Self-electrodes

Metal alloys frequently are formed into electrodes for spectrochemical analysis and usually are referred to as self-electrodes. Since metals are electrically conducting, this method is quite satisfactory. Spark analysis of self-electrodes is common since good precision can be attained and concentrations of metals in the alloy range are easily determined. The Petry stand can be used for flat metal surfaces if desired. 

An ac spark source provides much better reproducibility than does the dc arc. It is ideal for metallurgical analytical problems, using self-electrodes or the Petry point-to-plane technique. Detection limits using the ac spark are not as low as with the dc arc, but the ac spark can be used satisfactorily to higher concentration levels. The ac spark is very noisy and more ion lines appear with ac spark discharge than appear with dc arc excitation. Fractional distillation is not a problem with the ac spark. 

Metal alloy comparison standards can be made by mixing pure metals or metal alloys of known composition by heating in an induction furnace. The standards can then be formed into solid rods to serve as self-electrodes or flat disks if the point-to-plane technique is to be used. The National Bureau of Standards has available a number of metallurgical standards of certified composition. 

Since a wide variety of sample types is likely, it follows that sample preparation will differ markedly from one type of sample to another. Metals can be formed into self-electrodes, as previously mentioned, or into flat disks. Metal alloys may need to be remelted to obtain homogeneity and also to be formed into a suitable shape. Metal powders can be pressed into pellets and placed in cupped graphite or carbon electrodes. Metals also may be converted into ionic solutions by proper treatment with acids or other chemical agents. 

Metal samples frequently can be shaped to fit electrode holders and serve as self-electrodes. Spark excitation usually is used for metals and... 

Metal samples also may be prepared as flat disks and used in the Petry stand designed for this purpose. The counter electrode usually is a pointed carbon or graphite electrode. Excitation conditions are similar to those used with the self-electrodes. ... 

Figure C3.2.4. Plot of the log of photocurrent against number of methyl units in a alkylsilane based monolayer self-assembled on a n silicon electrode. The electrode is immersed in a solution witli an electron donor. Best fits of experimental data collected at different light intensities ( ) 0.3 mW cm ( ) 0.05 mW cm. From [10].

Electrolysis cell. This is shown in Fig. VI, 31, 1 and is almost self-explanatory. The cylindrical cell of Pyrex glass (6" long by 2 " diameter) is cooled by immersion in a cooling bath. The electrodes consist of two platinum plates (4 cm. X 2-5 cm. X 0-3 mm.), which are placed about 2 mm. apart. The temperature of the electrolyte is maintained at 30-35° by means of the internal cooling coil and also by immersion of the cell in ice-water. A current of 1 5-2 0 amperes is passed until the electrolyte becomes slightly alkaline, which normally takes about 20-50 per cent, longer than the calculated time on the basis of the current and the amounts of acid employed. It is advantageous to reverse the direction of the current occasionally. 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

Electromagnetic flow meters ate avadable with various liner and electrode materials. Liner and electrode selection is governed by the corrosion characteristics of the Hquid. Eor corrosive chemicals, fluoropolymer or ceramic liners and noble metal electrodes are commonly used polyurethane or mbber and stainless steel electrodes are often used for abrasive slurries. Some fluids tend to form an insulating coating on the electrodes introducing errors or loss of signal. To overcome this problem, specially shaped electrodes are avadable that extend into the flow stream and tend to self-clean. In another approach, the electrodes are periodically vibrated at ultrasonic frequencies. 

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. 

Self-baked carbon electrodes are those whose shapes are formed in situ (33). The carbonaceous mixture is placed into a hoUow tube-shaped metal casing. The upper end receives the unbaked mixture as a soHd block, small particles, or warm plastic paste. The casing contains inwardly-projecting longitudinal perforated fins that become surrounded by baked carbon as the casing is incrementally moved downward and through the contact plates. Casing and carbon are consumed in this furnace. 

Prebaked carbon electrodes are manufactured in all diameters up through 1500 mm. Some prebakes are produced as quadriforms to suit specific furnaces. Self-baking electrodes are in service through 2134 mm diameter. Electrode lengths are as needed for particular appHcations. Rounds are available in lengths up to 2794 mm and quadriforms as long as 3556 mm. Self-baked electrodes are continuous. 

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