Apparatus

The simplest form of sublimation apparatus consists of a beaker or porcelain dish on top of which is placed an upturned watch-glass. The beaker contains the solid to be sublimed and the underside of the watch-glass provides the surface upon which the sublimed components condense (Figure 1). A perforated filter paper is commonly placed between the beaker and the watch-glass to prevent sublimate falling back into the sample.

A variant upon the above system uses an upturned funnel instead of a watch-glass as the condensing

Figure 1 Simple apparatus for demonstrating the principles of sublimation. S, sample; P, sublimate (product); FP, perforated filter paper.

surface and an appropriately placed sealing ring improves the performance (Figure 2). Coils through which coolant is circulated can promote the sublimation process.

An early form of sublimation apparatus of which the above arrangements are derivatives (Figure 3)

Figure 2 Apparatus for simple sublimation at atmospheric pressures. Watch-glass, W, with sample, S, surmounted by filter funnel, FF, with cooling coils, C, glass wool, G, and collected sublimed product, P. A sealing ring, R, is included between the watch-glass and filter funnel.
Figure 3 Early form of sublimation apparatus. The heated crucible, CR, rests in the cooling device. The sublimate product, P, collects on the underside of the watch-glass, W. S, sample.

materials such as enzymes and the process has been termed lyophilization.

Sublimation of metallic elements from rock or ore samples requires high temperatures. The equipment used is based upon silica furnace tubes in order to withstand the necessary conditions. The silica tube is heated in a furnace and the sublimate condenses either on a cool part of the tube or on a cooled surface immediately after leaving the tube.

The conditions of sublimation must be chosen according to the requirements of the application. For simple purification the sample temperature is raised slowly, under reduced pressure if necessary, until sublimate is observed on the condensing surface. These established conditions should then be maintained until no further sublimation appears to be occurring, at which point the sample temperature can be raised again if other components of the sample can be further removed. At any point in this cycle the apparatus can be dismantled and the sublimate removed. This process allows selective separation or fractional sublimation to be carried out.

included a means of cooling the surface on which the sublimate condenses. Cooling can be achieved in a number of ways, for example, using filter papers moistened with cool water, or in the case of the upturned funnel, a suitably shaped coil of circulating coolant liquid can be placed around the surface.

Sublimation under reduced pressure uses a modified form of apparatus in which a sealed enclosure allows a vacuum to be applied. The cooled surface is orientated with respect to the sample so as to maximize the condensation once sublimation has occurred (Figure 4). Reducing the distance that the sublimed substance(s) must travel is beneficial provided the necessary temperature gradient between sample and condensing surface can be maintained. An alternative arrangement for sublimation applications is shown in Figure 5.

Freeze-drying, a special application of the sublimation principle, uses apparatus of a different kind (Figure 6). The sample is dispersed around the walls of a round-bottomed flask whilst it is frozen by immersion in a suitable freezing mixture, for example dry ice-acetone. The flask is then attached to the evacuating system which usually comprises an oil vacuum pump protected from the ice sublimate by a train of condenser traps. Over a period of typically several hours the ice sublimes from the sample and condenses in the traps. Air is then admitted to the apparatus and the dried sample can be removed whilst the sublimed ice is drained off as water through the drain tap. Frequently this system is used to dry heat-sensitive

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Solar Panel Basics

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