Cooling Baths and Cold Traps


When ice water is not cool enough for use as a bath, salt and ice may be used. For even lower temperatures, dry ice may be used with an organic liquid. An ideal cooling liquid for use with dry ice should have the following characteristics:

Nontoxic vapors
Low viscosity
Nonflammability
Low volatility

Ether, acetone, and butanone are too volatile and flammable. The final choice of a liquid will also depend on the temperature requirements. Although no substance meets all of the above criteria, the following are suggested.

Ethylene glycol or propylene glycol in a 3:2 ratio with water and thinned with isopropyl alcohol
Isopropyl alcohol
Some glycol ethers

Add the dry ice to the liquid, or the liquid to the dry ice in small increments. Wait for the foaming to stop before proceeding with further addition. The rate of addition can be increased gradually as the liquid cools.

Cryogenic coolants should always be used with caution. Cryogenic liquids must be handled in properly vented containers. Be aware that very low temperature coolants, such as liquid nitrogen, may condense oxygen and cause an explosion with combustible materials. Use gloves and face shields, slowly immerse the object to be cooled to avoid too vigorous boiling and overflow of the coolant. Glass Dewar flasks should be of borosilcate glass and protected by covering with cloth-backed friction or duct tape or encased in metal sheath to contain flying pieces in the event of implosion.

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 mild air pressure or a siphon. Metal or plastic Dewar type flasks are preferable and eliminate this problem. Never use a household Thermos bottle in place of a Dewar flask

Do not lower your head into a dry ice chest. Because no oxygen is present, suffocation can result. Do not handle the dry ice with bare hands; if the skin is even slightly moist, severe burns can result. Use dry leather or suitable cryo-gloves. When chipping dry ice, wear goggles.


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