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Cooling bath

Liquid mixture used to maintain low temperatures

Acooling bathorice 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 iceorice); (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[edit]

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,^[1] while mixtures of methanol and water span the −128 °C to 0 °C temperature range.^[2][3] 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.^[1]

Traditional cooling baths[edit]

Water and ice baths[edit]

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.^{[citation needed]}

Dry ice baths at −78 °C[edit]

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[edit]

The American Chemical Society notes^{[citation needed]} that the ideal organic solvents to use in a cooling bath have the following characteristics:

1. Nontoxic vapors.
2. Low viscosity.
3. Nonflammability.
4. Low volatility.
5. 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 § Further reading below).

See also[edit]

• List of cooling baths
• Pumpable ice technology

References[edit]

Further reading[edit]

• Jonathan M. Percy; Christopher J. Moody; Laurence M. Harwood (1998). Experimental Organic Chemistry: standard and microscale. Blackwell Publishing. ISBN 978-0-632-04819-9.
• Wilfred Louis Florio Armarego; Christina Li Lin Chai (2003). Purification of Laboratory Chemicals (5th ed.). Butterworth-Heinemann. ISBN 978-0-7506-7571-0.
• Kenneth P. Fivizzani (2003). Safety in Academic Chemistry Lab, by American Chemical Society, Volume 1: Accident Prevention for College and University Students (7th ed.). American Chemical Society. ISBN 9780841238633.

External links[edit]

• Carter Research Group. "Cooling Baths". Oregon State University.
• A. J. Meixner; et al. "10.5.2 Different Freezing Mixtures". University of Siegen.