Other Voltammetric Techniques Stripping Voltammetry

Anodic Stripping Voltammetry (ASV) is an extremely sensitive electro-analytical technique that can determine trace quantities of certain metals at the parts-per-billion level. The first phase of an ASV experiment involves a pre-concentration 

in which the analyte is deposited (i.e. in the case of metal analysis, reduced to its elemental form) at the working electrode by controlled potential electrolysis in a stirred solution at a suitable reduction potential as shown in Fig. 2.39. The process can be written as  

In the second phase, the potential of the working electrode is scanned so that the deposited metal is oxidised back to its ionic form, i.e. is anodically stripped from the electrode (see Fig. 2.39) by scanning the potential to a value where the following process occurs  

Additionally, as shown in Fig. 2.41, the potential—time profile can be used in ASV with a cleaning step (step A). This is usually applied in-between measurements to ensure that the deposited metal is fully stripped from the electrode surface so as to improve the reproducibility of the analytical measurement 

In addition to anodic stripping voltammetry there are also cathodic and adsorptive stripping voltammetry. In each case, similar to anodic stripping voltammetry, there is first a pre-conditioning step, which for cathodic stripping is either  

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Finally, the stripping procedure can also be applied to the interface between two immiscible electrolyte solutions [96]. By a proper polarization of the interface, a certain ion can be transferred from the sample solution into a small volume of the second solution. After this accumulation, the ion can be stripped off by linear scan voltammetry, or some other voltammetric technique. The shipping peak current is linearly proportional to the concentration of ions in the second solution and indirectly to the concentration of ions in the sample solution. The method is used for the determination of electroinactive ions, such as perchlorate anion [97]. The principles of the procedure are the same as in the case of faradaic reactions, and the differences arise from the particular properties of phenomena on the interface that are beyond the scope of this chapter. 

Polarography has been largely replaced by voltammetry with electrode materials that do not present the toxicity hazard of mercury. Principles described for the mercury electrode apply to other electrodes. Mercury is still the electrode of choice for stripping analysis, which is the most sensitive voltammetric technique. For cleaning up mercury spills, see note 18. 

The adsorption of ions and molecules on the surface of mercury electrodes is a thoroughly investigated phenomenon [51 ]. Surface-active substances are either electroactive [52] or electroinactive [53]. The former can be analyzed by adsorptive stripping voltammetry [54]. This is the common name for several electroanalytical methods based on the adsorptive accumulation of the reactant and the reduction, or oxidation, of the adsorbate by some voltammetric technique, regardless of the mechanisms of the adsorption and the electrode reaction [55, 56]. Frequently, the product of the electrode reaction remains adsorbed to the electrode surface. Hence, the term stripping should not be taken literally in all cases. Besides, some adsorbates may be formed by electrosorption reactions, so that their reduction includes covalently bound mercury atoms. The boundary between adsorption followed by reduction, on the one hand, and electrosorption, on the other, is not strictly defined. Moreover, it is not uncommon that, upon cathodic polarization, the current response is caused by a catalytic evolution of hydrogen, and not by the reduction of the adsorbate itself [57]. However, what is common to all methods is a hnear relationship between the surface concentration of the adsorbate and the concentration of analyte at the electrode surface ... 

M), which is suitable for monitoring ultralow levels of trace metals in natural water. Other advantages include multielements measurement, trace element speciation and distribution, and continuous in situ detection. The sensors for two voltammetric stripping techniques are discussed below anodic stripping voltammetry (ASC) and adsorptive cathodic stripping voltammetry (AdCSV). 

Square-wave voltammetry (SWV) is one of the four major voltammetric techniques provided by modern computer-controlled electroanalytical instruments, such as Autolab and pAutolab (both EcoChemie, Utrecht), BAS 100 A (Bioana-lytical Systems) and PAR Model 384 B (Princeton Applied Research) [1], The other three important techniques are single scan and cyclic staircase, pulse and differential pulse voltammetry (see Chap. II.2). All four are either directly applied or after a preconcentration to record the stripping process. The application of SWV boomed in the last decade, firstly because of the widespread use of the instruments mentioned above, secondly because of a well-developed theory, and finally, and most importantly, because of its high sensitivity to surface-confined electrode reactions. Adsorptive stripping SWV is the best electroanalytical method for the determination of electroactive organic molecules that are adsorbed on the electrode surface [2]. 

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