Types of Electrodes

Many different types of electrodes are used for potentio-metric applications. They include redox, ion-selective membrane (glass and polymer), and PCO2 electrodes. 

Redox potentials are the result of chemical equilibria involving electron transfer reactions  

In an electrochemical cell, electrons maybe accepted from or donated to an inert metaUic conductor (e.g., platinum). A reduction process tends to charge the electrode positively (remove electrons), and an oxidation process tends to charge the electrode negatively (add electrons). By convention, a heterogeneous redox equilibrium (equation 2) is represented by the cell 

A positive potential E 0) for this cell signifies that the cell reaction proceeds spontaneously from left to right E 0 signifies that the reaction proceeds from right to left and = 0 indicates that the two redox couples are at mutual equilibrium. 

The electrode potential (reduction potential) for a redox couple is defined as the couple s potential measured with respect to the standard hydrogen electrode, which is set equal to zero (see hydrogen electrode later). This potential, by convention, is the electromotive force of a cell, where the standard hydrogen electrode is the reference electrode (left electrode) and the given half-cell is the indicator electrode (right electrode). The reduction potential for a given redox couple is given by the Nernst equation  

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Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

In voltammetry we measure the current in an electrochemical cell as a function of the applied potential. Individual voltammetric methods differ in terms of the type of electrode used, how the applied potential is changed, and whether the transport of material to the electrode s surface is enhanced by stirring. 

Electrodes. Because of the numerous different processes, there are many different types of electrodes in use (9), eg, prefabricated graphite, prefabricated carbon, self-baking, and composite electrodes (see Carbon). Graphite electrodes are used primarily in smaller furnaces or in sealed furnaces. Prebaked carbon electrodes, made in diameters of <152 cm or 76 by 61 cm rectangular, are used primarily in smelting furnaces where the process requkes them. However, self-baking electrodes are preferred because of thek lower cost. 

Immersion electrodes are the most common glass electrodes. These are roughly cylindrical and consist of a barrel or stem of inert glass that is sealed at the lower end to a tip, which is often hemispherical, of special pH-responsive glass. The tip is completely immersed in the solution during measurements. Miniature and microelectrodes are also used widely, particularly in physiological studies. Capillary electrodes permit the use of small samples and provide protection from exposure to air during the measurements, eg, for the determination of blood pH. This type of electrode may be provided with a water jacket for temperature control. 

The choice of size, shape, and type of electrode is based on economic considerations and is usually determined by the characteristics of the gas and suspended matter and by mechanical considerations such as flue arrangement, the available space, and previous experience with the electrodes on similar problems. The spacing between collecting electrodes in plate-type precipitators and the pipe diameter... 

When electrical conductivity is used as the basis of the sorting process, contact of the particles is made by a brush type of electrode to generate the signal for analysis. Materials having a resistance difference of 2000 kn can be readily separated from material of 100 kH resistance. 

Ttu pci roniiance of this type of electrode grounding is almost the same as for the pipe grounding (Section 22.1.2) its is the viirialion in resistance to the ground with the length of the electrode as in Figure 22.4. 

Steady-state potential comparable with Type 2 reversible electrode Potentials of electropositive metals that react with solution to give sparingly soluble salts of the metal. Cu or Ag in NaCl or Ag in HCI giving an M/MX/X type of electrode... 

The response time of an electrode is defined as the time taken for the cell e.m.f. to a reach a value which is 1 millivolt from the final equilibrium value. The response time is obviously affected by the type of electrode, particularly with regard to the nature of the membrane, and is also affected by the presence of interfering ions, and by change in temperature. 

The quality of steam produced in electrode boilers is greatly affected by the quality of the electrolyte. For jet steam boilers, the BW should be demineralized water, with conductivity provided by adding an electrolyte. For other types of electrode boilers, the FW supplied should be fully softened, as a minimum. 

With both types of electrodes Imbihl and coworkers28,29 found good agreement with the work function-change potential-change equality... 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

Input power is not the energy consumption at the discharge-gap, but the presented value on the high-voltage power supply. Results show that type A electrodes offer the highest ener efficiency (LHV, 30.2% HHV, 30.8%). For that reason, the pair of coaxial needle type of electrodes was selected for subsequent experiments. 

At another type of active electrode, found in many batteries, the reaction is the conversion between a metal and an Insoluble salt. At the surface of this type of electrode, metal cations combine with anions from the solution to form the salt. One example is the cadmium anode of a rechargeable nickel-cadmium battery, at whose surface cadmium metal loses electrons and forms cations. These cations combine immediately with hydroxide ions in... 

Finally, a simple method for a rapid evaluation of the activity of high surface area electrocatalysts is to observe the electrocatalytic response of a dispersion of carbon-supported catalyst in a thin layer of a recast proton exchange membrane.This type of electrode can be easily obtained from a solution of Nafion. As an example. Fig. 11 gives the comparative... 

The type of electrode reaction that will occur depends on the electrode and electrolyte and also on external conditions the temperature, impurities that may be present, and so on. Possible reactants and products in these reactions are (1) the electrode material, (2) components of the electrolyte, and (3) other substances (gases, liquids, or solids) which are not themselves component parts of an electrode or the electrolyte but can reach or leave the electrode surface. Therefore, when discussing the properties or behavior of any electrode, we must indicate not merely the electrode material but the full electrode system comprising electrode and electrolyte as well as additional substances that may be involved in the reaction for example, ZnCl2, ag I (Clj), graphite [the right-hand electrode in (1.19)]. 

Unfortunately, a unique terminology for the various types of electrodes and reactions has not yet been established. Electrodes can be classified according to different distinguishing features ... 

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). 

In each case, the set up consist of two half cells, each containing an electrode dipping into a solution of an appropriate electrolyte, separated by a salt bridge or similar device. Atoms of elements having a greater tendency to lose electrons (Ni, Zn) are oxidized at the anode, giving up electrons which travel through the external circuit to the cathode, where they combine with the cation (Cu2+, H+) which is most readily reduced. An illustrative list of cells with different types of electrode is shown in Table 6.9, while an illustrative list of various types of half-cell is shown in Table 6.10. 

Table 6.9 Representation of cells with different types of electrode.

As has been pointed out, another interesting type of electrode-concentration cell is that comprising two hydrogen electrodes working at different pressures and remaining immersed in a solution of hydrochloric acid. The cell may be represented as ... 

In general, the physical state of the electrodes used in electrochemical processes is the solid state (monolithic or particulate). The material of which the electrode is composed may actually participate in the electrochemical reactions, being consumed by or deposited from the solution, or it may be inert and merely provide an interface at which the reactions may occur. There are three properties which all types of electrodes must possess if the power requirements of the process are to be minimized (i) the electrodes should be able to conduct electricity well, i.e., they should be made of good conductors (ii) the overpotentials at the electrodes should be low and (iii) the electrodes should not become passivated, by which it is meant that they should not react to form on their surfaces any compound that inhibits the desired electrochemical reaction. Some additional desirable requirements for a satisfactory performance of the cell are that the electrodes should be amenable to being manufactured or prepared easily that they should be resistant to corrosion by the elements within the cell that they should be mechanically strong and that they should be of low cost. Electrodes are usually mounted vertically, and in some cases horizontally only in some rare special cases are they mounted in an inclined manner. 

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

As an introduction to the working of electrochemical cells, we shall consider first the various types of electrodes and then cell features such as potentials at the electrode solution interface and solution electrolysis, if any. 

Table 4.6 indicates that depending of the type of electrode, the "practical Fredox range is appreciably more extended than the "ideal range, which according to Fig 4.17 amounts to... 

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