Electrochemical methods comparison

Economic Aspects. Several pubUcations probe the various areas of electroorganic process cost. CeUs (90), overaU process costs (41,91—93), economic optimization (94,95), and a comparison between the chemical and electrochemical methods (91,96) are aU discussed. 

Comparison of Chemical and Electrochemical Methods in Organic Synthesis... 

Liquid-phase electron-transfer reactions that lead to ion radicals can be reversible. The equilibria of these reactions can be managed to obtain the desired results. This chapter considers methods for such management. Electrochemical methods of ion radical generation are given in comparison with chemical ones. Chemically generated ion radicals can exist in solutions or, in some special cases, as solids. The peculiarities of all the methods used for ion radical generation are essential in understanding of ion radical reactivity. 

It follows from Equation 6.12 that the current depends on the surface concentrations of O and R, i.e. on the potential of the working electrode, but the current is, for obvious reasons, also dependent on the transport of O and R to and from the electrode surface. It is intuitively understood that the transport of a substrate to the electrode surface, and of intermediates and products away from the electrode surface, has to be effective in order to achieve a high rate of conversion. In this sense, an electrochemical reaction is similar to any other chemical surface process. In a typical laboratory electrolysis cell, the necessary transport is accomplished by magnetic stirring. How exactly the fluid flow achieved by stirring and the diffusion in and out of the stationary layer close to the electrode surface may be described in mathematical terms is usually of no concern the mass transport just has to be effective. The situation is quite different when an electrochemical method is to be used for kinetics and mechanism studies. Kinetics and mechanism studies are, as a rule, based on the comparison of experimental results with theoretical predictions based on a given set of rate laws and, for this reason, it is of the utmost importance that the mass transport is well defined and calculable. Since the intention here is simply to introduce the different contributions to mass transport in electrochemistry, rather than to present a full mathematical account of the transport phenomena met in various electrochemical methods, we shall consider transport in only one dimension, the x-coordinate, normal to a planar electrode surface (see also Chapter 5). 

Depending on the circumstances at hand, several different types of mean comparisons can be made. In this section we review the method for comparison of two means with independent samples. Other applications, such as a comparison of means with matched samples, can be found in statistical texts. Suppose, for example, we have two methods for the determination of lead (Pb) in orchard leaves. The first method is based on the electrochemical method of potentiometric stripping analysis [1], and the second is based on the method of atomic absorption spectroscopy [2], We perform replicate analyses of homogeneous aliquots prepared by dissolving the orchard leaves into one homogeneous solution and obtain the data listed in Table 3.1. 

Anodic cyanation has been shown to be a direct process by electrochemical methods in conjunction with the analysis of products from cpe experiments (nos. 11 and 34, Table 8). In addition, cyano radicals can be generated in homogeneous solution, and a comparison of processes initiated by radical cation and cyano radical initiated processes reveals the indiscriminate nature of the latter towards aromatic substrates (cf. also Williams, 1960). This is in contrast to the electrophilic nature of the radical cation process. 

In this expression, i is current density, p is density, n is the number of electron equivalents per mole of dissolved metal, M is the atomic weight of the metal, F is Faraday s constant, r is pit radius, and t is time. The advantage of this technique is that a direct determination of the dissolution kinetics is obtained. A direct determination of this type is not possible by electrochemical methods, in which the current recorded is a net current representing the difference between the anodic and the cathodic reaction rates. In fact, a comparison of this nonelectrochemical growth rate determination with a comparable electrochemical growth rate determination shows that the partial cathodic current due to proton reduction in a growing pit in A1 is about 15% of the total anodic current (26). 

The redox potentials for many organic ions and radicals are available in the literature [22, 82-86], or they can be estimated (usually to within 0.1 eV) by several simple electrochemical methods. The redox potentials of radical ion precursors are similarly available by direct measurement or by comparison with the redox data of appropriate electronically equivalent, but kinetically stable,... 

Fig. 9J Comparison of the radiotracer and the electrochemical methods for the adsorption of benzene on Pt. v = 1 Vis. Radiotracer method , electrochemical method a a A. Data from Duic, Bockris and Gileadi, Electrochim. Acta, 13, 1915, 1968).

An alternative approach to pK determination for very weak hydrocarbon acids is the electrochemical method of Breslow " . This method is thermodynamic in origin employing voltammetric reduction/oxidation of the cation (or anion) to radical thence to anion (or cation) and comparing the energetics of these steps to the triphenylmethyl system (including bond dissociation energies of the respective hydrocarbons). Values of p aS obtained for some weak carbon acids by this method are given in Table 1 for comparison with the Streitwieser results. 

Figure 20. Steady-state electrochemical method, (a) Concentration profiles of the product obtained upon electron transfer in the EC sequence in Eqs. (190) to (191) as a function of the dimensionless chemical rate constant k5 /D (numbers on the solid curves). The reactant concentration is shown for comparison as the dashed line, (b) Variations in the product electrode concentration as a function of k5 /D. The dashed curve corresponds to the approximation in Eq. (206).

Electrodeposition of Pt on multiwall carbon nanotubes (MWNT) has been reported by Wang et al.27 In their process, a layer of Co was first electrodeposited on a carbon paper. The purpose of Co film was to catalyze the growth of the subsequent MWNTs layer. A chemical vapor deposition (CVD) technique was used to deposit MWNTs on the Co film. Platinum was then electrodeposited on the MWNT layer. The electrolyte used contained H2PtCl6 and H2SO4. The electrodeposition of Pt was performed under potentiostatic condition using 0 V vs. saturate calomel electrode (SCE). The particle size achieved with this technique was approximately 25 nm that is too large in comparison with other chemical and electrochemical methods. 

Barek, J., V. Pacakova, K. Stulik, and J. Zima. 1985. Monitoring of aromatic amines by HPLC with electrochemical detection. Comparison of methods for destruction of carcinogenic aromatic amines in laboratory wastes. Talanta 52(4) 279-83. 

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