Silica in glass

Glass is resistant to attack by most chemicals. However, the silica in glass reacts with the strong Lewis base F from hydrofluoric acid to form fluorosilicate ions ... 

Hydrofluoric acid is used to etch glass by reacting with the silica in glass to form the volatile gas SiF4 ... 

Hydrofluoric acid solutions cannot be stored in glass containers because HE reacts readily with silica in glass to produce hexafluorosilicic acid (H2SiFg). 

Phosphorus trifluoride will react with carbon at high temperatures to produce tetrafluoroethyl-ene, F2C = CF2, and at temperatures above 500°C it will attack silica (in glass), producing substantial quantities of Sip4. Various metals react with PF3 at high temperatures to give fluorides and phosphides. Phosphorus trifluoride forms a complex with arsenic pentafluoride below -78°C but above this temperature it is fluorinated by the latter. 

Sandstone. A sedimentary rock which may be of several types, e.g. siliceous, calcareous, argillaceous, depending on whether the grains of silica in the rock are bonded with secondary silica, with lime or with clay. Siliceous and argillaceous sandstones both find some use in the refractories industry crushed sandstone is used as a source of silica in glass manufacture. 

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. 

Most striking colors are obtained in glasses containing 10—20 wt % K2O, 10—22 wt % ZnO, and 50—60 wt % silica. CaO and 2 3 present (6). To this batch is added 1—3 wt % CdS, CdSe, and/or CdTe. Melting must be under neutral or mildly reducing conditions. Otherwise, S, Se, and Te will be oxidized to SO2, Se02, or Te02, which are colorless. 

The reinforcing filler usually takes the form of fibres but particles (for example glass spheres) are also used. A wide range of amorphous and crystalline materials can be used as reinforcing fibres. These include glass, carbon, boron, and silica. In recent years, fibres have been produced from synthetic polymers-for example, Kevlar fibres (from aromatic polyamides) and PET fibres. The stress-strain behaviour of some typical fibres is shown in Fig. 3.2. 

Other factors must also be considered. Similarly the replacement of 1 % milling clay by /4% of the more colloidal bentonite is beneficial. Large additions of quartz at the mill improve heat resistance and, provided the firing temperature is increased to dissolve a sufficient quantity of this silica in the glass, the acid resistance is also enhanced. 

H.13 Hydrofluoric acid is used to etch grooves in glass because it reacts with the silica, Si02(s), in glass. The products of the reaction are aqueous silicon tetrafluoride and water. Write a balanced equation for the reaction. 

FIGURE 15-22 When a mixture of hydrofluoric acid and ammonium fluoride is swirled inside a glass vial (a i, reaction with the silica in the glass frosts the surface of the cover glass as well as the walls of the vial (b)... 

Celata et al. (2006) studied experimentally the drag in glass/fused silica microtubes with inner diameter ranging from 31 to 259 jam for water flow with Re > 300. The drag measurements show that the friction factor for all diameters agrees well with predictions of conventional theory A = 64/Re (for the smallest diameter 31 pm, the deviations of experimental points from the line A = 64/Re do not exceed... 

Water occurs in glass-ionomer and related cements in at least two different states (Wilson McLean, 1988 Prosser Wilson, 1979). These states have been classified as evaporable and non-evaporable, depending on whether the water can be removed by vacuum desiccation over silica gel or whether it remains firmly bound in the cement when subjected to such treatment (Wilson Crisp, 1975). The alternative descriptions loosely bound and tightly bound have also been applied to these different states of water combination. In the glass-poly(acrylic acid) system the evaporable water is up to 5 % by weight of the total cement, while the bound water is 18-28 % (Prosser Wilson, 1979). This amount of tightly bound water is equivalent to five or six molecules of water for each acid group and associated metal cation. Hence at least ten molecules of water are involved in the hydration of each coordinated metal ion at a carboxylate site. 

We are now in a position to discuss requirements for ionomer glasses further. Consider the case of the simple silica (SiOj) glass where we can represent the network diagrammatically thus ... 

Because a wide variety of TLC plates are available commercially, the most appropriate type of plate should be chosen considering several important factors such as the efficiency of the resolution of the lipid species of interest, cost of the plate, the size of the sample to be applied on the plate, and commercial availability of PTLC, depending on the amount of material to be separated. Silica plates are most commonly used for the analysis of hpids. If a large amount of sample material is needed to be separated, plates with a thicker layer of silica should be used. Commercial PTLC plates are available, and thicknesses of 200 to 2000 pm and 250 to 1000 pm are most commonly used in PTLC. Silica-coated glass plates can be of various sizes ... 

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