Distillation manometer

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. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Methyl trichlorosilane [75-79-6] M 149.5, b 13,7 /101mm, 64.3 /710.8mm, 65.5 /745mm, 66.1 /atm, d 1.263, n 1.4110. If very pure distil before use. Purity checked by Si nmr, 6 in MeCN is 13.14 with respect to Me4Si. Possible contaminants are other silanes which can be removed by fractional distillation through a Stedman column of >72 theoretical plates with total reflux and 0.35% take-off (see p. 441). The apparatus is under N2 at a rate of 12 bubbles/min fed into the line using an Hg manometer to control the pressure. Sensitive to H2O. [J Am Chem Soc 73 4252 7957 J Org Chem 48 3667 7955.]... 

In order to carry out a distillation, the apparatus is completely assembled, the water pump turned on to its maximum capacity, and the screw clip on the capillary tube in the Claisen flask adjusted so that a gentle stream of air bubbles through the liquid (see Section 11,19 for details of the preparation of the capillary tube). The barometric pressiue is read, and if the resulting vacuum determined from the reading on the manometer is satisfactory (as estimated from the temperature of the tap water), the flask may be heated in an air (Fig. II,... 

A 2 in. sch 40 pipe carries a 35° API distillate at 50°F (SG = 0.85). The flow rate is measured by an orifice meter which has a diameter of 1.5 in. The pressure drop across the orifice plate is measured by a water manometer connected to flange taps. 

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. 

Turn the knob on the manometer to disconnect it from the inlet. Don t leave the manometer permanently connected. Vapors from your distillation, water vapor from the aspirator, and so on, may contaminate the mercury. 

If a rubber tube connected to the vacuum source and the system (or manometer) collapses, you ve had it. The system is no longer connected to the vacuum source, and as air from the bleed tube or vapor from the liquid you re distilling fills your distillation setup, the pressure in the system goes up. Occasionally test the vacuum hoses and if they collapse under vacuum, replace them with sturdier hoses that can take it. 

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. 

For accurate measurements it is essential to degas the manometer fluid before charging the manometer with it, but if it is introduced by distillation in vacuo this is, of course, not necessary. 

Experience with many preparations of this acid chloride has shown that it is desirable to use, during the distillation under reduced pressure, a safety water-bottle between the receiving flask and the manometer. This avoids the passage of vapors of phosphorus oxychloride or acid chloride into the pump. 

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