Vacuum Distillation, Manometers

When working with glassware that is to be evacuated, you should wear safety glasses at all times. There is always danger of an implosion. 

Safety glasses must be worn at all times during vacuum distillation. 

It is a good idea to work in a hood when performing a vacuum distillation. If the experiment will involve high temperatures ( 220°C) for distillation or an extremely low pressure ( 0.1 mm Hg), for your own safety you should definitely work in a hood, behind a shield. 

Reduced-pressure microscale distillation (internal monitoring of 

NOTE Normally, you should not grease joints. It is necessary to grease the joints in a vacuum distillation only if you cannot achieve the desired pressure without using grease. 

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If you want to measure the pressure in your vacuum distillation setup, you ll need a closed-end manometer. There are a few different types, but they all work essentially the same way. I ve chosen a stick type (Fig. 75). This particular model needs help from a short length of rubber tubing and a glass T to get connected to the vacuum distillation setup. 

Suppose, by luck of the draw, you ve had to prepare and purify 1-octanol (B.P. 195°C). You know if you simply distill 1-octanol, you run the risk of having it decompose, so you set up a vacuum distillation. You hook your setup to a water aspirator and water trap and attach a closed-end stick manometer. You turn the water for the aspirator on full-blast and open the stick manometer. After a few minutes, nothing seems to be happening. You pinch the tubing going to the vacuum distillation setup, (but not to the manometer) closing the setup off from the source of vacuum. Suddenly, the mercury in the manometer starts to drop. You release the tube going to the vacuum distillation setup, and the mercury jumps to the upper limit. You have air leaks in your vacuum distillation setup. 

A typical vacuum distillation apparatus is illustrated in Fig. 1. It is constructed of a round-bottomed flask (often called the pot ) containing the material to be distilled, a Claisen distilling head fitted with a hair-fine capillary mounted through a rubber tubing sleeve, and a thermometer with the bulb extending below the side arm opening. The condenser fits into a vacuum adapter that is connected to the receiver and, via heavy-walled rubber tubing, to a mercury manometer and thence to the trap and water aspirator. 

Figure 6-20. Minimum-component vacuum distillation apparatus. (A) pump, (B) and (C) Variacs, (D) support plate, (E) terminal strip, (F) stirring motor, (G) heating mantle, (H) r. b. flask, (I) Vigreux distilling column, (J) distilling head, (K) thermometer, (L) Liebig condenser, (M) manostat, (N) stopcock, (O) stopcock, (P) bleeding valve, (Q) drying tower, (R) U-tube manometer, (S) trap, (T) Dewar flask, (U) support plate, (V) r. b. flasks, (W) cow, (X) vacuum adapter, (Y) stopcock, (Z) Hg overflow, (AA) trap, (BB) Dewar flask, (CC) McLeod gauge, (DD) cork rings.

Some dry AgF is charged into a silver reaction flask provided with a manometer and an inlet tube that can be closed off. The AgF can also be produced by the action of F g on the inner walls of the flask itself. After evacuation, pure, carefully dried Dg (see above, Dg) is admitted into the flask. The latter is then closed and heated to 110°C until the pressure ceases to change. The DF formed is frozen out of the reaction mixture by cooling with liquid nitrogen, and excess Dg is drawn off by suction after opening the flask. The.product is purified by high-vacuum distillation in which all connections and receivers must be of Ag or Cu. 

Using the procedure described in Technique 16, Section 16.4, distill the product by vacuum distillation using an apparatus fitted with a Hickman still and a water-cooled condenser (Technique 16, Figure 16.5). Place a small piece of a stainless steel sponge in the lower stem of the Hickman still to prevent bumpover and stir vigorously with a magnetic spin vane. Use an aspirator for the vacuum source and attach a manometer if one is available (Technique 16, Figure 16.10). You may use an aluminum block to heat the dishllation mixture. The aluminum block temperature will be about 130°C (with 20 mm Hg vacuum). If you have less than 0.75 mL, you should combine your product with that of another student. 

A vacuum distillation apparatus using the components of the traditional organic laboratory kit is shown in Figure 16.5. It uses the ebulliator tube, the Claisen head, and a thermometer for internal temperature monitoring. A water condenser is shown, but with high-boiling liquids, this apparatus may be simplified by removing the water condenser. A special vacuum adapter allows connection to the manometer and vacuum source. 

The principal device used to measure pressures in a vacuum distillation is the closed-end manometer Two basic types are shown in Figures 16.9 and 16.10. The manometer shown in Figure 16.9 is widely used because it is relatively easy to construct. It consists of a U-tube that is closed at one end and mounted on a wooden support. You can construct the manometer from 9-mm glass capillary tubing and fill it, as shown in Figure 16.11. 

The primary difference between the apparatus for simple and vacuum distillations is the attachment of the vacuum adapter to a vacuum source. Rather than being directly connected to the vacuum source, the adapter is normally linked to a heavy-walled sidearm flask. This flask has several functions. For example, if a water aspirator is being used to produce a vacuum, the flask serves as a trap to prevent water from backing up into the apparatus, which may occur if there is a sudden decrease in the water pressure in the aspirator. If it is equipped with a stopcock release valve, the flask also provides a means for connecting the apparatus and the manometer to the source of the vacuum and releasing the vacuum. Further discussion of traps is provided under the topic "Vacuum Filtration" in Section 2.17. 

The volume and density of the vapor formed by volatilization of a given amount of liquid are pressure-dependent. For example, the volume of vapor formed from vaporization of a drop of liquid will be about 20 times as great at 38 torr as it would be at 760 torr. During a vacuum distillation, large quantities of vapor are produced in the distillation flask, and these vapors enter the condenser with high velocity prior to condensing. Since the density of the vapor is much lower at reduced pressure than at atmospheric pressure, it may be difficult to control the rate of vaporization and thereby minimize the difference in pressure between the distillation flask and the manometer where the pressure must be accurately measured. 

When constructing a manometer of the type shown in Fig. 12(c), it is impor tant to apply a very high vacuum (e.g., with a Hy-Vac pump) to the manometer while the mercury in the left-hand (sealed) limb is heated until it boils unless this is done, traces of air will remain in this limb and cause inaccurate readings. During a distillation, the tap I should be kept closed except when a pressure reading is being taken if it is left open indefinitely, a sudden default by the distillation apparatus or by the pump may cause the mercury in the sealed limb of G to fly back and fracture the top of the limb. 

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