Electrode potentials 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. 

It is stated that in time the acidity (up to 2,5 units) of 0,1-1,0 M HMTA aqueous solutions changes maximally at 1°C, in comparatively to other temperatures (11, 16, 21°C). When the temperature arises the change of HMTA aqueous solutions pH values decreases in time. Formaldehyde and ammonium ions (end products of HMTA hydrolysis) have been fixed only in more diluted solutions (0,10 and 0,25M). The concentration of NH in them in some times is higher than H2C=0 concentration that is caused by oxidation of the last one to a formic acid, being accompanied by the change of the system platinum electrode potential. It is stated that concentration NH in solutions does not exceed 5% from HMTA general content. The conclusion the mechanism of HMTA destruction in H,0 to depend essentially on its concentration and temperature has been made. 

Pourbaix, M., Recent Applications of Electrode Potential Measurements in the Thermodynamics and Kinetics of Corrosion of Metals , Corros., 25, 267 (1969) de Nora, O., Gallone, P., Traini, C. and Meneghini, G., On the Mechanism of Anodic Chlorate Oxidation , J. Electrochem. Soc., 116, 147 (1969)... 

Otherwise, the effect of electrode potential and kinetic parameters as contained in the relevant expression for the PMC signal (21), which controls the lifetime of PMC transients (40), may lead to an erroneous interpretation of kinetic mechanisms. The fact that lifetime measurements of PMC transients largely match the pattern of PMC-potential curves, showing peaks in accumulation and depletion of the semiconductor electrode and a minimum at the flatband potential [Figs. 13, 16-18, 34, and 36(b)], demonstrates that kinetic constants are accessible via PMC transient measurements, as indicated by the simplified relation (40) derived for the depletion layer of an n-type electrode. 

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

Hence, it is important to remember that the products, reaction mechanism and the rate of the process may depend on the history and pretreatment of the electrode and that, indeed, the activity of the electrode may change during the timescale of a preparative electrolysis. Certainly, the mechanism and products may depend on the solution conditions and the electrode potential, purely because of the effect of these parameters on the state of the electrode surface. 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

The mechanical friction between the electrode surfaee under investigation and a suitable probe depends on the electrode potential showing a maximum at Ep-. Of various experimental setups reviewed previously [69Boc2] the most recent one is used to measure the frietion between a test eleetrode in wire shape and a cylindrical slider sitting on the wire [69Boel]. Results are interpreted in terms of the repulsion of double layers being present on the wire and the slider [69Boe2]. (Data obtained with this method are labelled F). 

It is sometimes said that this electrode is reversible with respect to the anion. This claim must be examined in more detail. An electrode potential that depends on anion activity still constitutes no evidence that the anions are direct reactants. Two reaction mechanisms are possible at this electrode, a direct transfer of chloride ions across the interface in accordance with Eq. (3.34) or the combination of the electrode reaction... 

Experimental studies in electrochemistry deal with the bulk properties of electrolytes (conductivity, etc.) equilibrium and nonequilibrium electrode potentials the structure, properties, and condition of interfaces between different phases (electrolytes and electronic conductors, other electrolytes, or insulators) and the namre, kinetics, and mechanism of electrochemical reactions. 

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. 

It is the basic task of electrochemical kinetics to establish the functional relations between the rate of an electrochemical reaction at a given electrode and the various external control parameters the electrode potential, the reactant concentrations, the temperature, and so on. From an analysis of these relations, certain conclusions are drawn as to the reaction mechanism prevailing at a given electrode (the reaction pathway and the nature of the slow step). 

This book seeks essentially to provide a rigorous, yet lucid and comprehensible outline of the basic concepts (phenomena, processes, and laws) that form the subject matter of modem theoretical and applied electrochemistry. Particular attention is given to electrochemical problems of fundamental significance, yet those often treated in an obscure or even incorrect way in monographs and texts. Among these problems are some, that appear elementary at first glance, such as the mechanism of current flow in electrolyte solutions, the nature of electrode potentials, and the values of the transport numbers in diffusion layers. 

On the basis of this argument, the mechanism for the current oscillation and the multilayer formation can be explained as follows. First note that U is kept constant externally with a potentiostat in the present case. In the high-current stage of the current oscillation, the tme electrode potential (or Helmholtz double layer potential), E, is much more positive than U because E is given hy E=U —JAR, where A is the electrode area, R is the resistance of the solution between the electrode surface and the reference electrode, andj is taken as negative for the reduction current. This implies that, even if U is kept constant in the region of the NDR, is much more... 

The presence of solution at a metal surface, as has been discussed, can significantly influence the pathways and energetics of a variety of catalytic reactions, especially electrocatalytic reactions that have the additional complexity of electrode potential. We describe here how the presence of a solution and an electrochemical potential influence the reaction pathways and the reaction mechanism for methanol dehydrogenation over ideal single-crystal surfaces. 

These facts indicate that two mechanisms are involved, depending on the electrode potential in the case of a-FePc whereas only one mechanism seems to be involved in the case of /3-FePc. 

Anderson AB, Neshev NM. 2002. Mechanism for the electro-oxidation of carbon monoxide on platinum, including electrode potential dependence—Theoretical determination. J Electrochem Soc 149 E383-E388. 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

It was mentioned on page 306 (see Fig. 5.24) that, even at room temperature, a crystal plane contains steps and kinks (half-crystal positions). Kinks occur quite often—about one in ten atoms on a step is in the half-crystal position. Ad-atoms are also present in a certain concentration on the surface of the crystal as they are uncharged species, their equilibrium concentration is independent of the electrode potential. The half-crystal position is of basic importance for the kinetics of metal deposition on an identical metal substrate. Two mechanisms can be present in the incorporation of atoms in steps, and thus for step propagation ... 

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