Halogen electrodes

In principle, a gas electrode consists of an inert material immersed into a melt and flushed with the respective gas. In the case of oxygen electrodes, the inert material is a noble metal (Pt, Au, etc.), and in the case of halogen electrodes it is graphite, and the respective gas is adsorbed on the surface of the electrode. The gas molecules are in equilibrium with the ions of the same gas in the melt. 

Halogen electrodes in molten salts are being used especially for the study of molten halides. In 1930 Hildebrand and Salstrom [98-100] were the first who applied a chlorine reference electrode in molten salts. Up to the present, other authors used and improved this electrode [101-105], The reversible bromine/ bromide electrode was set up for the first time by Hildebrand and Salstrom [98] and improved by Murgulescu et al. [106,107], In the following an example is shown of how to obtain and use a reversible gas electrode in molten salts for the determination of thermodynamic activity. 

Table 10 Exchange Current Densities and Coverage (0) for Halogen Electrodes in Molten Salts...

The case of chlorine or other halogen electrodes is similar, except that these elements ionize into anions, after dissociation, so that electrons must be supplied by the metallic wire, not withdrawn by it, when the gas passes into solution as ions. 

Halogen Electrodes.—The determination of the standard potentials of the halogens is simple in principle it involves measurement of the potential of a platinum electrode, coated with a thin layer of platinum or iridium black, dipping in a solution of the halogen acid or a halide, and surrounded by the free halogen. The uncertainty due to liquid junction can be avoided by employing the appropriate silver-silver halide or mercury-mercurous halide electrode as reference electrode. In practice, however, difficulties arise because of the possibility of the reactions... 

The standard electrode potential of an electrode is a measure of its tendency to gain electrons. If E is positive the electrode will tend to gain electrons if E is negative the electrode will tend to lose electrons. Both these tendencies are measured relative to hydrogen. The alkali metals have the most negative electrode potentials whereas the halogen electrodes are very... 

Ann. Phys., 1878, v, 182 1882, xvi, 561 ( die Thomson sche Theorie lasst sich dutch die Thatsachen widerlegen , 583) 1882, xvii, 593 (halogen electrodes) G. Wiedemann, Die Lehre von der Electricitdt, Brunswick, 1883, ii, 879-93 Ostwald, (i), 1887, ii, 509-19. Karl Ferdinand Braun (Fulda, 6 June 1850-New York, 20 April 1918) was associate professor of physics in Marburg (1877) and Strasbourg (1880), and professor of mathematics in the Karlsruhe Polytechnic (1883). 

This method is also applicable to the determination of standard electrode potentials of halogen electrodes thus, for the cell... 

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

Platinum is a beautiful silvery-white metal, when pure, and is malleable and ductile. It has a coefficient of expansion almost equal to that of soda-lime-silica glass, and is therefore used to make sealed electrodes in glass systems. The metal does not oxidize in air at any temperature, but is corroded by halogens, cyanides, sulfur, and caustic alkalis. 

The quaternary ammonium salts (QAS) are widely used as ionofores of ion-selective electrodes and extractants of metals halogenic anion complexes. The influence of the QASes nature with various methyl groups contents on the cadmium extraction from bromide media has been investigated. 

Despite the fact that the zinc/ ferricyanide system employs an alkaline electrolyte, the electrode reactions are quite similar to those in zinc/halogen batteries and battery constructions are usually bipolar too. 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

Carbon electrodes as employed with e.g. halogens are designated as C only irrespective of the actual composition. 

Increased pressure [hyperbaric chamber, respiratory ventilators (eg, PEEP or CPPB)] P02 polarographic electrode "error" due to halogenated hydrocarbons (eg, llalothane) t P02 usually <50... 

Poa polarographic electrode "error" due to halogenated hydrocarbons (eg, Halothane)... 

Gas These are constructed by placing a strip of nonreactive metals (usually platinum or gold) in contact with both the solution and a gas stream (a) the hydrogen electrode consists of a platinum strip exposed to a current of hydrogen, and partly immersed in an acid solution. A potential is set up between the gas and the solution, the equilibrium involved being H2 2H + 2 e (b) potentials also occur when the halogens are in contact with their ions in solution, the equilibrium in the case of chlorine being Cl2 + 2 e- 2 Cl". 

In general, the electrolysis of a molten salt at inert electrodes produces the metal at the cathode, e.g., calcium from calcium chloride (melting point 774 °C). The anion is often a halide ion which, on discharge, yields the halogen, e.g., chlorine from calcium chloride. 

Principles and Characteristics Combustion analysis is used primarily to determine C, H, N, O, S, P, and halogens in a variety of organic and inorganic materials (gas, liquid or solid) at trace to per cent level, e.g. for the determination of organic-bound halogens in epoxy moulding resins, halogenated hydrocarbons, brominated resins, phosphorous in flame-retardant materials, etc. Sample quantities are dependent upon the concentration level of the analyte. A precise assay can usually be obtained with a few mg of material. Combustions are performed under controlled conditions, usually in the presence of catalysts. Oxidative combustions are most common. The element of interest is converted into a reaction product, which is then determined by techniques such as GC, IC, ion-selective electrode, titrime-try, or colorimetric measurement. Various combustion techniques are commonly used. 

Francis [26] has recently described some modem methods for element determinations, including the oxygen flask technique and the determinations of C, H, O, N (Kjeldahl), halogens, S, P and F (ion-selective electrode). 

Coulometry. Even in water, controlled potential or potentiostatic coulometry is a difficult and often time-consuming technique, as the analyte must participate in a direct electrode reaction. Therefore, in non-aqueous media there are only a few examples of its application, e.g., the potentiostatic coulometry of nitro and halogen compounds in methanol (99%) with graphical end-point prediction, as described by Ehlers and Sease153. 

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