Electrode potentials, standard mechanism

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

De Souza et al. (1997) used spectroscopic ellipsometry to study the oxidation of nickel in 1 M NaOH. Bare nickel electrodes were prepared by a series of mechanical polishing followed by etching in dilute HCl. The electrode was then transferred to the spectroelectrochemical cell and was cathodicaUy polarized at 1.0 V vs. Hg/HgO for 5 minutes. The electrode potential was then swept to 0.9 V. Ellipsometry data were recorded at several potentials during the first anodic and cathodic sweep. Figure 27.30 shows a voltammogram for Ni in l.OM NaOH. The potentials at which data were recorded are shown. Optical data were obtained for various standard materials, such as NiO, a -Ni(OH)2, p-Ni(OH)2, p-NiOOH, and y-NiOOH. 

Fig. 3 Potential energy profiles for the concerted and the stepwise mechanism in the case of a thermal reductive process. E is the electrode potential for an electrochemical reaction and the standard potential of the electron donor for a homogeneous reaction. For an oxidative process, change - into + and donor into acceptor.

Fig. 6 Passage from the stepwise to the concerted mechanism upon decreasing the driving force. Left potential energy profiles. Right reaction of 4-nitrocumyl chloride with homogeneous donors diamonds 2-nitropropanate ion, squares duroquinone anion radical, circles RNu -. E electrode potential or standard potential of a homogeneous donor.

The stability of silicon electrodes contacting an aqueous electrolyte is a severe problem in regenerative solar systems. As mentioned previously, the standard electrode potential of a silicon element is negative enough to induce an electrochemical reaction mechanism, giving rise to an insulating surface silicon oxide in the absence of complexing reactants. On the... 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

The reason why chemical reactions take place and the mechanism by which they operate are matters of increasing interest and call for some knowledge of thermodynamics and kinetics. The brief excursions into these subjects are synoptic and intended to provide the reader with no more than he will need in pursuing the main part of the book. Information about the kinetics of inorganic reactions is still sparse, but there is a wealth of thermodynamical data, standard electrode potentials and dissociation constants and the student must acquire facility in using these figures. 

For references on electrochemical reaction kinetics and mechanism, see, e.g., Newman and Thomas-Alvea, Electrochemical Systems, 3d ed., Wiley Interscience, 2004 Bard and Faulkner, Electrochemical Methods Fundamentals and Applications, 2d ed., Wiley, 2001 Bethune and Swendeman, Table of Electrode Potentials and Temperature Coefficients, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, New York 1964, pp. 414-424 and Bethune and Swendeman, Standard Aqueous Electrode Potentials and Temperature Coefficients, C. A. Hampel Publisher, 1964. 

It is important to obtain experimental information on the thermodynamics of electrode processes to ascertain the tendency of a particular reaction to occur under a given set of experimental conditions namely temperature, pressure, system com H)sition and electrode potential. Such information is provided by the standard- or formal-electrode potentials for the redox couple under consideration. Appropriate combinations of these potentials enable the thermodynamics of homogeneous redox processes to be determined accurately. However, such quantities often are subject to confusion and misinterpretation. It is, therefore, worthwhile to outline their significance for simple electrochemical reactions. This discussion provides background to the sections on electrochemical kinetics which follow. The evaluation of formal potentials for various types of electrode-reaction mechanisms is dealt with in 12.3.2.2. 

A 1 5 soil/water extract is mechanically stirred during measurement to minimise changes in electrode potential associated with suspension effects and positioning of electrodes. Results by this procedure are commonly higher by about 0.5-0.6 of a pH unit (Baker etal. 1983) than those measured in the field by the mixed indicator/barium sulfate method of Raupach and Tucker (1959). The water should have a pH >6.5 but <7.5. If necessary, boil distilled or deionised water for 15 minutes and cool under C02-free conditions. EC should be <10 3dS/m. Standardisation of the equipment is undertaken using standard pH solutions usually pH 4.00, 7.0 and 9.183. Such solutions are normally purchased ready for use. 

Standard electrode potentials of alkyl halides and nucleophiles have been used to pi edict die rates of SKT alternatives to Ss2 substitution mechanisms 134.49.50.08.601. A reaction much faster than predicted on the basis of Marcus theory is a likelv S 2 process, while similarity between prediction and rate, favors SET. Despite some disagreement as to the proper treatment rtf a dissociative electron transfer 48.7(). this approach has proven useful in helping to classify reactions as probably SN2 or SET in mechanism (most successfully with large, delocalized nucleophiles). 

The considered electrochemical model has brought us to certain conclusions on the mechanism of polymer polarization in the M1-P-M2 systems. It is evident that definite electrode potentials are established on metals faces in contact with the polymer material. These potential values are affected by the work function of the electrons and metal afEnity to the corresponding structural elements of the polymer lining. These potentials are noncoincident with a standard electrochemical series of metals. This is natural, since the properties of polymer materials as electrolytes are not identical to those of salt solutions of the corresponding metals, for which the standard electrode potentials of metals are determined. The studied metals in pairs separated by the PVB lining are arranged in the following series ... 

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