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SECTION
7: Safe Lab Practices and Procedures
7F: CRYOGENICS
Cryogenic liquids have boiling points less than -73ºC (-100ºF).
Liquid nitrogen, liquid oxygen and carbon dioxide are the most common
cryogenic materials used in the laboratory. Hazards may include fire,
explosion, embrittlement, pressure buildup, frostbite and asphyxiation.
Many of the safety precautions observed for compressed gases also apply
to cryogenic liquids. Two additional hazards are created from the unique
properties of cryogenic liquids:
- Extremely Low Temperatures
–The cold boil-off vapor of cryogenic liquids rapidly
freezes human tissue. Most metals become stronger upon exposure to cold
temperatures, but materials such as carbon steel, plastics and rubber
become brittle or even fracture under stress at these temperatures.
Proper material selection is important. Cold burns and frostbite caused
by cryogenic liquids can result in extensive tissue damage.
- Vaporization - All cryogenic liquids
produce large volumes of gas when they vaporize. Liquid nitrogen will
expand 696 times as it vaporizes. The expansion ratio of argon is 847:1,
hydrogen is 851:1 and oxygen is 862:1. If these liquids vaporize in
a sealed container, they can produce enormous pressures that could rupture
the vessel. (See Anecdotes for an account
of such an incident.) For this reason, pressurized cryogenic containers
are usually protected with multiple pressure relief devices.
Vaporization of cryogenic liquids (except oxygen) in an enclosed area
can cause asphyxiation. Vaporization of liquid oxygen can produce an oxygen-rich
atmosphere, which will support and accelerate the combustion of other materials.
Vaporization of liquid hydrogen can form an extremely flammable mixture
with air.
Most cryogenic liquids are odorless, colorless, and tasteless when
vaporized. When cryogenic liquids are exposed to the atmosphere, the
cold boil-off gases condense the moisture in the air, creating a highly
visible fog.
- Always handle these liquids carefully to avoid skin burns and frostbite.
Exposure that may be too brief to affect the skin of the face or hands
may damage delicate tissues, such as the eyes.
- Boiling and splashing always occur when charging or filling a warm
container with cryogenic liquid or when inserting objects into these
liquids. Perform these tasks slowly to minimize boiling and splashing.
Use tongs to withdraw objects immersed in a cryogenic liquid.
- Never touch uninsulated pipes or vessels containing cryogenic liquids.
Flesh will stick to extremely cold materials. Even nonmetallic materials
are dangerous to touch at low temperatures.
- Cylinders and dewars should not be
filled to more than 80% of capacity, since expansion of gases during
warming may cause excessive pressure buildup.
- Check cold baths frequently to ensure they are not plugged with
frozen material.
Face shields worn with safety
glasses or chemical splash goggles are recommended during transfer
and handling of cryogenic liquids.
Wear loose fitting, dry insulated or leather gloves when handling objects
that come into contact with cryogenic liquids and vapor. Trousers should
be worn on the outside of boots or work shoes.
- Neither liquid nitrogen nor liquid air should be used to cool a flammable
mixture in the presence of air, because oxygen can condense from the
air, leading to an explosion hazard.
- Wear insulated, dry gloves and a face shield when handling dry ice.
- Add dry ice slowly to the liquid portion of the cooling bath to avoid
foaming over. Do not lower your head into a dry ice chest, since suffocation
can result from carbon dioxide buildup.
Traps that open to the atmosphere condense liquid air rapidly. If you
close the system, pressure builds up with enough force to shatter glass
equipment. Therefore, only sealed or evacuated equipment should use
liquid nitrogen cooled traps.
 Section
7G: Electrical Safety
 Section
7E: Compressed Gases
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