Ultrasonic immersion, solvent

We have observed a dependence of the yield, polymerization degree, and polydispersity of polysilanes on temperature and also on the power of ultrasonication. In the ultrasonication bath the simplest test of the efficiency of cavitation is the stability of the formed dispersion. It must be remembered that the ultrasonic energy received in the reaction flask placed in the bath depends on the position of the flask in the bath (it is not the same in each bath), on the level of liquid in the bath, on temperature, on the amount of solvent, etc. When an immersion probe is used the cavitation depends on the level of the meniscus in the flask as well. The power is usually adjusted close to 50% of the output level but it varies with the reaction volume, flask shape, and other rection conditions. The immersion-type probe is especially convenient at lower temperatures. 

In 1983 Suslick reported the effects of high intensity (ca. 100 W cm, 20 kHz) irradiation of alkanes at 25 °C under argon [47]. These conditions are of course, well beyond those which would be produced in a reaction vessel immersed in an ultrasonic bath and indeed those normally used for sonochemistry with a probe. Under these extreme conditions the primary products were H2, CH4, C2H2 and shorter chain alk-l-enes. These results are not dissimilar from those produced by high temperature (> 1200 °C) alkane pyrolyses. The principal degradation process under ultrasonic irradiation was considered to be C-C bond fission with the production of radicals. By monitoring the decomposition of Fe(CO)5 in different alkanes it was possible to demonstrate the inverse relationship between sonochemical effect (i. e. the energy of cavitational collapse) and solvent vapour pressure [48],... 

Ultrasonic extraction (USE) it is a simple extraction technique, in which the sample is immersed in an appropriate organic solvent in a vessel and placed in an ultrasonic bath. The efficiency of extraction depends on the polarity of the solvent, the homogeneity of the matrix and the ultrasonic time. The mixture of sample and organic solvent is separated by filtration [40,41],... 

The cleaning may be carried out by solvent vapours, by immersion in the liquid phase—and by both methods in combination. Immersion also should overcome static attraction, but if this phase is not used a gun emitting deionized air under low pressure may be employed to clean individual parts and to take away debris. Agitation of components by ultrasonic means can be helpful in removing contamination from blind holes and recesses. 

After exposure, the samples are immersed in a suitable solvent, their container immersed in an ultrasonic bath, sonicated and the resultant solution analyzed. It is strongly recommended that each sample be run in triplicate and the results compared to those of three similar unexposed controls kept in the dark for the same exposure duration. This protocol is outlined in Figure 10. 

Ultrasonic systems using less harmful and corrosive materials have been successfully used as an alternative to vapor cleaning. For example, stainless steel, which used to be cleaned with distillate-spray wash and vapor rinse using 1,1,1 -trichlo-roethane, has been replaced by immersion in ultrasonic bath containing trichloro-trifluoroethane and methanol. Hence the beneficial substitution of one organic solvent by a more environmentally friendly cleaner is possible. 

Despite the automated degassing, it is still advisable to filter each of the components of the mobile phase under vacuum before use. Degassing can be performed manually using a combination of vacuum filtration (see Chapter 3 for filter compatibility information), followed by immersion in an ultrasonic bath for a short period of time. Care should be taken when using an ultrasonic bath because of the generation of heat that can alter the mobile phase composition by evaporation of the more volatile organic solvent. 

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