Chemical Research Safety Note #12
Procedures for Working with Liquid Nitrogen
Liquid nitrogen is frequently used in chemical research laboratories for the purpose of cooling. Liquid nitrogen is a valuable coolant because of its low boiling point (bp -196 ÉC), inexpensive price, and low toxicity. In comparison to liquid air, which was previously used as a popular coolant, liquid nitrogen has the advantage that it does not support combustion.
Handling Liquid Nitrogen: Personal Protection
Cryogenic liquids such as liquid nitrogen can cause very severe burns upon eye or skin contact. Splashes are common when handling liquid nitrogen, and safety goggles must therefore be worn at all times when working with this material. In addition, protective gloves that can easily be removed in the event of a spill should be worn when handling liquid nitrogen (alternatively, potholders may sometimes be more convenient for handling small containers of cryogenic materials). Particular care must be taken to prevent uninsulated vessels containing liquid nitrogen from coming into contact with unprotected parts of the body, since extremely cold materials can become firmly bonded to the skin such that separation is not possible without serious injury.
Contact of the skin with liquid nitrogen can cause severe cryogenic burns; the tissue damage that results is similar to that caused by frostbite or thermal burns. Since small amounts of liquid nitrogen quickly evaporate from the surface of exposed skin, some inexperienced workers may mistakenly underestimate the risk of cryogenic burns when working with this material. In fact, it is not unusual for spills and splashes of liquid nitrogen to become trapped under rings, bracelets, watchbands, or inside gloves, and this can result in serious and painful burns.
Containers for Liquid Nitrogen
The properties of some materials (including metals) change drastically when exposed to cryogenic liquids such as liquid nitrogen. Containers for such liquids must therefore be selected carefully to ensure that they can withstand the temperatures and pressures they may be exposed to. Liquid nitrogen is commonly stored in Dewar flasks which should be taped to minimize the hazard in the event of an implosion.
Cold Traps Cooled with Liquid Nitrogen
A common use of liquid nitrogen is as a coolant for traps incorporated in vacuum lines. Extreme care must be employed when using liquid nitrogen as a cold trap coolant. Systems including liquid nitrogen traps must never be opened to the atmosphere until the trap is removed from the coolant. Oxygen has a higher boiling point (-183 ÉC) than nitrogen (-196 ÉC), and will condense out of the atmosphere and collect in a liquid-nitrogen cooled vessel open to the air. Liquid oxygen forms highly explosive mixtures with many organic materials. If you suspect liquid oxygen has condensed in a cold trap, then shield the trap (with an explosion shield, closed hood window, etc.), post a sign indicating the danger, and allow the trap (vented to the atmosphere) to slowly warm to room temperature.
Liquid Nitrogen and Condensed Argon
Argon, a gas commonly employed as an "inert atmosphere" for chemical reactions, distillations, and other laboratory operations, also has a boiling point (-186 ÉC) which is higher than that of nitrogen. Consequently, liquid argon will condense in a reaction vessel under an argon atmosphere which is cooled with liquid nitrogen. This creates an extremely hazardous situation, since if the vessel is then removed from the coolant, the liquid argon will instantly vaporize, expanding in volume by a factor of 847! Even if the vessel is vented (e.g. to an inert gas line), an explosion is very likely due to the rapid increase in pressure in the vessel. Consequently, never cool an apparatus that is under an argon atmosphere using liquid nitrogen.
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