential of the current peak on the stripping step can be used to identify the analyte. The magnitude of the current of the stripping peak is proportional to the concentration of the analyte in the mercury electrode. Since the concentration of the analyte in the electrode is related to its concentration in solution, the stripping peak current is therefore proportional to the solution concentration.
A number of different wave forms have been used for the stripping step, including linear sweep voltammetry (LSSV), differential pulse voltammetry (DPSV), and square wave voltammetry (SWSV). SWSV and DPSV are more commonly used, due to their lower detection limits. The parameters required for each of these wave forms are described in detail elsewhere (click the appropriate link in the previous sentence).
As noted above, it is the concentration of lead in the mercury electrode that is directly measured in the stripping step rather than the concentration of lead in solution. The electrode concentration can be increased by increasing the Deposition Time and/or the rate of stirring. The values required for these two parameters depends on the sensitivity of the mercury electrode, which is determined by the surface area to volume ratio (i.e., how many of the deposited lead atoms are on the mercury surface and hence are detectable in the stripping step). This ratio is considerably higher for the TMFE, so a shorter Deposition Time is required. In addition, faster stirring can be used with the TMFE due to the relative mechanical instability of the HMDE (i.e., the mercury drop can fall off if the stirring is too fast). The signal resolution is also better with the TMFE, which can be important if there is more than one metal ion present.
However, the greater sensitivity of the TMFE can also be a disadvantage, since the solubility limit of the metal in the mercury can be exceeded more readily. This can lead to the formation of intermetallic compounds, which can affect the accuracy of the experimental results (due to e.g., shifts in the stripping potentials and depression of the stripping currents). One pair of metals that readily combine is zinc and copper.
In order to be of use as a quantitative analytical technique, the results of a stripping experiments must be reproducible. Therefore, the experimental conditions must be reproducible. A second disadvantage of the TMFE is the relatively poor reproducibility of the film. Since the film is deposited on the surface of a glassy carbon electrode, it is sensitive to the microstructure of the glassy carbon surface, which can be affected by the method used to prepare the surface. In contrast, an HMDE is highly reproducible. Whatever the chosen mercury electrode, great care must be taken in sample preparation, cleaning of glassware, etc. The rate of stirring during the deposition step must also remain constant.
The above method is called anodic stripping voltammetry (ASV), since the stripping current is anodic. This method can be used for metal ions that can be readily reduced to the metallic state and reoxidized - about 20 metal ions, including lead, copper, cadmium, and zinc. This is not as many as can be detected using atomic absorption spectroscopy (AAS), although the sensitivity of ASV is comparable with, and sometimes better than AAS. The advantage of ASV over AAS is its ability to detect several metal ions simultaneously. In addition, different oxidation states of a given metal can be detected (e.g., arsenic and antimony).
Other stripping voltammetric techniques include cathodic stripping voltammetry (CSV) and adsorptive stripping voltammetry (AdSV). The basis for CSV is the oxidation of mercury followed by the formation of an insoluble film of HgL (L is the analyte) on the surface of the mercury electrode during the deposition step. CSV is most commonly used for detection of sulfur-containing molecules (e.g., thiols, thioureas, and thioamides), but it has also been used for molecules such as riboflavin and nucleic acid bases (e.g., adenine and cytosine).
AdSV is different from ASV and CSV in that the deposition step is non-electrolytic, and occurs via the adsorption of molecules on the surface of the working electrode (the HMDE is most commonly used). The stripping step can be either anodic or cathodic. AdSV has been used for organic molecules (e.g., dopamine, chlorpromazine, erythromycin, dibutone, and ametryne) and for metal complexes of metals not amenable to detection by ASV (e.g., cobalt and nickel).
