Ideal Electrode

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

The industrial realisation and development of this process, however, has been long delayed owing to the unavailability of suitable GDE. The ideal electrode, when... 

With an ideal electrode (5.26) and (5.27) could be used directly to compute the titration curve. However, with an ISE it is necessary to consider interferences and to calculate the potential from the Nikolsky equation. If it is assumed that interferents are present both in the sample and in the titrant, then the electrode potential is given by [133, 134],... 

With ideal electrodes, the AE should be zero, but with real electrodes this value must be determined by filling both half-cells with the same solutioa... 

Almost all analyte ion inside the membrane in Figure 15-8b is bound in the complex LC+, which is in equilibrium with a small amount of free C+ in the membrane. The membrane also contains excess free L. C+ can diffuse across the interface. In an ideal electrode, R cannot leave the membrane, because it is not soluble in water, and the aqueous anion A-cannot enter the membrane, because it is not soluble in the organic phase. As soon as a few C+ ions diffuse from the membrane into the aqueous phase, there is excess positive charge in the aqueous phase. This imbalance creates an electric potential difference that opposes diffusion of more C+ into the aqueous phase. 

The primary objective of the discussion that follows is to establish a basis for choosing and applying carbon electrodes for analytical applications. As with any electrode material or electroanalytical technique, the choice depends on the application there is no ideal electrode for all situations. We first discuss the criteria that drive the chemist s choice of electrode or procedure. These criteria include background current, potential limits, and electrode kinetics, and may be considered dependent variables that are ultimately controlled by the properties of the carbon surface. Then we consider the independent variables that determine electroanalytical behavior. These include the choice of carbon material, surface roughness, cleanliness, etc. By considering the dependence of electroanalytical behavior on surface variables that the user can control, it should be possible to make rational choices of electrodes and procedures to lead to the desired analytical objective. 

A number of other carbon materials have been used for electrochemical detection [6] however, at this point none of these appear to have a clear advantage over the electrodes described earlier. Nevertheless, these alternative materials certainly do work and there is little doubt that we will continue to see additional entries as the search for the ideal electrode continues. The chapter on carbon electrodes by McCreery and Kneten (Chap. 10) is a good place to review the fundamental issues. 

The ion removal medium is generally a sodium chloride solution, while the ideal electrode-rinsing solution contains sodium sulfate rather than... 

The electrochemical and chemical stability of diamond makes it an ideal electrode material for electrochemical fluorination reactions. The installation of fluorine a to heteroatom-substituted positions can be anodically performed by hydrogen fluoride/triethylamine mixtures. The Fuchigami group studied several electrode materials for the fluorination of oxindole 20. In this transformation to 21 only a... 

Electrode materials. The ideal electrode material for the degradation of organic pollutants should be totally stable in the electrolysis medium cheap and exhibit high activity toward organic oxidation and low activity toward secondary reactions (e.g., oxygen evolution). 

Potentiometric methods have eliminated the problems that beset earlier studies, due to the high electrolyte concentrations required for ideal electrode behavior. Following the so-called constant ionic medium principle [91], a large excess of an indifferent (or inert or swamping) electrolyte is added, so that the activity coefficients of the species can be considered constant when their concentration (very low compared to that of the indifferent electrolyte) are changed over a wide range. 

Scatchard plot analysis of data for river water titrations was conducted for values of [Gujot] 2 10 H. This was done because of apparent non-ideal electrode behavior at [Cujot]... 

In practice, however, deviation from the ideal electrode behavior is common, and an additional contribution to the total measured ion activity has to be considered due to the presence of the interfering ion j in the sample solution. Under these conditions the ion-selective electrode potential can be approximated by the Nikolsky-Eisenman equation ... 

This indicates that potentiostatic plating is better than galvanostatic plating for fabricating fern-shaped deposits, which are, for example, ideal electrodes for Zn-air batteries due to the relatively large specific area. 

What electrochemical properties for redox measurements would the ideal electrode have ... 

Local Probing of Electrochem ical Processes at Non-ideal Electrodes 3... 

In the present contribution, the possibilities of local in-situ STM and SFM probing at non-ideal electrodes are illustrated with recent SPM work performed in the electrochemistry group of the University of Bern STM studies of underpotential deposition of Pb and Tl at non ideal (chemically polished) Ag(l 11) electrodes are presented to show the influence of the nanometer-scale morphology of the non-ideal Ag(lll) substrate upon the local progress of adsorbate formation and the long-term stability of the resulting adsorbates. More detailed reports of the experiments are given elsewhere [3,4]. 

Local Probing of Electrochemical Processes at Non-ideal Electrodes 5... 

Figure 5.2.3 Diffusion fields at a) long and b) short times at a rough electrode. Depicted here is an idealized electrode where the roughness is caused by parallel triangular grooves cut on lines perpendicular to the page. Dotted lines show surfaces of equal concentration in the diffusion layer. Vectors show concentration gradients driving the flux toward the electrode surface.

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