Metal electrodes, special purpose

One of the primary uses of iridium is in the manufacture of alloys. An alloy is made by melting and mixing two or more metals. An alloy s properties differ from those of the elements that make it up. Iridium is often combined with platinum, for example, to provide a stronger material than the platinum itself These alloys are very expensive and are used for only special purposes. For instance, the spark plugs used in helicopters are made of a platinum-iridium alloy. Such alloys are also used for electrical contacts, special types of electrical wires, and electrodes. 

ISEs it is common practice to use potential measuring instruments with input impedances >10 Cl to ensure that there is no error in the potential measurement. Most modern pH/mV meters constructed with field-effect transistor-type input amplifiers fulfill this requirement. However, as the electrode surface area becomes smaller, the resistance of the ISE increases dramatically. Thus, for microsized electrodes, specially designed amplifier circuits with even higher input impedances are required to obtain accurate intracellular ion values and to help eliminate noise. In many instance, the micro-type measurements must also be made within the confines of a Faraday cage to reduce noise further by shielding the electrodes finm environmental noise. In automated clinical chemistry analyzers, confinement of the electrodes within the outer metal cabinet of the instrument serves a similar purpose. 

In the electrochemical studies reported so far, NMR has been applied as an ex situ technique, where a powdered metal is used as an electrode in an electrochemical cell and then the metal powder is transferred, usually with electrolyte, to a NMR sample tube for observation (151-154). For example, the formation of surface CO from methanol on Pt was studied (153). High-surface-area Pt (24 m /g) was placed in a Pt boat that served as the working electrode, and a solution of 0.1 M C-enriched methanol in 0.5 M sulfuric acid was used as the electrolyte. The electrode was held at the desired potential, then a 0.2 g sample of the Pt was removed, mixed with glass beads, and placed in a glass NMR sample tube. The spectrum showed the presence of about 10 spins in the form of CO. So far only special purpose NMR instruments have been used in such studies. 

While the development of primary cells with a lithium anode has been crowned by relatively fast success and such cells have filled their secure rank as power sources for portable devices for public and special purposes, the history of development of lithium rechargeable batteries was full of drama. Generally, the chemistry of secondary batteries in aprotic electrolytes is very close to the chemistry of primary ones. The same processes occur under discharge in both types of batteries anodic dissolution of lithium on the negative electrode and cathodic lithium insertion into the crystalline lattice of the positive electrode material. Electrode processes must occur in the reverse direction under charge of the secondary battery with a negative electrode of metallic lithium. Already at the end of the 1970s, positive electrode materials were found, on which cathodic insertion and anodic extraction of lithium occur practically reversibly. Examples of such compounds are titanium and molybdenum disulfides. 

The formation of new nuclei and of a fine-crystalline deposit will also be promoted when a high concentration of the metal ions undergoing discharge is maintained in the solution layer next to the electrode. Therefore, concentration polarization will have effects opposite those of activation polarization. Rather highly concentrated electrolyte solutions, vigorous stirring, and other means are employed to reduce concentration polarization. Sometimes, special electrolysis modes are employed for the same purposes currents that are intermittent, reversed (i.e., with periodic inverted, anodic pulses), or asymmetric (an ac component superimposed on the dc). 

In this respect the special role of irreversible adsorption of foreign metal adatoms should be mentioned as this type of adsorption provides the possibihty of producing stable modified electrodes for electrocatalytic purposes (for instance Sn, Bi adatom modified electrodes). In some cases, the irreversibly adsorbed adatom undergoes well-defined surface redox processes that could be structure sensitive (for instance. As, Sb, and Bi adatoms on platinum substrates or two-dimensional Bi -i- As alloys supported on Pt(lll). ... 

To begin this study, it is necessary to obviate the phenomena linked to the electrodes. Thus, for the purpose of being able to separate and analyze the different contributions of the metal on the interactions between gas and solid, a reactor has been specially designed. This reactor mrrst be able to by turns isolate and bring into contact particnlar zones of the sample. These zones are pinpointed by the presence or absence of electrodes. The samples have to be elaborated and conditioned in a specific way. ... 

Surface-modified electrodes — In order to alter the properties of an electron-conducting substrate, i.e., a metal or graphite or semiconductor used as a part of an electrode, different chemical compounds are produced/deposited/attached/immobilized on the surface. These electrodes are most frequently called surface-modified, chemically-modified, or polymer-modified electrodes, depending on the methods and materials used for the modification. The obvious purpose of these efforts is the production of electrodes with novel and useful properties for special applications, but also to help gain a better understanding of the fundamental charge transfer processes at the interfaces. Usually the enhancement of the rate of the electrode reaction... 

Consider a three-electrode electrochemical cell comprising an electrolyte solution of metal ions with a bulk concentration c, an inert, ideally polarizable working electrode, a counter electrode and a reference electrode with a fixed potential Eref. The whole system is kept at a constant temperature T. For the sake of convenience, we select a reference electrode made of the bulk metal M whose ions are present in the solution. The only requirement that we have to meet for the purpose is the bulk metal M to have a stable and reversible equilibrium potential JE x(c). That being the case. Ere/ = Ej(c) and any external potential E which we apply to the working electrode polarizes it directly to the electrochemical overpotential t] = EJ/f) - E, which is a measurable quantity with a clear physical significance. Certainly we can use any other reference electrode with a fixed potential Ere/ and this is what we should do if the bulk metal M has no reversible potential in the particular experimental system or if this potential is not known. However, then the external potential E will polarize the working electrode to the overpotential rjre/ = Ere/ - E, which relates to rj according to //re/ = // + AEref where AEre/= Ere/- Eao(c). Namely the difference Ere/-EJ/S) is which we eliminate with the special choice of our reference electrode. 

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