Electrode oxygen

The oxygen electrode reaction with accompanying standard potentials at 25°C may be represented as 

The marked irreversibility of the oxygen evolution and reduction reactions in aqueous solutions has imposed severe limitations on the mechanistic information which can be obtained for both reactions. In general, at the current densities normally employed for kinetic studies, the current-potential data are insensitive to the back reaction, which normally occurs early on in the multi-step reaction sequence. Further, the reduction and oxidation processes are usually studied only at widely separated potentials. Thus, the surface conditions, whether in the case of metals or bulk oxides, probably differ sufficiently such that the reduction and oxidation pathways may not be complementary. The situation is complicated further by the large number of possible pathways for both reactions. 

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The reaction can be followed by measurement of dye formation. The rate of oxygen depletion can be measured using an oxygen electrode. Additional reagents can be added to prevent the formation of oxygen from the generated HpOp. The reactions ate as follows ... 

The dissolved oxygen concentrations are determined immediately and after five days. The method for dissolved measurement involves either a modified Winkler titration or a membrane-covered oxygen electrode. The difference between initial and final dissolved oxygen multiphed by the dilution factor is the BOD value. 

FIG. 24-14 Dissolved-oxygen electrodes a) polarographic (impress breakdown voltage for oxygen measure current) (h) voltametric (measure electromotive force). 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

These subsystems profoundly affect the fuel cell system performance. As an example, the inherently slow air (oxygen) electrode reaction must be acceler-... 

The electrode potential behaviour of copper in various solutions has been investigated and discussed in considerable detail by Catty and Spooner . According to these workers a large part of the surface of copper electrodes in aerated aqueous solutions is normally covered with a film of cuprous oxide and the electrode potential is usually close to the potential of these film-covered areas. The filmed metal simulates a reversible oxygen electrode at... 

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

The apparatus is sometimes referred to as an oxygen electrode , but it is actually a cell. Although the Teflon membrane is impermeable to water and, therefore, to most substances dissolved in water, dissolved gases can pass through, and gases, such as chlorine, sulphur dioxide and hydrogen sulphide, can affect the electrode. The apparatus can be made readily portable and it is, therefore, of value for use in the field and can be used to monitor the oxygen content of rivers and lakes (see Ref. 53). 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

Cells of cylindrical geometry are produced mainly in four sizes D (LR-20), C (LR-14), AA (LR-6), and AAA (LR-03). The two other alkaline cells in this section (using HgO or an oxygen electrode as cathode) are almost exclusively produced as small button cells. 

The concentration of dissolved oxygen in a fermenter is normally measured with a dissolved oxygen electrode, known as a DO probe. There are two types in common use galvanic... 

Damjanovic, A. Mechanistic Analysis of Oxygen Electrode Reactions 5... 

W. Zipprich, H.-D. Wiemhofer, U. Vohrer, and W. Gopel, In-situ Photoelectron-Spectroscopy of Oxygen Electrodes on Stabilized Zirconia, Ber. Buns. Phys. Chem. 99, 1406-1413 (1995). 

Similarly to aqueous electrochemistry, potentials in solid state electrochemistry utilizing YSZ are expressed in terms of the potential of a reference metal electrode exposed to P02 = 1 atm at the temperature T of interest. Thus a standard oxygen electrode scale (soe) can be defined. Similarly to equation (7.2) one has ... 

There is an important point regarding the absolute standard oxygen electrode scale defined by Eq. (7.27). The U (abs) value is defined by the equilibrium ... 

Figure 7.12 shows the relationship between the standard oxygen electrode (soe) scale of solid state electrochemistry, the corresponding standard hydrogen electrode (she) scale of solid state electrochemistry, the standard hydrogen electrode (she) scale of aqueous electrochemistry, and the physical absolute electrode scale. The first two scales refer to a standard temperature of 673.15 K, the third to 298.15 K. In constructing Figure 7.12 we have used the she aqueous electrochemical scale as presented by Trasatti.14... 

Description of the cell composition is based - as far as possible - on the Stockholm convention (1953), i.e. the left-hand electrode constitutes the negative terminal of the cell. Cells are listed according to the metallic constituent of the electrode mentioned first which is involved in the electrode reaction establishing the respective electrode potential. Contact materials and conductive additives may be mentioned first before the actual element of interest only for the sake of correct materials sequence. The sequence of electrode components is stated as reported in the original publications. When an oxygen electrode is used as reference electrode an oxygen partial pressure of 0.21 atm is assumed. 

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