Conventional electrode

Clinical Applications Perhaps the area in which ion-selective electrodes receive the widest use is in clinical analysis, where their selectivity for the analyte in a complex matrix provides a significant advantage over many other analytical methods. The most common analytes are electrolytes, such as Na+, K+, Ca +, H+, and Ch, and dissolved gases, such as CO2. For extracellular fluids, such as blood and urine, the analysis can be made in vitro with conventional electrodes, provided that sufficient sample is available. Some clinical analyzers place a series of ion-selective electrodes in a flow... 

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

Additional information on the rates of these (and other) coupled chemical reactions can be achieved by changing the scan rate (i.e., adjusting the experimental time scale). In particular, the scan rate controls the tune spent between the switching potential and the peak potential (during which the chemical reaction occurs). Hence, as illustrated in Figure 2-6, i is the ratio of the rate constant (of the chemical step) to die scan rate, which controls the peak ratio. Most useful information is obtained when the reaction time lies within the experimental tune scale. For scan rates between 0.02 and 200 V s-1 (common with conventional electrodes), the accessible... 

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

To circumvent high overvoltage and fouling problems encountered with the direct oxidation of NADH at conventional electrode (equation 6-11), much work has been devoted to the development of modified electrodes with catalytic properties for... 

Future trends may include the commercialization of ISE s for other clinically significant ions such as bicarbonate, magnesium and phosphate. Solid contact electrodes and ISFET s may allow for mass production of smaller, less expensive devices. However, a high standard of performance must be achieved before conventional electrodes become obsolete. 

Since the absolute and the conventional electrode potentials differ only by an additive constant, the absolute potential depends on the concentration of the reactants through the familiar Nernst s equation. This dependence is implicitly contained in Eq. (2.6) the real Gibbs energies of solvation contain an entropic term, which depends on the concentration of the species in the solution. 

To learn that mediators are redox species used to effect redox chemistry on biological species that are electroactive, yet inert at most conventional electrodes the charge employed in electro-converting the mediator is the same as would have been used in converting the analyte if it was electroactive. To appreciate that mediators can be electro-reduced or electro-oxidized, yet will effect a chemical redox reaction with the analyte. 

An electrode of surface area 100 pm or less is called a microelectrode and provides a means of decreasing the double-layer capacitance which can affect our coulometry experiments so badly. Microelectrodes are also useful when the cell considered is also tiny, as, for example, is the case when performing in vivo voltammetry (see next chapter) with biological samples. For example, a nerve ending is typically 10-100 pm in diameter, so electroanalytical experiments using a conventional electrode would be impossible. 

It is true that only a minute amount of electrolysis can occur at the electrode because of its size. However, because the electrode is small, the layer around the electrode that is depleted of analyte is quite thin - certainly far thinner than that around a conventional electrode. 

CNT conductive surface modification Both SWNTs and MWNTs can be deposited directly from a CNT dispersion as a random network or thin film on conventional electrodes. From the point of view of their construction such electrodes are very easy to prepare but they may suffer from mechanical instability, thus limiting their application. 

Fig. 12. Position of the band edges of various semiconductors in absolute and conventional electrode potential scale at pH = 7 GaP (40). CdSe (41), CdS (42), ZnO (43), T1O2 (44, 45), SnC>2 (46) and the electron transfer terms of the excited rhodaminc in solution...

Extensive studies have been carried out concerning ion transfers, electron transfers and combinations of ion and electron transfers at liquid-liquid interfaces. Po-larography and voltammetry at liquid-liquid interfaces are of analytical importance, because they are applicable to ionic species that are neither reducible nor oxidizable at conventional electrodes. They are also usefid in studying charge-transfer processes at liquid-liquid interfaces or at membranes solvent extractions, phase transfer catalyses, ion transport at biological membranes, etc. are included among such processes. 

It is therefore intriguing to understand what is the particular role of the platinum/electrolyte interface in the Kolbe synthesis favoring that reaction path—Eqs. (39a)-(39c)—which is thermodynamically disfavored and unlikely to occur. A closely related reaction whose kinetics are easier to investigate with conventional electrode kinetic methods is the anodically initiated addition of N3 radicals to olefins, discovered by Schafer and Alazrak (275). The consecutive reactions, which follow the initial generation of the reactive intermediate, an Na radical, are somewhat slower than that of the Kolbe radicals, so that their rate influences the shape and potential of the current voltage curves which can be evaluated in terms of reaction rates and rate laws. 

Numerous ingenious compound electrodes using enzymes have been built.29 These devices contain a conventional electrode coated with an enzyme that catalyzes a reaction of the analyte. The product of the reaction is detected by the electrode. 

Because of the difficulties described earlier, electroanalytical studies are usually performed separately from radical generation studies. But a flat cell has been designed [26] (Fig. 29.19) to permit simultaneous monitoring of the electrochemical and EPR response of a free-radical system (SEEPR). The auxiliary electrode extends along the edges of the working electrode, which diminishes the problems of iR drops and provides better uniformity of current density than is possible with conventional electrode placement. This cell is used primarily for short-term (on the order of seconds) electrochemical experiments, such as... 

In recent years, considerable effort has gone into the development of a new class of electrochemical devices called chemically modified electrodes. While conventional electrodes are typified by generally nonspecific electrochemical behavior, i.e., they serve primarily as sites for heterogeneous electron transfer, the redox (reduction-oxidation) characteristics of chemically modified electrodes may be tailored to enhance desired redox processes over others. Thus, the chemical modification of an electrode surface can lead to a wide variety of effects including the retardation or acceleration of electrochemical reaction rates, protection of electrodes, electro-optical phenomena, and enhancement of electroanalytical specificity and sensitivity. As a result of the importance of these effects, a relatively new field of research has developed in which the... 

The electrohydrodimerisation of acrylonitrile to give adiponitrile (a one-electron process at high substrate concentrations, Scheme 1.8A and Chapter 6) is an example of how an industrially important electrosynthetic process has been investigated following recent instrumental developments, viz. the application of ultramicroelectrodes at low-voltage sweep rates. Use of conventional electrodes would have required substrate concentrations in the mM range but, under these conditions, acrylonitrile undergoes a different reaction - a two-electron electrochemical reduction of the alkene residue (Scheme 1.8B). The switchover between the two reactions occurs at about 1 mol dm-3 substrate concentration. 

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