the area. Any reaction of this type should be carried out in a hood and labeled with signs that indicate the contents of the reaction vessel and the explosion risk.

Glass tubes with high-pressure sealers should be no more than three-quarters full. Appropriate precautions using the proper shielding must be taken for condensing materials and sealing tubes. Vacuum work can be carried out on a Schlenck line as long as proper technique is used. The sealed glass tubes can be placed either inside pieces of brass or iron pipe capped at one end with a pipe cap or in an autoclave containing some of the reaction solvent (to equalize the pressure inside and outside the glass tube). The tubes can be heated with steam or in a specially constructed, electrically heated ''sealed-tube" furnace that is controlled thermostatically and located such that the force of an explosion would be directed into a safe area. When the required heating has been completed, the sealed tube or bottle should be allowed to cool to room temperature. Sealed bottles and tubes of flammable materials should be wrapped with cloth toweling, placed behind a safety shield, and then cooled slowly, first in an ice bath and then in dry ice. After cooling, the clamps and rubber stoppers can be removed from the bottles prior to opening. Personal protective equipment and apparel, including shields, masks, coats, and gloves, should be used during tube-opening operations. It should be noted that NMR tubes are often thin-walled and should only be used for pressure reactions in a special high-pressure probe or in capillary devices.

Newly fabricated or repaired glass equipment for pressure or vacuum work should be examined for flaws and strains under polarized light. Corks, rubber stoppers, and rubber or plastic tubing should never be relied on as relief devices for protection of glassware against excess pressure; a liquid seal, Bunsen tube, or equivalent positive relief device should be used. When glass pipe is used, only proper metal fittings should be used.

6.E.2 Liquefied Gases and Cryogenic Liquids

Cryogenic liquids are materials with boiling points of less than -73 °C (-100 °F). Liquid nitrogen, helium, and argon, and slush mixtures of dry ice with isopropanol are the materials most commonly used in cold traps to condense volatile vapors from a system. In addition, oxygen, hydrogen, and helium are often used in the liquid state.

The primary hazards of cryogenic liquids are fire or explosion, pressure buildup (either slowly or due to rapid conversion of the liquid to the gaseous state), embrittlement of structural materials, frostbite, and asphyxiation. The extreme cold of cryogenic liquids requires special care in their use. The vapor that boils off from a liquid can cause the same problems as the liquid itself.

The fire or explosion hazard is obvious when gases such as oxygen, hydrogen, methane, and acetylene are used. Air enriched with oxygen can greatly increase the flammability of ordinary combustible materials and may even cause some noncombustible materials to burn readily (see Chapter 5, sections 5.G.4 and 5.G.5). Oxygen-saturated wood and asphalt have been known to literally explode when subjected to shock. Because oxygen has a higher boiling point (-183 °C) than nitrogen (-195 °C), helium (-269 °C), or hydrogen (-252.7 °C), it can be condensed out of the atmosphere during the use of these lower-boiling-point cryogenic liquids. With the use of liquid hydrogen particularly, conditions may develop for an explosion. (See Chapter 5, sections 5.F.3 and 5.G.2, for further discussion.)

It is advisable to furnish all cylinders and equipment containing flammable or toxic liquefied gases (not vendor-owned) with a spring-loaded pressure-relief device (not a rupture disk) because of the magnitude of the potential risk that can result from activation of a non-resetting relief device. Commercial cylinders of liquefied gases are normally supplied only with a fusible-plug type of relief device, as permitted by DOT regulations. Pressurized containers that contain cryogenic material should be protected with multiple pressure-relief devices.

Cryogenic liquids must be stored, shipped, and handled in containers that are designed for the pressures and temperatures to which they may be subjected. Materials that are pliable under normal conditions can become brittle at low temperatures. Dewar flasks, which are used for relatively small amounts of material, should have a dust cap over the outlet to prevent atmospheric moisture from condensing and plugging the neck of the tube. Special cylinders insulated and vacuum-jacketed with pressure-relief valves and rupture devices to protect the cylinder from pressure buildup are available in capacities of 100 to 200 liters (L).

A special risk to personnel is skin or eye contact with the cryogenic liquid. Because these liquids are prone to splash in use owing to the large volume expansion ratio when the liquid warms up, eye protection, preferably a face shield, should be worn when handling liquefied gases and other cryogenic fluids. The transfer of liquefied gases from one container to another should not be attempted for the first time without the direct supervision and instruction of someone experienced in this operation. Transfers should be done very slowly to minimize boiling and splashing.

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