cooling bath

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.[https://chemtips.wordpress.com/2015/02/09/methanolwater-mixtures-make-great-cooling-baths/ Methanol/Water mixtures make great cooling baths]. Chemtips.wordpress.com. Retrieved on 2015-02-23.[https://chemtips.wordpress.com/2015/02/23/the-ridiculously-thorough-guide-to-making-a-meohwater-bath/ The ridiculously thorough guide to making a MeOH/Water bath]. Chemtips.wordpress.com. Retrieved on 2015-02-23. 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.

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.{{Reference needed|date=March 2018}}

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{{cn|date=February 2020}} 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 {{section link|#Further reading}} below).

• List of cooling baths
• Pumpable ice technology

{{reflist}}

• {{cite book |author1=Jonathan M. Percy |author2=Christopher J. Moody |author3=Laurence M. Harwood |title=Experimental Organic Chemistry: standard and microscale |publisher=Blackwell Publishing |year=1998 |isbn=978-0-632-04819-9 |url-access=registration |url=https://archive.org/details/experimentalorga0002harw }}
• {{cite book |author1=Wilfred Louis Florio Armarego |author2=Christina Li Lin Chai | title = Purification of Laboratory Chemicals | publisher = Butterworth-Heinemann | year = 2003 | isbn = 978-0-7506-7571-0 | edition = 5th}}
• {{cite book | author = Kenneth P. Fivizzani | title = Safety in Academic Chemistry Lab, by American Chemical Society, Volume 1: Accident Prevention for College and University Students | publisher = American Chemical Society | year = 2003 | isbn = 9780841238633 | edition = 7th}}

• {{cite web | url = http://oregonstate.edu/dept/chemistry/carter/cooling-bath-table | title = Cooling Baths | author = Carter Research Group | publisher = Oregon State University}}
• {{cite web | title = 10.5.2 Different Freezing Mixtures | author = A. J. Meixner | display-authors=etal | url = http://www2.uni-siegen.de/~pci/versuche/english/v105-2.html | publisher = University of Siegen}}

Category:Laboratory techniques

Category:Cryogenics

Category:Cooling technology