Silica vessel

This has so far only been observed for the heterogeneous decomposition in glass or silica vessels at temperatures between 250 and 400 °C 39. However, the formation of small quantities of hydrogen in these experiments indicates that (a) is accompanied by a pyrolysis corresponding to... 

The existence of siloxane bonds on the surface of silica has been inferred mainly from the fact that the number of observed silanol groups is not sufficient for complete surface coverage. Practically no silanol groups are present in silicas heated to high temperatures. The siloxane bonds are quite unreactive. Actually, this is the cause of the inertia of fused silica vessels towards chemical attack. When siloxane bonds are opened, the process usually will not stop at the surface and dissolution of silica will take place. 

When, for example, a weighed amount of metallic gold contained in a fused silica vessel was treated with excess of bromine trifluoiide, it dissolved rapidly. Excess of solvent, together with free bromine, were then evaporated in vacuum and a white solid remained which had the empirical formula AuBrFg. It was freely soluble in bromine trifluoride and enhanced the conductivity of the latter, so that it may reasonably be formulated as an acid (BrFjAuFJ in the bromine trifluoride solvent system. The compound lost bromine trifluoride when heated to 200° and gave auric fluoride AuFg. 

The methods used to purify the Es have been reported (2). Portions of the purified Es chloride solution were evaporated to dryness in fused silica vessels and the residues taken up in 10 ... 

Bistrifluoromethyl sulfide XLVII undergoes photolysis with ultraviolet light in a silica vessel, but not in Pyrex glass, yielding bistrifluoromethyl sulfide (XLIX) and sulfur.27 Possibly, irradiation leads to isomerization of XLVII and XLVIII, followed by S-S bond fission to give two CFjS- free radicals a view supported by the formation of bis(trifluoromethylthio)mercury (L) in the presence of mercury. The structures assigned for XLVII and XLIX find support from spectroscopic data.28... 

Oxidations were carried out under static conditions in a thermo-statted 30-cc. silica vessel mounted in a furnace. Hydrocarbon-oxygen mixtures, prepared in a storage bulb at room temperature, were admitted to the reaction vessel by momentarily opening a tap. 

This conclusion was based only upon a supposed increase in the heat of activation with temperature, which was interpreted as proof that the wall reaction which predominates at lower temperatures is superseded by a true gas reaction. Hinshelwood and Topley f found, by varying the ratio surface/ volume, that the reaction taking place in a silica vessel was predominantly heterogeneous up to 1,044° abs. at least. Since the total velocity as measured in these experiments was actually rather less than that in the experiments of Trautz and Bhandarkar, it seems clear that the reaction measured by the latter observers must have been heterogeneous also. Re-examination of their experimental data, moreover, fails to substantiate the conclusion that there is any real increase in the heat of activation with temperature. 

Two years later Thompson and Hinshelwood (40) after studying the kinetics of the oxidation of ethylene in silica vessels at temperatures between 400° and 500° and finding that the rate is affected by the total pressure approximately in a reaction of the third order, the effects depending very much more on the partial pressure of the ethvlenes than on that of oxygen, suggested as a via media that while there is no doubt that Bone s interpretation of the course of the oxidation as a process of successive hydroxylations is essentially correct... the first stage in the reaction is the formation of an unstable peroxide if this reacts with more oxygen the chain ends but if it reacts with ethylene, unstable hydroxylated molecules are formed which continues the chain. It should be noted, however, that they adduced no experimental proof of the actual initial formation of the assumed peroxide. 

The methyl radical reacts either by addition to or by chlorine abstraction from the substrate as represented in steps 8 and 9. The free-radical process may not be a surprise, since the reaction was carried out in a silica vessel and in the absence of a chain suppressor. 

Platinum Resistance Thermometer. Platinum resistance thermometers are based on the electrical resistance of Pt. This resistance for Pt wires wound on a mica support and enclosed in a glass or silica vessel can be manufactured to be either 25.5 Q at 0°C or 2.5 Q at 0°C. The electrical resistance, typically measured in a Wheatstone or Mueller bridge, increases by about 0.1 Q or 0.01 Q, respectively, per degree centigrade (0.4%/°C). Empirical equations convert Pt resistance and its small nonlinearities to temperature ... 

Thus in vessels of fused quartz the silanes do not react with water,9 but the alkali extracted by the water from an ordinary glass flask (or from chips of ordinary glass introduced into the silica vessel) suffices to catalyze the reaction. The hydrogen is liberated quantitatively from any silion-hydrogen bond by dilute solutions of the alkalies, one molecule of hydrogen for each silicon-hydrogen bond. 

This would represent a reversible oxidation. The reaction with H2 could then be [M-02]it -f 112( 7) —> [M()2H]w H(gf). Studies of the exchange of H2 with D2 at temperatures near 600°C [G. Boati et al., Nuovo Cimento, 10, 993 (1953)] show that atoms are initiated at the walls of a silica vessel by O2 diffusing through the walls of the flask ... 

Interesting additional inert gas effects have been observed by Biron and Nalbandjan [17]. Using silica vessels they confirmed the above behaviour but in experiments under conditions analogous to those used by Semenova, already discussed, they found the critical explosion pressures to be unaffected by argon addition. Their results are given in Table 4. The conditions used by Semenova correspond with extremely low chain breaking efficiency at the vessel wall, and under these conditions the rates... 

Similar general behaviour to that described for boric acid coated vessels has also been reported by Dixon-Lewis et al. [63], who used silica vessels washed with hydrofluoric acid and distilled water, and has been found also with H3PO4 coated, HNO3 washed and HF washed Pyrex vessels [53, 62]. 

Compared with the reaction between hydrogen and oxygen, there have been relatively few studies of the deuterium—oxygen system. Early studies by Hinshelwood et al. [243] dealt with the second explosion limits and the slow reaction in silica vessels while at about the same time Frost and Alyea [14] measured the second limits in a KCl coated Pyrex vessel of 20 mm diameter. More recently Linnett and Selley [244] have determined the relative efficiencies of a number of molecules in reaction (ivD) D + O2 + M = DO2 + M (ivD)... 

Fig. 50. Explosion limits of mixtures of hydrogen and nitrous oxide (after Navailles and Destriau [295]). Curve 1, pyi /Pf o = 1/7 curve 2,j h2/PN20 = 1/10 curve 3, PH2/PN2O = 1/15 curve 4, PH2/PN2O = 1/18 curve 5, PH2/PN2O = 1/20 curve 6, pure N2O. Silica vessel, 1.8 cm diameter. (By courtesy of La Societe Chimique de France.)...

During a study of the oxidation of carbon at lower temperatures, Strickland-Constable [490], and Madley and Strickland-Constable [491] found in subsidiary experiments that CO and N2O did not react together appreciably in Pyrex or silica vessels at around 620 K. Oxidation of CO did, however, occur in a vessel in which N2O was already reacting with charcoal at 593 K. The latter reaction (without added CO) gave CO2 as the principal gaseous product, with very little CO formed. Thus in the presence of added CO two reactions occur simultaneously... 

Fig. 19. Oxidation of formaldehyde in nitric acid-washed and in aged silica reaction vessels at 337 °C [30] (a) Nitric acid-washed vessel O2 79.3 torr, HCHO 127.4 torr (b) aged silica vessel O2, 68.6 torr, HCHO 127.6 torr.

OcUfluorona rfithaleiie Hexafluororbeiuite(V). O tafluoronaphthalene (0.42 mmol) was dissolved in dry trichlorofluoromethane (0.5 mL) at 273 K in a fused silica vessel. A slight excess of rhenium hexafluoride was admitted to the reaction vessel, causing the deep green octafluoro-naphthalene hexafluororhenate(V) to precipitate at the vapor-liquid in-... 

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