Absence of Oxidants

FIGURE 7.3. pH dependence of the etch rate of n-Si(III) in 1M NH4F. After Matsumura and Fukidome. (Reproduced by permission of The Electrochemical Society, Inc.) 

FIGURE 7.4. pH dependence of etch rate and ratio of the contribution of electrochemical reaction to that of chemical reaction, RJR, on n-Si. At pH 14 the solution is 2M NaOH. (Reprinted from Allonque et al. 1995, with permission from Elsevier Science.) 

FIGURE 7.5. Etch rate in concentrated HF as a function of time. After Willeke and Kellermann.  

In general, the dissolution of silicon involves two essential steps, namely, oxidation followed by dissolution  

A high etch rate is only obtained when both rates of oxidation and dissolution are high. The etch rate can be increased either by addition of effective oxidizing agents into the solution or by anodic polarization which increases the surface concentration of carriers. As shown by the i-V curve (Fig. 5.1) the anodic dissolution of silicon in HF solutions consists of three regions. At potentials below the current peak, nonuniform anodic 

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Thermodynamic calculations for reactions forming carbon disulfide from the elements are compHcated by the existence of several known molecular species of sulfur vapor (23,24). Thermochemical data have been reported (12). Although carbon disulfide is thermodynamically unstable at room temperature, the equiHbtium constant of formation increases with temperature and reaches a maximum corresponding to 91% conversion to carbon disulfide at about 700°C. Carbon disulfide decomposes extremely slowly at room temperature in the absence of oxidizing agents. 

Decomposition Flame Arresters Above certain minimum pipe diameters, temperatures, and pressures, some gases may propagate decomposition flames in the absence of oxidant. Special in-line arresters have been developed (Fig. 26-27). Both deflagration and detonation flames of acetylene have been arrested by hydrauhc valve arresters, packed beds (which can be additionally water-wetted), and arrays of parallel sintered metal elements. Information on hydraulic and packed-bed arresters can be found in the Compressed Gas Association Pamphlet G1.3, Acetylene Transmission for Chemical Synthesis. Special arresters have also been used for ethylene in 1000- to 1500-psi transmission lines and for ethylene oxide in process units. Since ethylene is not known to detonate in the absence of oxidant, these arresters were designed for in-line deflagration application. 

It is instructive to compare, insofar as possible, the gaseous products of PETN decompn and expln. This is done on a water-free basis in Table I . Note the absence of oxides of nitrogen in the expln products and the increase in C02 from decompn to expln... 

Four dichloro isomers, the 1,6-, 2,3-, 2,7-, and 2,8-dichlorodibenzo-p-dioxins, were studied. These compounds also dissolve in TFMS acid, forming cation radicals in the absence of oxidizing agents or UV irradiation. The 2,8-isomer (Figure 5) exhibited a three-line spectrum, in agreement with the two equivalent protons in the 3,6 positions. The 2,7-isomer should also exhibit a three-line spectrum, similar to the 2,8-... 

An iodine solution can be employed as oxidizing agent in place of potassium hexacyanoferrate(lll). 4-Aminoantipyrine also produces colored zones with 1- and 1,4-unsaturated 3-ketosteroids (pregnadienediol derivatives) in the absence of oxidizing agents. 

Although not essential, one often uses a previously coated AgCl-Ag electrode or a silver-plated Pt wire coated electrolytically in KC1 solution with a thin deposit of AgCl. Such dry AgCl-Ag electrodes are much in favour as reference electrodes (although in the absence of oxidants), in addition to calomel electrodes (Pt wire in contact with Hg, covered with calomel paste in contact with KC1 solution), which also belong to the second kind, viz.,... 

ESR spectroscopy, however, revealed that 75, which one may call a ketyl radical, was not present in alkaline solutions of 77 in absence of oxidizing agents. The signal actually observed was that of 9-hydroxy-N,N -dimethyl-9,9 -biacridan radical 76. It was suggested that this radical was formed by one-electron transfer from N-methyl-acridone radical anion 75 to lucigenin monocarbinol 77, or by addition of hydroxide ion to lucigenin radical cation 78 (formed from lucigenin by one-electron transfer by the ketyl 75). 

In the absence of oxidants (O2 or other), the back electron transfer between the reduced complex and the radical of the base takes place. 

We don t observe oxidative deamination in the absence of oxidants. We may boil ethylenediamine with copper indefinitely without any chemical changes taking place. This observation is in addition to the lack of chloride-induced deamination, which conflicts with the hydride transfer mechanism. Moreover, if this mechanism did occur, it would imply a copper-induced exchange of hydrogen between the methylene group and water. 

Galactose oxidase of P. circinatus was apparently inhibited by traces of BESOD. It can be inactivated by H Oj produced in the reaction unless catalase was added. There was an activation by traces of O J. In the absence of oxidants the reaction usually showed an induction period The enzyme, used at very low concentrations in the assays, was protected by proteins like serumalbumin. SOD did, however, not alter the reaction rate when added after 15 min This was interpreted by an inactivation of SOD by the H O accumulated in the reaction but it could just as well mean that SOD had no effect on the active enzyme, but that it did lower the activation in the induction period. Peroxidase activated galactose oxidase and suppressed the effect of SOD It did protect the enzyme against H O inactivation and could have been responsible for appreciable amounts of OJ, produced from O and from radicals formed in its action on a substrate. 

The fire clay for making the bricks which line the furnace, and which is generally found in the same mine with the ironstone, varies very considerably in composition in different localities, having properties more or less suitable for the purpose, according to the absence of oxides, and the quantity of silica and alumina. Tav-lok, in his analysis of the rocks of the coal-measures at the Hartley pit, gives the following... 

Stilbenes and associated molecules provide very good examples of the formation of intermediate unstable isomers which give a chemical route for internal conversion. Upon irradiation, stilbenes undergo a cis-trans isomerization as the predominant reaction. However, under oxidative conditions phenanthrene is also formed.12 It was shown that the phenanthrene came only from c/s-stilbene (13),61 and that an intermediate unstable isomer, nms-dihydrophenanthrene (14), was the precursor of the phenanthrene.62-64 The dihydrophenarithrene was in its ground state, but vibrationally excited, and was formed by a process calculated to be endothermic by 33 10 kcal/mole-1.02 Oxygen or other oxidants converted it to phenanthrene (15), but in the absence of oxidants it was either collisionally stabilized or reverted to m-stilbene. 

Urbain and Botjrion [186] first prepared the divalent europium salt EuGla. In weakly acidic aqueous solution Eu2+ is quite stable in the absence of oxidizing agents. The divalent ions of Sm, Eu and Yb are strong reducing agents and follow the order Sm2+ > Yb2- > Eu2+. The oxidation potentials (E°) for the reaction... 

At 425°, as shown in Fig. 8e, the rate in the absence of oxide corresponded to nearly complete conversion of the oxygen to water, but the rate was much slower when oxide was present. Apparently new factors, which must be investigated further, enter into the reaction at this temperature. 

Therefore, in the absence of oxidation and biodegradation reactions in anaerobic zones of soil and sediment, the half-life of elemental phosphorus could be 10-10,000 years... 

It is shown that low-temperature liquid-phase oxidation of resinous acids, their salts and dicarboxylic acids, as well as their mixtures with hydrocarbons is accompanied by decarboxylation, the latter not proceeding in the absence of oxidation. The chemical conjugation mechanism in decarboxylation becomes of importance, associated with the production of fatty acids and organics oxidation. 

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