Cooling bath

  A cooling bath is a mixture used in a laboratory when low temperatures are needed, for example to conduct low-temperature chemical reactions (such as when kinetic control of the reaction is desired) to collect highly volatile liquids from distillation, or in cold traps. It usually consists of a solid that melts or sublimes at a low temperature, or a liquid that boils at a low temperature, mixed with some other substance that can help modulate the temperature of the bath or improve heat conduction.

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Types

The simplest and cheapest cooling bath is an ice/water mixture, which maintains a temperature of 0 °C. For lower temperatures, there are three main types of cooling baths:

Ice

A slurry of ice, and an inorganic salt such as sodium chloride or calcium chloride. Can reach temperatures down to about −40 °C. The temperature depends on the amount and type of salt used, based on the freezing point depression effect.

Dry ice

A slurry of dry ice and an organic solvent. Can reach temperatures down to about −78 °C.

Liquid nitrogen

Liquid nitrogen, either used as-is, or with an organic solvent. Can reach temperatures down to about the boiling point of pure N₂, −196 °C. The temperature depends on the freezing point of the organic solvent.

Pros and cons

Several factors influence the choice of the cooling bath composition. The first is availability — both dry ice and liquid nitrogen are comparatively inexpensive, but dry ice is more easily stored. Locations which do not have existing facilities for supplying and handling liquid nitrogen may prefer dry ice.

The toxicity and flammability of the composition also matters. For example, many of the solvents used with dry ice are flammable. That said, at the working temperatures, evaporation of most of these solvents are sufficiently low not to be hazardous.^[1] In most cases, a relatively non-toxic solvent such as acetone or diethyl ether is preferred, than something more toxic like bromomethane. When the cooling bath is no longer needed, the waste solvents also need to be stored and disposed off.

Liquid nitrogen is usually considered the best cooling bath refrigerant because it is non-toxic, is cheap, and leaves no residue. However, it suffers from the drawback of being cold enough to condense liquid oxygen from the air. This is important when the cooling bath is used for a cold trap, where solvents and oxygen may accumulate to give a potentially explosive mixture.^[1]

References

1. ^ ^{a b} Alvin B. Kaufman and Edwin N. Kaufman. Cold Traps. Ohio State University.

• Jonathan M. Percy, Christopher J. Moody, Laurence M. Harwood (1998). Experimental Organic Chemistry: standard and microscale. Blackwell Publishing. ISBN 978-0632048199. 
• Wilfred Louis Florio Armarego, Christina Li Lin Chai (2003). Purification of Laboratory Chemicals, 5th Edition, Butterworth-Heinemann. ISBN 978-0750675710.