Mercury diffusion

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

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. 

Mercury diffusion pumps are standard pieces of laboratory equipment and, for most cases, quite adequate ones can be constructed from glass. In general, Pyrex glass (or similar glass) is necessary because hot mercury vapour circulates and differential strain is... 

Very Simple Glass Mercury Diffusion Pump... 

By reacting dicarboxylic acids with 5 % excess of diols, Carothers and Arvin obtained a range of polyesters with molecular weights up to about 4000 [19]. One of the collaborators in this work was J. W. Hill, who constructed a molecular still attached to a mercury diffusion pump that was capable of reducing the pressure in the reaction vessel to 10 5 mm of mercury [20], He made a polyester by reacting... 

Mercury is a very suitabie pump fluid, it is a chemicai eiement that during vaporization neither decomposes nor becomes strongiy oxidized when air is admitted. However, at room temperature it has a comparativeiy high vapor pressure of 10 mbar. if iower uitimate totai pressures are to be reached, coid traps with iiquid nitrogen are needed. With their aid, uitimate totai pressures of 10 ° mbar can be obtained with mercury diffusion pumps. Because mercury is toxic, as aiready mentioned, and because it presents a hazard to the environment, it is nowadays hardiy ever used as a pump fluid. LEYBOLD suppiies pumps with mercury as the pump fluid oniy on request. The vapor pressure curves of pump fluids are given in Fig. 

Mercury diffusion and vapor-jet pumps are less sensitive to air ingress than oil diffusion pumps. The oxidation of the hot mercury caused by the air ingress is negligible in regard to the operating characteristics of the pump when compared with the mercury loss in the forepump line. 

The reactor is evacuated with a two-or three-stage oil or mercury diffusion pump, backed by a rotary mechanical pump. A large removable liquid-nitrogen-... 

The vacuum line used in the following preparations is similar to that described by Shriver.14 It consists of a pump station, a main reaction manifold with six reaction stations, a fractionation manifold with four U-traps and a reaction station at each end, a McLeod Guage, and a Topler pump. The pump station employs a two-stage mechanical forepump and a two-stage mercury diffusion pump. Operating vacuum is 1.0 X 10"S torr. Teflon valves are employed throughout. 

Mercury diffusion pumps are normally constructed from quartz or heat-resistant glass and are therefore a possible source of hazard should they Sreak, especially whilst they are hot. However, during over 40 years of working with such pumps, the author has neither experienced nor heard of such an accident. The major real disadvantage of mercury pumps is the relatively high vapour pressure of mercury at room temperature (ca. 10 Torr), which makes its necesssary to ensure that the cold traps prevent efficiently the mercury vapour from diffusing forward into the line. 

If one expects the alcohol to have a limited thermal stability (e.g. CHjCsCCeCCI OH, RSOCCH2OH), 20-30 ml of paraffin oil should be added (see Chap. 1-2.7). The use of a mercury diffusion pump (p < 0.01 mmHg) is advised. One should not try to redistil the product at water-pump pressure. 

It is generally advisable to first carry out a "rough" disdllation at oil-pump or mercury diffusion-pump pressure, collecting the products in a strongly cooled single receiver (fig. 14). 

Vacimm Technique. There is a growing literature on ultra high vacuum technique by conventional methods (22). It will suffice to say that vacua of the order of 10 to 10 mm. Hg can be obtained readily with the use of either oil or mercury diffusion pumps. The crucial points are ... 

This main bulb was connected to a second bulb containing a modern ion gauge which measures or records gas pressures in a fraction of a second. It can measure pressures from about 10" ° to 10" mm. The system was connected to a liquid air trap and mercury diffusion pump through a movable glass plate with a small hole. The effective area of this hole was 0.010 cm. , and the pump speed was 0.12 liter/sec. This pump speed could be increased more than tenfold by removing the glass plate. 

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