Microscale vacuum distillation

Oh. Check out the O-ring cap seal discussion for a perspective on microscale vacuum distillation (see Chapter 6, Microscale Jointware ). 

A basic apparatus similar to the one shown in Figure 16.1 may be used for microscale vacuum distillations. As is the case for simple distillation, this apparatus uses the Hickman head as a means to reduce the length of the vapor path. The major difference to be found when comparing this assembly to one for simple distillation (Technique 14, Figure 14.8) is that the opening to the atmosphere has been replaced by a connection to a vacuum source (top right-hand side). The usual sources of vacuum are the aspirator (Technique 8, Section 8.5), a mechanical vacuum pump, or a "house" vacuum line (one piped directly to the laboratory bench). The aspirator is probably the simplest of these sources and the vacuum source most likely to be available. However, if pressures below 10-20 mm Hg are required, a vacuum pump must be used. 

That s clear enough. You lose lots of your microscale product with a reduced-pressure (vacuum) distillation. As far as I know, no microscale laboratory manual has anyone perform this. Yes, you might be asked to... 

So, since nobody actually has any microscale reduced-pressure distillations, and with certain sizes of equipment the O-rings can squeeze out from under the plastic caps, and since I don t use them for reduced-pressure (vacuum) distillation anyway (I switch to a semi-micro apparatus with real 14/20 joints and—gasp —grease), I don t know how vacuum tight these seals really are. 

The following set of instructions is a step-by-step account of how to carry out a vacuum distillation. The microscale apparatus illustrated in Figure 16.1 will be used however, the procedures apply to any vacuum distillation. 

Until recently, there was no way you could duplicate an ordinary distillation of any class in microscale (re-read the material on the different classes in Chapter 20, Distillation ), because the equipment either didn t exist or was fabricated with glass joints that didn t match up. Recently, Ace Glass (and others for all I know) has produced an all T14/10 joint microscale kit. Unfortunately neither a three-way nor a vacuum takeoff adapter is included. These pieces are, however, available separately. 

Choose either Experiment 14A or Experiment 14B, but not both. The semimicroscale procedure requires the use of equipment not found in the typical microscale kit a 20-mL round-bottom flask, a distillation head, and a vacuum takeoff adapter. The purpose of Experiment 14B is to allow an alternative to the use of a Hickman head for the distillation step. 

Another common method for removing solvent is by simple microscale distillation (Sec. 2.13). Discontinue heating when only a small amount of solvent remains, and do not overheat the stillpot, as your product may decompose if heated too strongly. The stillpot should be cooled to room temperature. Remove the Hickman stillhead and water condenser and insert an air condenser into the vial. Attach a thermometer adapter fitted with a glass tube to the top of the condenser and attach the tube to a vacuum source (Fig. 2.73). If you use a water aspirator, be sure to place... 

Apparatus Glass apparatus from the microscale experimental procedure of Section 19.2, 3-mL conical vial, two screw-cap centrifuge tubes, 1-mL plastic syringe, Pasteur pipet with 0.5- and 1.0-mL calibration marks, ice-water bath, and apparatus for magnetic stirring, simple distillation, vacuum filtration, Craig tube filtration, and fiameless heating. 

The most powerful microscale distillation system currently available is the 2.5-in. vacuum-jacketed microscale spinning-band distillation column (see Fig. 3.16 and Experiment [3C] for description and details).This still is designed for convention downward distillate collection and nonstopcock reflux control. The column is rated at 10 theoretical plates. 

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