Glass, organic fused silica

The high-purity water thus produced typically has a conductance of about 0.5 x 10-6fi-1cm-1 (0.5juScm-1) and is suitable for use under the most stringent requirements. It will meet the purity required for trace-element determinations and for operations such as ion chromatography. It must however be borne in mind that such water can readily become contaminated from the vessels in which it is stored, and also by exposure to the atmosphere. For the determination of organic compounds the water should be stored in containers made of resistant glass (e.g. Pyrex), or ideally of fused silica, whereas for inorganic determinations the water is best stored in containers made from polythene or from polypropylene. 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

A very good technique for the characterization of the smoothness of a substrate is the well known stylus technique. With this technique, the surface waviness and surface roughness can be determined very rapidly, and a quantitative measure of the surface quality is also obtained. Polished crystal plates such as sapphire and polished vitreous materials as, for example, fused silica, silicate glasses and organic polymers show very uniform surfaces and yield smooth stylus traces. Drawn glass also has smooth surfaces, with occasional surface irregularities of approximately 100 nm in... 

FIGURE 2.11. Schematic representation of the Photo-CREC Water-II Reactor (1) MR or BL lamp, (2) replaceable 3,2-cm-dianieter glass inner tube, (3) replaceable 5.6-cm-diameter glass inner tube, (4) fused-silica windows, (5) UV-opaque polyethylene outer cylinder, (6) stirred tank, (7) centrifugal pump, and (8) air injector, (Reprinted with permission from Ind. Eng. Chem. Res., 40(23), M. Salaices, B. Seiraiio and H.l. de Lasa, Photocatalytic conversion of organic pollutants Extinction coefficients and quantum efficiencies, 5455-5464. Copyright 2001 American Chemical Society). 

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