Other Reduced Pressure Apparatus

A simple apparatus assembled from ordinary laboratory parts (Fig.2.10) is suitable for operation at ordinary laboratory vacuum. It can be used for preparative and for analytical purposes. A is a graduated separating funnel fitted with a rubber bung B 

A more elaborate system can be evacuated before mixing the reactants and is essentially similar to the above apparatus. Details appear in M.A. Malati, Lab.Prac. (1982)1172. 

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With the aid of the columns and other apparatus described it is possible to separate liquid mixtures continuously at normal and reduced pressures as well as at small overpressures (chap. 5.4.5) as sharply a.s in batch distillation. This is clear from the distillation curves of the products obtained in the continuous distillation of a mixture of crude fatty acids in the C4 to Ci range (Fig. 168). A further example shows that by the use of control devices it was possible to separate a mixture of phenols into its main components with great constancy. Fig. 169 illustrates the results of the first separation in this sequence, which was made between the ortho- and Twcfa/pom-cresol fractions. 

In practically all modem apparatus for laboratory distillation certain functions are now automatic. Heating is often controlled by contact thermometers, and the rate of evaporation of the charge with differential manometers. Reduced pressures are also frequently kept constant with automatic units. In fact, the present trend is towards fully automatic control of the conditions of distillation and simultaneous recording of the data measured, an object that has already been reached in many industrial installations. The ideal procedure is for the mixture to be merely introduced into the apparatus and the latter switched on it then automatically yields the required products of distillation and at the same time prepares a diagram of the top and bottom temperatures and other data that may be needed. 

Vacuum distillations require special precautions. Safety spectacles or, rather, a face shield should be worn for work at reduced pressures. Heaters give some protection from flask and column. In addition, glass-wool sheaths may be applied to the other ])art.s for insulation and protection. If this is not possible for experimental reasons, for instance because the separation process should be visible throughout its course a wire gauze or plane glass screen must be placed in front of the apparatus. 

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 the 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. 

Low-pressure operations are common in many laboratories. Laboratories are often equipped with a house vacuum system, vacuum pumps, rotary evaporators, water aspirators, vacuum concentrators, vacuum ovens, and other apparatus that operate under reduced pressure. Some labs build elaborate vacuum racks... 

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