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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
Unprotected parts of the body should not be in contact with uninsulated vessels or pipes that contain cryogenic liquids because extremely cold material may bond firmly to the skin and tear flesh if separation or withdrawal is attempted. Even very brief skin contact with a cryogenic liquid can cause tissue damage similar to that of frostbite or thermal burns, and prolonged contact may result in blood clots that have potentially very serious consequences. Gloves must be impervious to the fluid being handled and loose enough to be tossed off easily. A potholder may be a desirable alternative. Objects that are in contact with cryogenic liquids should also be handled with tongs or potholders. The work area should be well ventilated. Virtually all liquid gases present the threat of poisoning, explosion, or, at a minimum, asphyxiation in a confined space. Major harmful consequences of the use of cryogenic inert gases, including asphyxiation, are due to boiling off of the liquid and pressure buildup, which can lead to violent rupture of the container or piping.
In general, liquid hydrogen should not be transferred in an air atmosphere because oxygen from the air can condense in the liquid hydrogen, presenting a possible explosion risk. All precautions should be taken to keep liquid oxygen from organic materials; spills on oxidizable surfaces can be hazardous. Though nitrogen is inert, its liquefied form can be hazardous because of its cryogenic properties and because displacement of air oxygen in the vicinity can lead to asphyxiation followed by death with little warning. Rooms that contain appreciable quantities of liquid nitrogen (N2) should be fitted with oxygen meters and alarms. Liquid nitrogen should not be stored in a closed room because the oxygen content of the room can drop to unsafe levels.
Cylinders and other pressure vessels used for the storage and handling of liquefied gases should not be filled to more than 80% of capacity, to protect against possible thermal expansion of the contents and bursting of the vessel by hydrostatic pressure. If the possibility exists that the temperature of the cylinder may increase to above 30°C, a lower percentage (e.g., 60%) of capacity should be the limit.
6.E.2.1 Cold Traps and Cold Baths
Cold traps should be chosen that are large enough and cold enough to collect the condensable vapors in a vacuum system. Cold traps should be checked frequently to make sure they do not become plugged with frozen material. Cold traps in a reduced-pressure system should be taped or placed in a metal can filled with vermiculite. After completion of an operation in which a cold trap has been used, the system should be vented in a safe and environmentally acceptable way. Otherwise, pressure could build up, creating a possible explosion and sucking pump oil into the system. Cold traps under continuous use, such as those used to protect inert atmosphere dry boxes, should be cooled electrically and monitored by low-temperature probes.
Appropriate gloves and a face shield should be used to avoid contact with the skin when using cold baths. Dry gloves should be used when handling dry ice. Lowering of the head into a dry ice chest is to be avoided because carbon dioxide is heavier than air and asphyxiation can result. The preferred liquids for dry ice cooling baths are isopropyl alcohol or glycols, and the dry ice should be added slowly to the liquid portion of the cooling bath to avoid foaming. The common practice of using acetone-dry ice as a coolant should be avoided. Dry ice and liquefied gases used in refrigerant baths should always be open to the atmosphere. They should never be used in closed systems, where they may develop uncontrolled and dangerously high pressures.
Extreme caution should be exercised in using liquid nitrogen as a coolant for a cold trap. If such a system is opened while the cooling bath is still in contact with the trap, oxygen may condens from the atmosphere. The oxygen could then combine with any organic material in the trap to create a highly explosive mixture. Thus, a system that is connected to a liquid nitrogen trap should not be opened to the atmosphere until the trap has been removed. Also, if the system is closed after even a brief exposure to the atmosphere, some oxygen (or argon) may have already condensed. Then, when the liquid nitrogen bath is removed or when it evaporates, the condensed gases will vaporize, producing a pressure buildup and the potential for explosion. The same explosion hazard can be created if liquid nitrogen is used to cool a flammable mixture that is exposed to air.
6.E.2.2 Selection of Low-Temperature Equipment
Equipment used at low temperatures should be selected carefully. Temperature can dramatically change characteristics of materials. For example, even the impact strength of ordinary carbon steel is greatly reduced at low temperatures, and failure can occur at points of weakness, such as notches or abrupt changes in the material of construction, in cold equipment. When combinations of materials are required, it is important that the temperature dependence of their volumes be considered so that leaks, ruptures, and glass fractures can be avoided. For example, O-rings that provide a good seal at room temperature may lose resilience and fail to function on chilled equipment.