Vacuum line simple

CAUTION. Fluorine is a strong oxidizer and a very corrosive material. An appropriate vacuum line made from copper or Monel in a well-ventilated area should be constructed for working with this element. The reactions themselves were carried out in Teflon vessels. If elementary precautions are taken, work with fluorine is relatively simple. 

In order to be able to design even the most elementary vacuum line, it is necessary to know something of the basic theory concerning the movement of molecules within the system. No attempt will be made here to instruct the reader in the details of vacuum physics and therefore the formulae given below have been kept simple. A more detailed discussion of the theory of gaseous flow can be found in Bushman s excellent review (Bushman, 1962). The object of the following discussion is simply to allow the reader to assess, without too much effort, the approximate efficiency of the system he is planning. 

The freeze valve Another simple valve which is extremely effective and especially useful when one needs to seal off a unit containing a volatile liquid from the vacuum line, is the freeze-valve. This is a glass U-tube of not more than 5 mm i.d. in the duct connecting the two parts which need to be separated. If some of the liquid is frozen into the U so that the tube is blocked, the low pressure side of the U can be sealed off without the necessity of freezing the bulk of the liquid on the other side. This technique can be especially useful when the flask containing the bulk liquid is very large or if... 

A simple, quantitative, vacuum-line method for the preparation of millimole quantities of 13C-enriched difluoromethane and chlorofluoromethane by heating (13C)dichloromethane with mercury(II) fluoride at 230 C for 1 hour is known. The volatile product is estimated to be 75% difluoromethane 2 and 25% chlorofluoromethane l.54... 

Many variations of the design of the cells shown in Figure 9.3 have been reported. Reasonable cost, simple construction, flexibility, and ease of correct use have led to widespread acceptance of these general designs. These cell designs fit most aqueous and nonaqueous sample requirements where the presence of oxygen or water is not critical. When the removal and exclusion of these contaminants is required, special care must be taken to work in an inert atmosphere. A dry box (Chap. 19) or a cell that can be interfaced to a vacuum line (Chap. 18) may be required. 

There are numerous designs for vacuum electrochemical cells, ranging from very simple to extremely complex. In operation, the vacuum electrochemical cell parts are first cleaned, washed with solvent, and dried in an oven at 200°C. The hot cell parts should then be quickly assembled and evacuated on the vacuum line for several hours or overnight. Once the electrochemical cell has been pumped down, it should be closed off and transferred to a dry box, where the air-sensitive sample and the electrolyte can be added to it. Alternatively, the solid electrolyte could be added into the electrochemical cell before assembly. 

Fig. 5.1. Simple vacuum line. A iineof this sort might be used to transfer volatile samples to infrared gas cells, NMR tubes, etc. It might also be used for loading tubes for sealed-tube reactions and for vacuum sublimations.

B. The Transfer Of Condensable Gases. Condensable gases are conveniently separated and transferred in a vacuum line by vaporization and condensation. Prior evacuation of the system and a suitably low temperature are necessary to quantitatively transfer the material. The vapor pressure corresponding to quantitative condensation of a compound will depend on the volume of the system and the quantity of material being transferred as a general rule, a maximum vapor pressure of 10 3 torr is satisfactory. For simple transfer operations, liquid nitrogen (bp — 196°C) is the most commonly used refrigerant. 

Volatile reactive liquids present few problems since they generally can be distilled from the vacuum line into a tube equipped with a serum bottle cap (Fig. 9.26). Nitrogen is then admitted to the tube, and the sample is taken with a syringe. Another scheme involves an inlet with a capillary tube or ampule breaker.33 35 This method is potentially useful for vacuum line work, since it is relatively simple to fill and seal off a sample tube attached to the vacuum line. 

A. Typical Systems. A simple system for the transfer of samples to an infrared gas cell or to a NMR sample tube consists of a fore pump, diffusion pump, trap, and manifold (Fig. 5.1). At the other extreme is a general-purpose chemical vacuum line, which permits the separation of volatile compounds, transfer of noncondensable gases, and storage of reactive gases and solvents (Fig. 5.2). When attack of stopcock grease is a serious problem, grease-free de-... 

A. Mercury Manometers for Routine Work. The pressure-volume-temperature measurement of gases is the backbone of quantitative chemical vacuum line work. For these measurements an error of a few percent is frequently sufficient and may be attained by a simple U-manometer attached to a calibrated volume and read with ah inexpensive cathetometer. 

If gas is to be delivered to a reaction flask which has an unobstructed outlet, a simple flow control valve on the high-pressure cylinder will provide adequate regulation of the gas delivery. In this case a needle valve is attached to the cylinder, or to a pressure regulator which in turn is attached to the cylinder. It also is possible to deliver gas to a closed system, such as a vacuum line, with a flow control valve. In this case the pressure within the apparatus must be carefully monitored by means of a manometer and the system should also be equipped with a means of pressure relief, such as a mercury bubbler manometer (Fig. 7.2). 

The useful screw-thread connector is becoming widely available. The simplest is the screw-capped adapter (Fig. 2.32) where the screw-capped joint is associated with a ground glass cone. The figure shows the silicone rubber ring and PTFE washer, and the adapter is useful for the insertion of gas tubes, thermometers or stirrer shafts. The screw-thread connection principle has been applied to the connection of water hoses to condensers and vacuum lines to Buchner flasks, to securing simple sealing septa to flasks, etc. 

A very simple apparatus for reaction in an inert atmosphere is a one-necked flask equipped with a magnetic stirrer and three-way stopcock (Fig. 2.76). The vertical arm of the stopcock is fitted with a rubber septum and the horizontal arm leads to a second three-way stopcock giving access to the nitrogen supply and to the vacuum line. This apparatus is suitable for reactions which take place at or below room temperature, do not require addition of solid reagent once the... 

Most commonly known as a Schlenk line (see Fig. 7.19), this vacuum line/appa-ratus allows easy evacuation and purging of Schlenk items (see Fig. 7.20). The concept for Schlenk procedures is very simple One simply evacuates an item connected by a thick-walled tube to the bottom of the three way stopcock. Then by... 

On the other hand, a liquid trap is essential between any liquid vacuum gauge and the manifold itself. The simplest accident can cause hours of needless delay as the vacuum line is cleaned out. Normally a simple splash trap is sufficient (see Sec. 7.4.6). Vacuum systems that go below 1CT5 torr should consider to have a cold trap placed between their liquid vacuum gauges and vacuum systems. This placement will keep vapors from contaminating either side of the system. 

Since these compounds are extremely reactive toward moisture and oxygen, the equipment used to handle them will be discussed briefly. (A high degree of experimental skill and extreme care are required for safe manipulation of these compounds ) A simple vacuum line, consisting of a manifold equipped with a mercury blowout manometer, a gas inlet tube, and several standard-taper joints, is employed. A worthwhile introduction to the use of vacuum-line techniques in synthetic chemistry is given by Jolly and by Sanderson. The sublimation apparatus shown in Fig. 2 and the vacuum filtration apparatus and... 

Many potential variations exist on these simple themes. For example, the high capacity pumping station used in our laboratory can be connected to any of five different vacuum lines by attaching standard flanges to an octahedral cross welded to the face plate flange bolted to the diffusion pump. Similar variations are possible with glass systems fitted with the appropriate hardware. 

The first range can be adequately handled with a simple mercury manometer. ( 1 ) However, absolute pressure transducers, such as the Baratron 200 Series manufactured by MKS Instruments, offer much greater accuracy over a wider pressure range.(8) If accurate and precise pressure measurements are commonly conducted on the vacuum line, the transducers are the sensors of choice. If not, mercury manometers are the economical alternative. 

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