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Unformatted text preview: dry ice one at a time, or add the
liquid in small increments to dry ice. Either way, wait for the foaming to stop before
proceeding with the next addition. Cryogenic Liquid Cooling Baths and Cold Traps
Exercise caution when using liquid cryogenic coolants. Follow the precautions
described in the MSDS for the cryogenic liquid you will be using. Be aware that very
low temperature coolants, such as liquid nitrogen, will condense oxygen from the air
on standing and can then cause an explosion if they come in contact with combustible materials. These precautions are essential:
● Use gloves and face shields. Immerse the object to be cooled slowly to avoid too
vigorous boiling and overflow of the coolant.
● Use only properly vented containers when handling cryogenic liquids.
● Glass Dewar flasks should be made of borosilicate glass and protected by covering
with cloth-backed friction or duct tape or by encasing in a metal sheath to contain
flying pieces in the event of implosion.
● The edge of a glass Dewar flask is fragile. Avoid pouring cold liquid onto the edge of
a glass Dewar flask when filling because the flask may break and implode. For the
same reason, do not pour a cryogenic liquid out of a glass Dewar flask; use a siphon.
Consider using a metal or plastic Dewar flask to eliminate this problem.
● Never use a household Thermos bottle or other insulated container in place of a
Dewar flask. Thermos bottles and other insulated containers are designed to keep
consumable liquids cold; they are not sturdy enough for laboratory use. Working with Reduced Pressure
Protect vacuum desiccators by covering with cloth-backed friction or duct tape or by
enclosing in a sturdy box or in an approved shielding device to restrict flying fragments in case of an implosion. Store only chemicals being dehydrated or protected
from moisture in a desiccator. Before opening a desiccator that is under reduced pressure, make sure that atmospheric pressure has been restored. Occasionally, a vacuum
desiccator lid will be found to be “frozen” after atmospheric pressure has been
restored. Try using a single-edge razor blade as a wedge; tap it gently with a wooden
block to loosen the lid.
Surround apparatus that is under reduced pressure with...
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This document was uploaded on 02/27/2014 for the course PHYS 1B at UCSD.
- Spring '07