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.

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]

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]}

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.

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).

• List of cooling baths
• Pumpable ice technology

• 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.

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