Electrode construction

For increased power requirements, electrode constructions have been developed which bring the electronic conductors in closer contact with the active material particles first, around 1930, the sinter electrode [110], recently in sealed cells largely replaced by the nichel-foam electrode, and then, around 1980, the fiber structure electrode [111]. In order to take full advantage of their increased perform-... 

Any inert metallic component of an electrode is written as the outermost component of that electrode in the cell diagram. For example, a hydrogen electrode constructed with platinum is denoted H+(aq) H2(g) Pt(s) when it is the right-hand electrode in a cell diagram and Pt(s) H2(g) H+(aq) when it is the left-hand electrode. An electrode consisting of a platinum wire dipping into a solution of iron(II) and iron(III) ions is denoted either Fe3+(aq),Fe2 (aq) Pt(s) or Pt(s) Fe3+(aq),Fe2+(aq). In this case, the oxidized and reduced species are both in the same phase, and so a comma rather than a line is used to separate them. Pairs of ions in solution are normally written in the order Ox,Red. 

The silver chloride electrode is preferable for precise measurements. The standard potentials of some electrodes of the second kind are listed in Table 3.4. Various electrode constructions are shown in Fig. 3.8. 

The main advantage of enzyme electrodes lies in the simplicity of the method of analysis although the presence of interfering ions may present a problem, particularly when using cation-sensitive electrodes. Methods of enzyme electrode construction are discussed in the section on biosensors. 

In the electrode construction, this anisotropic material can be cut either parallel to the basal benzenoid-like carbon planes formed by C/C covalent bonds (plane ab), or perpendicular to the carbon layers where the C/C bonds are van der Waals type (planes ac and be). 

In the overwhelming majority of analyses, an electrochemist will choose an electrode constructed from platinum, gold, silver, mercury or some... 

Dialysis units provided highly efficient means for increasing selectivity in a dynamic system by placement in front of a lithium-selective electrode constructed by incorporating 14-crown-4 ether 3-dodecyl-3 -methyl-1,5,8,12-tetraoxacyclotetradecane into a PVC membrane that was in turn positioned in a microconduit circuit by deposition on platinum, silver or copper wires. The circuit was used to analyse undiluted blood serum samples by flow injection analysis with the aid of an on-line coupled dialysis membrane. For this purpose, a volume of 200 pL of sample was injected into a de-ionized water carrier (donor) stream and a 7 mM tetraborate buffer of pH 9.2 was... 

The sintered electrode constructions are gradually replaced with structures of higher capacity as, for example, felted nickel fibril or foam structures [350-352]. An open nickel foam structure can be obtained by vapor deposition of nickel from nickel carbonyl into a bed of urethane foam and then burning off the polymer. The porosity increases from 80% typical for sinter electrodes and reaches 90-95% for felted or foam structures. Application of polymer bonded cadmium electrodes significantly reduces environmentally dangerous dusting during cell production [350]. 

By media variables we mean the solvent, electrolyte, and electrodes employed in electrochemical generation of excited states. The roles which these play in the emissive process have not been sufficiently investigated. The combination of A vV-dimethylformamide, or acetonitrile, tetra-n-butylammonium perchlorate and platinum have been most commonly reported because they have been found empirically to function well. Despite various inadequacies of these systems, however, relatively little has been done to find and develop improved conditions under which emission could be seen and studied. Electrochemiluminescence emission has also been observed in dimethyl sulfite, propylene carbonate, 1,2-dimethoxyethane, trimethylacetonitrile, and benzonitrile.17 Recently the last of these has proven very useful for stabilizing the rubrene cation radical.65,66 Other electrolytes that have been tried are tetraethylam-monium bromide and perchlorate1 and tetra-n-butylammonium bromide and iodide.5 Emission has also been observed with gold,4 mercury,5 and transparent tin oxide electrodes,9 but few studies have yet been made1 as to the effects of electrode construction and orientation on the emission character. 

These electrodes must be distinguished from biosensors with enzymes at their surface. Electrodes constructed from field effect transistors (see 19.7) lead to voltametric detection. 

The goal of this chapter is to describe the application of hydrodynamic electrodes to the study of electrode kinetics and the kinetics of electrode and coupled homogeneous reactions. In order to do this, it is important to describe first the mass transport and how to fulfil experimentally the conditions described by the mass transport equations, i.e. electrode construction and operation. 

This derivation makes a number of assumptions. Firstly, we assume that there is no disruption to the laminar flow pattern due to a finite disc surface, finite cell size, or eccentricity in disc rotation. To what extent design factors affect measured currents will be discussed further in the section on electrode construction. It is sufficient at this point to say that the criteria for negligible disruption can be met. 

Despite its title, ASTM D99119 is a resistivity method for sheet rubber material, not for products. It specifies the four electrode method and, hence, is similar to ISO 1853 and BS 2040, method 2, but the electrode construction is different and care would need to be taken to avoid leakage between the current and potentiometric electrodes. 

Figure 9.8 Experimental arrangement for use of thin-layer electrode constructed with glass or quartz plates. W, Working electrode R, reference electrode A, auxiliary electrode a, inert-gas inlet.

Figure 10.6 Electrode construction (A) for pyrolytic graphite, where edge or basal plane orientation may be exposed to solution (edge is shown) (B) for GC, WIGE, or carbon paste.

Both disk-shaped and cylinder electrodes can be fashioned with metal microwires and carbon fibers. Two practical considerations that must be kept in mind when working with small wires and fibers are their resistivity and their delicacy. The former consideration demands that only short sections of the microwire or fiber be used for electrode construction, while the latter mainly represents a significant source of frustration for the novice electrode maker. A 5-/ m-radius platinum wire at times seems to break just because you ve looked at it With a little practice, however, handling these small materials becomes straightforward. 

The use of mercury is nearly ideal for working electrode construction for several reasons. Mercury has a large liquid range (-38.9 to 356.9°C at normal pressure), and therefore electrodes of various shapes may be easily prepared either in pure form or by deposition of mercury on the conducting support. The surface of such electrodes is highly uniform and reproducible if the mercury is clean. 

For laboratory cells, minimizing the energy consumption and optimizing the space-time yield are not as important. It is more important that the different reaction parameters like electrode materials, diaphragms, and the working potentials can be varied easily. For electrolyses under potential control, a three-electrode construction has to be used, which is schematically shown in Figure 22.8. 

Figure 22.8 Schematic representation of an electrolysis cell (three-electrode construction W = working electrode AUX = auxiliary electrode R = reference electrode Q = coulometer D = diaphragm, if necessary CS = current source A = amperometer V = voltmeter).

The relative standard deviation was lower than 3.0% for the solution containing 4.50 x 10 4 mol L 1 catechol in — 10) and 5.0% among three different electrodes constructed using the same composition. 

For reasons of ease of manufacture, the majority of solid electrodes have a circular or rectangular form. External links are through a conducting epoxy resin either to a wire or to a solid rod of a metal such as brass, and the whole assembly is introduced by mechanical pressure into an insulating plastic sheath (Kel-F, Teflon, Delrin, perspex, etc.) or covered with epoxy resin. It is very important to ensure that there are no crevices between electrode and sheath where solution can enter and cause corrosion. Examples of electrodes constructed by this process will be shown in Chapter 8. 

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