Cooling bath. A cooling bath or ice bath, in laboratory chemistry practice, is a liquid mixture which is used to maintain low temperatures, typically between 13 °C and −196 °C.  These low temperatures are used to collect liquids after distillation, to remove solvents using a rotary evaporator, or to perform a chemical reaction below room temperature (see Kinetic control).

Cooling baths are generally one of two types: (a) a cold fluid (particularly liquid nitrogen, water, or even air) — but most commonly the term refers to (b) a mixture of 3 components: (1) a cooling agent (such as dry ice or ice); (2) a liquid "carrier" (such as liquid water, ethylene glycol, acetone, etc.), which transfers heat between the bath and the vessel; (3) an additive to depress the melting point of the solid/liquid system.

A familiar example of this is the use of an ice/rock-salt mixture to freeze ice cream. Adding salt lowers the freezing temperature of water, lowering the minimum temperature attainable with only ice.

Mixed-solvent cooling baths 
Mixing solvents creates cooling baths with variable freezing points.  Temperatures between approximately −78 °C and −17 °C can be maintained by placing coolant into a mixture of ethylene glycol and ethanol, while mixtures of methanol and water span the −128 °C to 0 °C temperature range.  Dry ice sublimes at −78 °C, while liquid nitrogen is used for colder baths.

As water or ethylene glycol freeze out of the mixture, the concentration of ethanol/methanol increases.  This leads to a new, lower freezing point.  With dry ice, these baths will never freeze solid, as pure methanol and ethanol both freeze below −78 °C (−98 °C and −114 °C respectively).

Relative to traditional cooling baths, solvent mixtures are adaptable for a wide temperature range.  In addition, the solvents necessary are cheaper and less toxic than those used in traditional baths.

Traditional cooling baths

Water and ice baths 
A bath of ice and water will maintain a temperature 0 °C, since the melting point of water is 0 °C.  However, adding a salt such as sodium chloride will lower the temperature through the property of freezing-point depression.  Although the exact temperature can be hard to control, the weight ratio of salt to ice influences the temperature:
 −10 °C can be achieved with a 1:2.5 mass ratio of calcium chloride hemihydrate to ice.
 −20 °C can be achieved with a 1:3 mass ratio of sodium chloride to ice.

Dry ice baths at −78 °C 
Since dry ice will sublime at −78 °C, a mixture such as acetone/dry ice will maintain −78 °C. Also, the solution will not freeze because acetone requires a temperature of about −93 °C to begin freezing.

Safety recommendations 
The American Chemical Society notes that the ideal organic solvents to use in a cooling bath have the following characteristics:
 Nontoxic vapors.
 Low viscosity.
 Nonflammability.
 Low volatility.
 Suitable freezing point.
In some cases, a simple substitution can give nearly identical results while lowering risks. For example, using dry ice in 2-propanol rather than acetone yields a nearly identical temperature but avoids the volatility of acetone (see  below).

See also 
 List of cooling baths
 Pumpable ice technology

References

Further reading

External links 
 
 

Laboratory techniques
Cryogenics
Cooling technology