Stripping analysis, voltammetric

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. 

Voltammetric sensors Here, detection is based on the redox behaviour of the analyte on the electrode. However, when the analyte is bound to suspended particles or present in complexes that are chemically inert, direct determination is generally not possible. Therefore, voltammetric sensors provide information on the species that are chemically available (labile). Uniquely, these sensors typically involve a necessary preconcentration step in which the analyte is usually reduced and accumulated for a certain time at the electrode. This process is followed by its oxidation and stripping from the electrode. Whole family of methods has emerged based on the different potential-current profiles for the stripping step, all having common name Stripping Analysis (SA). 

The technique of voltammetric stripping analysis is one of the most sensitive techniques available for the determination of metal ions in complex sample matrices. Traditionally such techniques have been undertaken at Hg electrodes, but in recent years, a large number of reports have focused on the use of SPCEs in this area. Reports prior to 2003 have been reviewed recently by the present authors [3]. 

Different versions of stripping analysis have been successfully employed in connection to bismuth-film electrodes. While most early studies have been devoted to conventional stripping-voltammetric measurements, recent activity expanded the scope of bismuth elec-... 

However, Nafion-modified electrodes also exhibited several disadvantages. For example, the response time of the Nafion-coated sensors increases due to a reduced diffusion coefficient value in the film [7], This can pose a serious disadvantage for in vivo work where dopamine and other neurotransmitter releases often occur on a sub-second time scale. In addition, Nafion coatings perform well for applications such as stripping analysis, but their use for direct voltammetric analysis is complicated by slow equilibration of the film with solution species [5 7], Therefore, there is a need for a modification system that can allow for rapid and selective permeation of the ions of interest. 

For chemical monitoring, a list of priority substances has been established that includes metals such as cadmium, lead, and nickel. As far as metals are concerned, voltammetric techniques and more precisely electrochemical stripping analysis has long been recognized as a powerful technique in environmental samples. In particular, anodic stripping voltammetry (ASV) coupled with screen-printed electrodes (SPEs) is a great simplification in the design and operation of on site heavy metal determination in water, for reasons of cost, simplicity, speed, sensitivity, portability and simultaneous multi-analyte capabilities. The wide applications in the field for heavy metal detection were extensively reviewed (Honeychurch and Hart, 2003 Palchetti et al., 2005). 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

Adsorptive stripping voltammetry (ASV) is another specialised technique where the SMDE electrode is used for reducible species and carbon paste electrodes for oxidisable ones. This allows enrichment (by factors of 100-1000) of ions at the working electrode before stripping them off for measurement this improves the detection limits. This technique is rapid, sensitive (10 "M), economical and simple for trace analysis. The basic instrumentation for stripping analysis is apotentiostat (with voltammetric analyser), electrode and recorder. While voltammetry is generally very useful for compounds that do not have a chromophore or fluorophore, stripping analysis is the best analytical tool for direct, simultaneous determination of metals of environmental concern, e.g. lead, cadmium, zinc and copper in sea water. 

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltammetric measurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanomolar levels and below, with detection limits comparable to those for trace metal determination with ASV. 

Because of its remarkable sensitivity (down to 10-11 M) stripping analysis is the most widely used voltammetric technique for trace metal determination in clinical samples (Wang, 1982A). Stripping analysis can be considered as a two-step technique. In the first step, metals in solution are effectively preconcentrated onto the working electrode by electrodeposition ... 

Eriksen (1979) compared different wet digestion procedures for voltammetric stripping analysis of urine and preferred the use of a mixture of nitric, sulfuric, and perchloric acids. Satisfactory recovery (92-100%) of lead and cadmium from spiked samples was obtained. An effective procedure based on a rapid (20 min.) digestion of freeze-dried urine samples coupled with voltammetric analysis was reported (Golimowski et al., 1979). 

At present other electroanalytical methods, e.g., tensammetry, are of no general interest in inorganic analysis in the pharmaceutical industry. Amperometric titrations are used occasionally for certain determinations, e.g., hydrogen peroxide, and of course for the endpoint determination of the Karl Fischer method. The use of potentiometric stripping analysis as an alternative to the voltammetric approach is not generally used for pharmaceutical compounds. 

Three broad classifications of electrochemical methods are used in this chapter. Po-tentiometric methods include zero-current potentiometry and methods in which current of controlled magnitude is apphed to the working electrode, such as in potentiometric stripping analysis (PSA). Amperometric methods consider all techniques in which current is measured these include constant-potential amper-ometry and amperometric measurements made in response to a variety of applied potential waveforms in voltammetric methods. Impedimetric methods comprise a final classification in these methods, faradaic currents are generally absent, and impedance, conductance, or capacitance is the measured property. 

Anodic stripping analysis The accumulation is usually carried out by the electrolysis at controlled potential for a definite time, t cc under reproducible mass-transport conditions. The deposition potential, E, imposed to the working electrode should be adjusted at the potential adequate to the plateau of the reduction voltammetric wave characteristic for the test metal (see Figs. 62A and 62B for a mixture of two tested species). Before adjusting the experiment, it is recommended to know the solubility data and diffusion coefficients of metals in mercury and their half-wave potentials (see Table 4). 

Hutton EA, Hocevar SB, Ogorevc B, Smyth MR (2003) Bismuth flhn electrode for simultaneous adsorptive stripping analysis of trace cobalt and nickel using constant current chronopotentiometric and voltammetric protocol. Electrochem Commun 5 765... 

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