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Unformatted text preview: t and ice may be
used. For lower temperatures, dry ice or dry ice with an organic liquid may be used.
Still lower temperatures require cryogenic liquids.
Dry Ice Cooling Baths and Cold Traps
Follow these precautions when using dry ice. It is often but incorrectly said that dry
ice can “burn” the skin. Correctly stated: Unless precautions are taken, dry ice will
damage fingers or hands, for example, by freezing them. Do not handle dry ice with
bare hands; if your skin is even slightly moist, severe freezing can result. Use tongs or
a folded cloth pad, or wear leather or cryo-gloves. Never put dry ice in your mouth.
Wear goggles while chipping dry ice. Dry ice sublimes, forming gaseous carbon dioxide, an asphyxiating gas. Be sure that you are protected by an adequate ventilation
system when using or handling dry ice. Do not lower your head into a dry ice chest;
no oxygen is present, and suffocation can result.
Sometimes it may be desirable to use a dry ice–organic liquid cooling bath.
Additional precautions are then necessary. For example, to be used safely with dry ice,
an organic liquid ideally should have the following five characteristics:
1. Nontoxic vapors
2. Low viscosity
4. Low volatility
5. Suitable freezing point
The final choice of a liquid will also depend on the temperature requirements. No
liquid meets all of the above criteria. The following are examples of liquids that
should never be used (numbers in parentheses signify which of the above criteria are
not met); they are too flammable and volatile:
30 students short index ●
● 1/15/03 12:45 PM Page 31 Ethyl ether (3 and 4)
Acetone (3 and 4)
Butanone (3 and 4) Of course, other liquids that are both quite flammable and volatile (3 and 4) are
unsuitable for use as cooling bath liquids. One of these would be a better choice:
● 60% Ethylene glycol, 40% water (2)
● 60% Propylene glycol, 40% water (2)
● Isopropyl alcohol (3)
● Ethanol (1 and 3)
After selecting a liquid, add a few small pieces of...
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This document was uploaded on 02/27/2014 for the course PHYS 1B at UCSD.
- Spring '07