Electrodes renewable surfaces

Fe electrodes with electrochemically polished (cathodically pretreated for 1 hr) and renewed surfaces have been investigated in H20 + KF and H20 + Na2S04 by Rybalka et al.721,m by impedance. A diffuse-layer minimum was observed at E = -0.94 V (SCE) in a dilute solution of Na2S04 (Table 19). In dilute KC1 solutions E,njn was shifted 40 to 60 mV toward more negative potentials. The adsorbability of organic compounds (1-pentanol, 1-hexanol, cyclohexanol, diphenylamine) at the Fe electrode was very small, which has been explained in terms of the higher hydro-philicity of Fe compared with Hg and Hg-like metals. 

Studies in the field of electrochemical kinetics were enhanced considerably with the development of the dropping mercury electrode introduced in 1923 by Jaroslav Heyrovsky (1890-1967 Nobel prize, 1959). This electrode not only had an ideally renewable and reproducible surface but also allowed for the first time a quantitative assessment of diffusion processes near the electrode s surface and so an unambiguous distinction between the influence of diffusion and kinetic factors on the reaction rate. At this period a great number of efectrochemical investigations were performed at the dropping mercury efectrode or at stationary mercury electrodes, often at the expense of other types of electrodes (the mercury boom in electrochemistry). 

Metals are the most important electrode materials. Because of the readily renewable surface of mercury electrodes, they have been for several decades and, to a certain degree, still remain the most popular material for theoretical electrochemical research. The large-scale mercury electrode also plays a substantial role in technology (brine electrolysis) but the general tendency to replace it wherever possible is due to the environmental harmfulness of mercury. 

The dropping ISE described by Skobets and coworkers [160, 161 ], where the ion-exchanger solution drops from a glass capillary, permits regular renewal of the electrode surface. A nitrate ISE with a renewable surface [162a] is shown in fig. 4.4. 

Double-layer properties in aqueous, propylene carbonate and formamide solutions have been studied at room temperature for liquid Ga-Pb alloy (0.06 atom % of Pb) [15], as a model of Pb electrode with renewable surface. The electrode behaves as an ideally polarizable electrode in a wide potential range, and its capacitance is intermediate between that of Ga and Hg electrodes and is independent of the solvent. This electrode is much less lipophilic than Ga. Adsorption of anions on this electrode increases in the sequence -BP4 = S042 < Gl < Br < r. 

Polarography and Voltammetry Both methods are the same in that current-potential curves are measured. According to the IUPAC recommendation, the tenn polarography is used when the indicator electrode is a liquid electrode whose surface is periodically or continuously renewed, like a dropping or streaming mercury electrode. When the indicator electrode is some other electrode, the term voltammetry is used. However, there is some confusion in the use of these terms. 

Polish the surface of the GEC electrode prior to each use, first with abrasive paper, and then with alumina paper (polishing strips 301044-001, Orion) to give a smooth mirror finish with a fresh renewable surface. When not in use, the electrodes can be stored in a dried place at room temperature. 

The DME presents special features derived from its homogeneous and isotropic drops, small size, and periodical renewed surface so that the current on each drop rises from zero to its maximum value toward the end of the drop life. Moreover, it is well known that mercury has the highest overpotential for hydrogen evolution, which enables polarization of the electrode to very negative potentials. 

The mercury-pool electrode is especially attractive because of its renewable surface and because it allows the use of a stirring bar to vigorously stir the electrode-solution interface. Its major limitation relative to a platinum electrode is its high mass and the awkwardness of rinsing and of weighing a liquid electrode relative to a solid electrode. 

Polarography is a subclass of voltammetry in which the working electrode is dropping mercury. Because of the special properties of this electrode, particularly its renewable surface and wide cathodic potential range (see Chapters 3-5 for details), polarography has been widely used for the determination of... 

The solvent, and the solution, are made to flow, one in a column down the centre of a vertical tube, the other down the walls, so that there are constantly renewed surfaces of solvent and solution, of large area, in fairly close proximity. The difference in the air-liquid potential between the two liquids (which is the surface potential of the solute) causes a difference in potential between the liquids, and since the liquids are constantly renewed, current must be supplied to one liquid, and taken from the other, in order to maintain this difference. This current is large enough to be measurable by an electrometer. A circuit is therefore constructed with reversible electrodes in contact with the insulated reservoirs containing a supply of each of the liquids, an electrometer to detect the flow of current, and a potentiometer to impose any desired potentials on the liquids. The potentiometer is adjusted until the electrometer shows no flow of current then the applied potential is equal to the difference in... 

One of the inherent problems associated with any heterogeneous technique such as voltammetry is the reproducibility of the properties and nature of an electrode surface. Traditionally, studies in which this was crucial utilized a hanging mercury drop, or dropping mercury electrode which ensured a continuously renewable surface. Cardwell et al. (1996) have described improved techniques for polishing electrodes which will go some way towards providing more reproducible electrode surfaces. Reproducibility may be assisted by the development of disposable electrodes (for example, Wang and Chen, 1994) that have developed disposable enzyme microelectrode array strips for glucose and lactate detection. 

D. C. Grahame, who is revered by electrochemists even today for his meticulous measurements and detailed and careful analysis of the data, believed that starting from double-layer-capacitance data yields more accurate results. He based his argument on the fact that capacitance can be measured on the dropping mercury electrode, with its periodically renewed surface, whereas eleclrocapillary measurements are taken on a stationary interphase, which is more prone to contamination. Also,... 

The most reliable data are from studies of hydrogen evolution on mercury cathodes in acid solutions. This reaction has been studied most extensively over the years. The use of a renewable surface (a dropping mercury electrode, in which a new surface is formed every few seconds), our ability to purify the electrode by distillation, the long range of overpotentials over which the Tafel equation is applicable and the relatively simple mechanism of the reaction in this system all combine to give high credence to the conclusion that p = 0.5. This value has been used in almost all mechanistic studies in electrode kinetics and has led to consistent interpretations of the experimental behavior. It... 

Most biosensors based on AChE have the enzymes immobilized on the surface of the sensor. The inhibition reaction being irreversible, the membrane with immobilized enzyme has to be replaced after several measurements or the biosensor can be use for only one determination. Due to this fact, the researchers tried to realize pesticide biosensors with a renewable surface or disposable biosensors based on screen-printed electrodes (SPE). The screen-printing technology provides a simple, fast and inexpensive method for mass production of disposable biosensors for different biomolecules starting with glucose, lactate and finishing with environmental contaminants as pesticides (Kulys et al., 1991) and herbicides (Skladal, 1992). 

Pt and Pd electrodes with a renewed surface obtained by cutting with a ruby knife under the working nonaqueous solution have been studied by Petrii et al. [69, 70] and by other investigators (Table 6). A more detailed discussion of data obtained is presented in Ref. [5]. 

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