Voltammetry, automation

L.B.O. dos Santos, J.C. Masini, Square wave adsorptive cathodic stripping voltammetry automated by sequential injection analysis potentialities and limitations exemplified by the determination of methyl parathion in water samples. Anal. Chim. Acta 606 (2008) 209—216. 

From the foregoing treatment of voltammetry it is clear that for its application sophisticated apparatus is required, especially the electronics moreover, the construction of the electrodes and their mutual positions within the measuring vessel, of adapted size and without or with stirring, requires great care and experience. For this reason it is often advisable to purchase commercial apparatus, which has reached a high level of sophistication and reliability. Here the most desirable property is smooth recording of the voltammetric curve, which necessitates the kind of automation inherently required in the voltammetric method this is different from advanced automation, which is treated in Part C. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Gutes and coworkers presented an automated electronic tongue based on sequential injection analysis (SIA) and linear sweep voltammetry, for the simultaneous determination of glucose and ascorbic acid, by means of ANN regression. The models were evaluated with an external test set (Gutes et al., 2006). 

Several techniques arising from cyclic voltammetry help the interested reader to peer into the future. Derivative polarograph (di/dV against Vt) increases the sharpness of detection of dissolved radicals and molecular fragments. Microelectrodes can be used with potential sweep circuitry. The use of varying electrical wave forms (instead of the linear potential variation) offers much to be learned in the future. Automation and the use of pattern recognition in mechanism evaluations... 

Voltammetry experiments are not often performed in flow cells for analytical purposes. One reason for this is the special problem of ohmic potential losses (iR drops) at an electrode in a confined stream. Another reason is the problem of precisely pumping solution at a carefully controlled velocity. In general, rotating electrodes are more easily controlled and do not involve serious plumbing problems. On the other hand, flow cells operated at a fixed potential (i.e., at one point along the steady-state voltammetric curve) are eminently useful for electrosynthesis, chromatographic detection, and automated analysis systems. These features will be described in later chapters. 

Linear sweep voltammetry, capacitance-voltage and automated admittance measurements have been applied to characterize the n-GaAs/room temperature molten salt interphase. Semiconductor crystal orientation is shown to be an important factor in the manner in which chemical interactions with the electrolyte can influence the surface potentials. For example, the flat-band shift for (100) orientation was (2.3RT/F)V per pCl" unit compared to 2(2.3RT/F)V per pCl" for (111) orientation. The manner in which these interactions may be used to optimize cell performance is discussed. The equivalent parallel conductance method has been used to identify the circuit elements for the non-illum-inated semi conductor/electrolyte interphase. The utility of this... 

Agemian and Bedak [42] have described a semi-automated method for the determination of total arsenic in soils. Chappell et al. [43] have described an inexpensive but effective method for the quantitative determination of arsenic species in contaminated soils. Chappell found that the extraction efficiency varied with the ratio of soil to acid and with the concentration of the acid. Rurikova and Beno [346] accomplished speciation of arsenic(III) and arsenic(V) in soils by cathodic stripping voltammetry. Wenclawiak and Krah [347] used reactive supercritical fluid extraction in speciation studies of inorganic and organic arsenic in soils. In this method, derivatisation with thioglycollic acid methyl ester was performed in supercritical carbon dioxide. Various other workers have discussed the determination of arsenic in soils [44-46]. 

Flow-injection analysis is also well-suited for the automation of anodic stripping voltammetry. Metals can be plated from the sample solution as it passes over the electrode. Stripping is then carried out in the deox-ygenated carrier stream (15, 34). The sample itself does not have to be deox-ygenated. Detection limits of 3 nM have been reported for lead by this technique (34). 

Automated Anodic Stripping Voltammetry for the Measurement of Copper, Zinc, Cadmium, and Lead in Seawater... 

ZERiNO AND LiEBERMAN Automated Anodic Stripping Voltammetry 85... 

To summarize, the analysis of seawater samples of representative composition for copper, zinc, cadmium, and lead by AASV with standard addition should yield reasonably accurate values for the concentrations of the metals. Although nickel and silver are present in seawater in concentrations high enough to interfere with the determinations of zinc and copper, the error caused by these metals is expected not to exceed 10 or 15%. Nevertheless, the composition of samples cannot always be guaranteed, and the analysis is always made on the assumption that the standard partitions are present in the sample and in the film in exactly the same manner as the metals originally present in the sample. Because this cannot be known with certainty, particularly when a field survey is being conducted, automated ASV with the thin film must at present be considered a semiquantitative indicator of trace metal activity in the water. Thorough intercomparisons between thin-film voltammetry and other techniques are needed to establish fully the quantitative aspects of this method. 

Table 14.3 A comprehensive list of the NO peak potentials (mV)/oxidation currents (pA) determined for all individual member of the metalloporphyrin library (left general chemical structure) as assessed by means of automated differential pulse voltammetry (right Ryabova et al.75...

Fig. 14.27 Automated adsorptive stripping voltammetry (AdSV) of Ni2+ ions (a) calibration plot derived from the peak current of DPVs of automated AdSV using the standard addition method (b) Long-term Ni2+ ions release from surfaces of electropolished NiTi shape memory alloys kept in NaCl solution. NaCl concentrations were either 15wt.% solid line) or 0.9wt.% dotted line).78 (Reproduced by permission of Wiley-VCH Verlag GmbH Co KGaA)...

Electrochemical methods of analysis have grown greatly in application and importance over the last 40 years, and this has been largely due to the development and improvement of electronic systems permitting refinements in the measurement of the critical characteristics mentioned in the foregoing. In addition to this, the measurement systems and the advanced electronics now permit much of the work in electroanalytical chemistry to be automated and controlled by microprocessors or computers. Some electroanalytical techniques have become very widely accepted others, such as polarography/voltammetry, less so. This has been due to early problems with equipment. Despite the fact... 

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