Oxidation atmospheric oxygen

The oxidation of silicon to SiO is not dependent on the kind of the oxidizing agent used, and the reaction may be carried out with metal oxides, atmospheric oxygen, etc. The finer the grain of the silicon, the better the reaction. 

Aluminum powder is frequently added to explosives and propellants to improve their performance. The addition of aluminium results in considerable gain in heat of explosion because of the high heat of formation of aluminia (1658 kJ/mole, 16260 kJ/kg) leading to higher temperatures of the fumes. Aluminium not reacted in the detonation front might be oxidized atmospheric oxygen to induce post-heating in the fume zone and to increase the -> air blast or even to initiate a delayed secondary explosion. 

The irradiation environment plays an important role in the evolution of polymer stability. While unsaturated hydrocarbon like acetylene [61] or divinyl benzene [62] is present in the material surrounding and provides radicals for the formation of intermolecular bridges, oxidative atmosphere, oxygen or air, promotes oxidation as the result of diffusion inside the polymer matrix. The distribution profile for carbonyl products that generated during irradiation takes a parabolic form [63]. The source of radicals may be one of the components of blends, which presents a lower stability. This case can be illustrated by various blends, EPDM/PP [64], EPDN-NR [65]. These polymer mixture show the maximum level of crosslinking at about 120-150 kGy. 

Oxidation. Atmospheric oxygen can oxidize polymer chains through the well known peroxide and hydroperoxide chain reaction. Free radicals have to be initiated to start this chain, and these are often formed by UV light. In practice it is usually necessary to incorporate some form of UV absorber and antioxidant in a polymeric material for service out of doors. 

Commercially, maleic anhydride is prepared more cheaply by the catalytic vapour phase oxidation (in the presence of vanadium pentoxide at about 400°) of benzene with atmospheric oxygen ... 

Donor substituents on the vinyl group further enhance reactivity towards electrophilic dienophiles. Equations 8.6 and 8.7 illustrate the use of such functionalized vinylpyrroles in indole synthesis[2,3]. In both of these examples, the use of acetyleneic dienophiles leads to fully aromatic products. Evidently this must occur as the result of oxidation by atmospheric oxygen. With vinylpyrrole 8.6A, adducts were also isolated from dienophiles such as methyl acrylate, dimethyl maleate, dimethyl fumarate, acrolein, acrylonitrile, maleic anhydride, W-methylmaleimide and naphthoquinone. These tetrahydroindole adducts could be aromatized with DDQ, although the overall yields were modest[3]. 

The reaction follows a free radical mechanism and gives a hydroperoxide a compound of the type ROOH Hydroperoxides tend to be unstable and shock sensitive On stand mg they form related peroxidic derivatives which are also prone to violent decomposi tion Air oxidation leads to peroxides within a few days if ethers are even briefly exposed to atmospheric oxygen For this reason one should never use old bottles of dialkyl ethers and extreme care must be exercised m their disposal... 

A sensitive method for the flow injection analysis of Cu + is based on its ability to catalyze the oxidation of di-2-pyridyl ketone hydrazone (DPKH) by atmospheric oxygen. The product of the reaction is fluorescent and can be used to generate a signal when using a fluorometer as a detector. The yield of the reaction is at a maximum when the solution is made basic with NaOH. The fluorescence, however, is greatest in the presence of HCl. Sketch an FIA manifold that will be appropriate for this analysis. 

The chemical properties of phthalocyanines depend mosdy on the nature of the central atom. Phthalocyanines are stable to atmospheric oxygen up to approximately 100°C. Mild oxidation may lead to the formation of oxidation iatermediates that can be reduced to the original products (29). In aqueous solutions of strong oxidants, the phthalocyanine ring is completely destroyed and oxidized to phthalimide. Oxidation ia the presence of ceric sulfate can be used to determine the amount of copper phthalocyanine quantitatively (30). 

Transparent yeUow pigment is composed of needle particles of EeO(OH) having a thickness of 2—5 nm, a width of 10—20 nm, and a length of 50—100 nm. They are prepared by the precipitation process from a very diluted solution of ferrous salt, eg, 6 wt % ferrous sulfate, foUowed by the oxidation of the precipitate with atmospheric oxygen at a temperature of less than 25°C. The precipitate is left to mature for about one day, then filtered, dried, and milled. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

The preparation of triaryknethane dyes proceeds through several stages formation of the colorless leuco base in acid media, conversion to the colorless carbinol base by using an oxidising agent, eg, lead dioxide, manganese dioxide, or alkah dichromates, and formation of the dye by treatment with acid (Fig. 1). The oxidation of the leuco base can also be accompHshed with atmospheric oxygen in the presence of catalysts. 

Other alkaline primary cells couple zinc with oxides of mercury or silver and some even use atmospheric oxygen (zinc—air cell). Frequendy, zinc powder is used in the fabrication of batteries because of its high surface area. Secondary (rechargeable) cells with zinc anodes under development are the alkaline zinc—nickel oxide and zinc—chlorine (see Batteries). 

It is pmdent to perform zone melting in a dry inert atmosphere. Oxygen causes most organic melts to oxidize slowly. Oxygen and moisture not only oxidize metals and semiconductors, but often enhance sticking to the container. Molten salts attack sUica more rapidly in the presence of moisture. Oxygen and water are considered impurities in some inorganic compounds. 

Antioxidants are used to retard the reaction of organic materials with atmospheric oxygen. Such reaction can cause degradation of the mechanical, aesthetic, and electrical properties of polymers loss of flavor and development of rancidity ia foods and an iacrease ia the viscosity, acidity, and formation of iasolubles ia lubricants. The need for antioxidants depends upon the chemical composition of the substrate and the conditions of exposure. Relatively high concentrations of antioxidants are used to stabilize polymers such as natural mbber and polyunsaturated oils. Saturated polymers have greater oxidative stabiUty and require relatively low concentrations of stabilizers. Specialized antioxidants which have been commercialized meet the needs of the iadustry by extending the useflil Hves of the many substrates produced under anticipated conditions of exposure. The sales of antioxidants ia the United States were approximately 730 million ia 1990 (1,2). 

Finely divided barium is susceptible to rapid, violent combination with atmospheric oxygen. Therefore, in powdered form it must be considered pyrophoric and very dangerous to handle in the presence of air or other oxidising gases. Barium powder must be stored under dry argon or helium to avoid the possibihty of violent explosions. Massive pieces of barium, however, oxidize relatively slowly and present no explosion hazard if kept dry. 

A fresh surface of siUcon carbide is thus constantiy being exposed to the oxidizing atmosphere. Active oxidation takes place at and below approximately 30 Pa (0.23 mm Hg) oxygen pressure at 1400°C (66). Passive oxidation is determined primarily by the nature and concentration of impurities (67). 

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