Vacuum apparatus

For more efficient drying at elevated temperatures, the vacuum apparatus (Fig. 48(A)) is often used. The sample to be dried is placed in an inner tube surrounded by a heating jacket. 

A colorless, colloidal precipitate was formed and stirred thoroughly for about 15 minutes, whereupon it was filtered by suction. The raw product thus obtained was washed with water until It contained only about Va% water-soluble salts. After drying for 12 hours In a vacuum apparatus at 60°C and under a pressure of 12 mm Hg, the product had the form of hard pieces. The pieces were comminuted to powder in a ball mill and the powder was passed through a sieve (3,600 meshes per cm ). The small residue on the sieve was again pulverized and passed through the same sieve. The yield was 870 g, or 99% of theoretical, calculated on the assumed formula... 

In the usual high-vacuum apparatus I placed aluminum borohydride onto a sample of uranium tetrafluoride. I observed an immediate reaction, with formation of green crystals which could be moved about the vacuum apparatus. As it happened, on the very day I was performing the synthesis, we had a visit from Professor H. C. Urey, who was in charge of the overall study. He immediately asked us to increase our research effort and undertake to prepare uranium tetraborohydride on a relatively large scale, adequate for testing. 

Figure 1. An ultrahigh vacuum apparatus used for the study of single crystal catalysis before and after operation at atmospheric pressure in a catalytic reactor.

When constructing high vacuum apparatus, waxed tools should only be used when the apparatus under construction can be thoroughly cleaned afterwards. Otherwise wax may be present in the vacuum system, and will be removed by pumping only with extreme slowness. 

A casual study of any laboratory vacuum apparatus will show how commonly T-joints must be made with either one or both of the tubes clamped. Generally speaking this is a much easier technique than that previously described, as it removes the necessity for synchronized manipulation of the glass. 

Mercury cut-offs are used in vacuum apparatus instead of taps when tap grease is undesirable. A frequently used type is shown in Figure 63. A two-way tap is attached at the top of a reservoir for mercury. 

The deposition of a wide range of materials using beams of elemental sources in high-vacuum apparatus (10-4—10-8 torr), essentially by physical methods, is known as molecular beam epitaxy (MBE)8 12 and atomic layer epitaxy (ALE). These methods will be mentioned where there is an overlap with CVD techniques, but will not be fully reviewed. (They are mentioned also in Chapter 9.15). 

The necessity for a co-catalyst with BF3 was subsequently confirmed rigorously by Evans and Meadows [8-10] by means of an all-glass vacuum apparatus which established a characteristic style of experimentation which I adopted and adapted. They showed that under rigorously anhydrous conditions isobutene and BF3 could be mixed without the isobutene polymerising, and that the addition of water did initiate a polymerisation. Fairbrother and Frith [11] reported very briefly that the polymerisation of isobutene by (Ta-Nb)F5 required a co-catalyst - without stating which one they used. 

Tritiated water (from the Radiochemical Centre, Amersham) was diluted to an activity of 5 102 mChml"1 curie = 3.7 x 1010 s 1. 2-Propanol, labelled with 14C (from Radiochemical Centre, Amersham), was diluted to 12.5 mCnml"1) with AnalaR 2-propanol, in a vacuum apparatus. 

In order to ascertain the nature of the propagating species in the polymerisation of styrene catalysed by perchloric acid in methylene dichloride we have investigated the behaviour of this system by (a) calorimetric, (b) spectroscopic, and (c) conductimetric techniques. All experiments were carried out in high-vacuum apparatus, with highly purified and dried reagents which were mixed by breaking phials or break-seals magnetically. 

The problem is solved by prefiltering all mobile phases and samples before beginning the experiment. For mobile phases and large sample volumes, this involves utilizing a vacuum apparatus that draws the liquid through a 0.5-pim filter. Since such filtration involves a vacuum, the mobile phase is automatically degassed as well, so the filtration need not be a separate step. An efficient operation would be to filter a mobile phase with a vacuum apparatus while simultaneously sonicating. See Workplace Scene 13.1. 

Fig. 1. An ultrahigh vacuum apparatus for studying single crystal catalysts before and after reaction at elevated pressures.

Refractory bricks composed of oxides of magnesium, chromium, aluminum and iron and trace amounts of silica and calcium oxide are used in roofs of open hearths, sidewalls of electric furnaces and vacuum apparatus and copper converters. Such refractories are made in an arc furnace by fusing mixtures of magnesite and chrome ore. 

TURBOVAC pumps are precisely balanced and may generally be connected directly to the apparatus. Only in the case of highly sensitive instruments, such as electron microscopes, is it recommended to install vibration absorbers which reduce the present vibrations te a minimum. For magnetically suspended pumps a direct connection to the vacuum apparatus will usually do because of the extremely low vibrations produced by such pumps. 

For around 20 years now, numerous standards and recommendations have been drawn up at national and international level and revised, whenever necessary, in accordance with the state of the art. These standards and recommendations must be observed whenever use is made of vacuum equipment (pumps, gauges, valves, etc.) and vacuum apparatus, systems and plants are assembled. They not only contain specifications applying specially to vacuum technology, but also go beyond this specific field and involve, for example, physical units, formulas, noise protection regulations, etc. 

Apparatus. A conventional vacuum apparatus was connected to the reaction vessel. A Pace gage could be used to measure oxygen uptake during runs. 

Figure 1. Reaction vessel Borosilicate glass = 25 mm. i.d., filled in nitrogen atmosphere and attached to vacuum apparatus by ball and socket joint...

Apparatus. A conventional static vacuum apparatus was used. Reactants were introduced into cylindrical borosilicate glass reaction vessels (250-ml. capacity) suspended in a furnace maintained at a temperature within 0.1°C. Pressure measurements were made by a transducer (Consolidated Electrodynamics) linked to a recorder (RE 511 Servo-scribe). 

For smaller scale operations or when catalytic amounts of impurities are important, a vacuum line is almost always more suitable and efficient. Although it takes longer to become familiar with a vacuum apparatus, once a certain degree of competence and confidence has been achieved, there is little to choose in terms of experimental time between h.v.t. and the rigorous use of a dry-box. The choice is often made simply on the basis of laboratory tradition (i.e. inertia) and know-how. 

Some of the more important uses of distillation in high vacuum apparatus which will be encountered later in this hook, are transfer of a liquid by means of a cold finger into a vessel which cannot itself be cooled blocking access to vessels by means of a temporary, or freeze , valve washing reaction products repeatedly and progressively with a single batch of solvent washing the interior of ducts free from solids. 

Some items not usually described in elementary glass blowing texts, but frequently useful in vacuum apparatus, are magnetic seal-breakers, universal joints, and metal wire seals. Therefore their construction is described here. 

Recent developments in fibre optics have made it possible to measure the spectrum in any kind of vessel, including any which form part of a vacuum apparatus, so that the problem of fitting a cell which is part of a vacuum rig into a spectrophotometer is now effectively obsolete. However, for single or occasional measurements the devices discussed here are simpler and very much cheaper than fibre-optic gadgetry. 

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