Stripping voltammetry techniques

Abrasive stripping voltammetry — Technique where traces of solid particles are abrasively transferred onto the surface of an -> electrode, followed by an electrochemical dissolution (anodic or cathodic dissolution) that is recorded as a current-voltage curve [i]. It allows qualitative and quantitative analysis of metals, alloys, minerals, etc. The technique is a variant of - voltammetry of immobilized particles [ii]. 

A variety of spectrographic, colorimetric, polarographic, and other analytical techniques are used for routine measurement of silver in biological and abiotic samples. The detection limit of silver in biological tissues with scanning electron microscopy and X-ray energy spectrometry is 0.02 xg/kg and sometimes as low as 0.005 xg/kg. In air, water, and soil samples, the preferred analytical procedures include flame and furnace atomic absorption spectrometry, plasma emission spectroscopy, and neutron activation. Sensitive anodic stripping voltammetry techniques have recently been developed to measure free silver ion in surface waters at concentrations as low as 0.1 xg/L. 

Stripping Voltammetry One of the most important quantitative voltammetric techniques is stripping voltammetry, which is composed of three related techniques anodic, cathodic, and adsorptive stripping voltammetry. Since anodic strip-... 

Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions. For example, in stripping voltammetry, sensitivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that obtained with a linear potential scan. 

The methods of investigation of metal species in natural waters must possess by well dividing ability and high sensitivity and selectivity to determination of several metal forms. The catalytic including chemiluminescent (CL) techniques and anodic stripping voltammetry (ASV) are the most useful to determination of trace metals and their forms. The methods considered ai e characterized by a low detection limits. Moreover, they allow detection of the most toxic form of metals, that is, metal free ions and labile complexes. 

Pretreatment of the collected particulate matter may be required for chemical analysis. Pretreatment generally involves extraction of the particulate matter into a liquid. The solution may be further treated to transform the material into a form suitable for analysis. Trace metals may be determined by atomic absorption spectroscopy (AA), emission spectroscopy, polarogra-phy, and anodic stripping voltammetry. Analysis of anions is possible by colorimetric techniques and ion chromatography. Sulfate (S04 ), sulfite (SO-, ), nitrate (NO3 ), chloride Cl ), and fluoride (F ) may be determined by ion chromatography (15). 

From the nature of the process described above it has been referred to as stripping polarography , but the term anodic stripping voltammetry is preferred. It is also possible to reverse the polarity of the two electrodes of the cell, thus leading to the technique of cathodic stripping voltammetry. 

Electrodes. The Hanging Mercury Drop Electrode is traditionally associated with the technique of stripping voltammetry and its capabilities were investigated by Kemula and Kublik.51 In view of the importance of drop size it is essential to be able to set up exactly reproducible drops, and this can be done as explained in Section 16.8 for the S.M.D.E. 

Anodic stripping voltammetry (ASV) has been used extensively for the determination of heavy metals in samples of biological origin, such as lead in blood. ASV has the lowest detection limit of the commonly used electroanalytical techniques. Analyte concentrations as low as 10 M have been determined. Figure 16 illustrates ASV for the determination of Pb at a mercury electrode. The technique consists of two steps. The potential of the electrode is first held at a negative value for several minutes to concentrate some of the Pb " from the solution into the mercury electrode as Pb. The electrode process is... 

A hanging electrolyte drop has also been applied to determine ionic species in solution using differential-pulse-stripping voltammetry procedures [69]. Particular emphasis was given to assessing the selectivity and sensitivity of the method. The technique of current-scan polarography has also been applied in the study of electron-transfer [70] and coupled electron-transfer-ion-transfer [71,72] reactions at the ITIES in this configuration. 

It is an advantage of electroanalysis and its apparatus that the financial investment is low in comparison, for instance, with the more instrumental spectrometric methods real disadvantages are the need to have the analyte in solution and to be familiar with the various techniques and their electrochemistry it is to be regretted that the knowledge of chemistry and the skill needed often deter workers from applying electroanalysis when this offers possibilies competitive with more instrumental methods (cf., stripping voltammetry versus atomic absorption spectrometry). 

Since 1978, the microprocessor has been increasingly used in electrochemical apparatus34. For instance, the above-mentioned microprocessor-controlled pH meters appeared on the market in about 1979 and the first application of a microprocessor in the differential pulse technique was even reported in 197735, in stripping voltammetry in 197836 and in combinations of various techniques37 41 from 1980 to 1982. 

The Model 384B (see Fig. 5.10) offers nine voltammetric techniques square-wave voltammetry, differential-pulse polarography (DPP), normal-pulse polar-ography (NPP), sampled DC polarography, square-wave stripping voltammetry, differential pulse stripping, DC stripping, linear sweep voltammetry (LSV) and cyclic staircase voltammetry. 

Van den Berg [131] used this technique to determine nanomolar levels of nitrate in seawater. Samples of seawater from the Menai Straits were filtered and nitrite present reacted with sulfanilamide and naphthyl-amine at pH 2.5. The pH was then adjusted to 8.4 with borate buffer, the solution de-aerated, and then subjected to absorptive cathodic stripping voltammetry. The concentration of dye was linearly related to the height of the reduction peak in the range 0.3-200 nM nitrate. The optimal concentrations of sulfanilamide and naphthyl-amine were 2 mM and 0.1 mM, respectively, at pH 2.5. The standard deviation of a determination of 4 nM nitrite was 2%. The detection was 0.3 nM for an adsorption time of 60 sec. The sensitivity of the method in seawater was the same as in fresh water. 

Measurement techniques that can be employed for the determination of trace metals include atomic absorption spectrometry, anodic stripping voltammetry, differential pulse cathodic stripping voltammetry, inductively coupled plasma atomic emission spectrometry, liquid chromatography of the metal chelates with ultraviolet-visible absorption and, more recently, inductively coupled plasma mass spectrometry. 

Other techniques that have been used include subtractive differential pulse voltammetry at twin gold electrodes [492], anodic stripping voltammetry using glassy-carbon electrodes [495,496], X-ray fluorescence analysis [493], and neutron activation analysis [494],... 

---