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.
| % Glycol in EtOH | Temp (°C) | % H2O in MeOH | Temp (°C) |
|---|---|---|---|
| 0% | −78 | 0% | −97.6 |
| 10% | −76 | 14% | −128 |
| 20% | −72 | 20% | N/A |
| 30% | −66 | 30% | −72 |
| 40% | −60 | 40% | −64 |
| 50% | −52 | 50% | −47 |
| 60% | −41 | 60% | −36 |
| 70% | −32 | 70% | −20 |
| 80% | −28 | 80% | −12.5 |
| 90% | −21 | 90% | −5.5 |
| 100% | −17 | 100% | 0 |
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
| Cooling agent | Organic solvent or salt | Temp (°C) |
|---|---|---|
| Dry ice | p-xylene | +13 |
| Dry ice | Dioxane | +12 |
| Dry ice | Cyclohexane | +6 |
| Dry ice | Benzene | +5 |
| Dry ice | Formamide | +2 |
| Ice | Salts (see: left) | 0 to −40 |
| Liquid N2 | Cycloheptane | −12 |
| Dry ice | Benzyl alcohol | −15 |
| Dry ice | Tetrachloroethylene | −22 |
| Dry ice | Carbon tetrachloride | −23 |
| Dry ice | 1,3-Dichlorobenzene | −25 |
| Dry ice | o-Xylene | −29 |
| Dry ice | m-Toluidine | −32 |
| Dry ice | Acetonitrile | −41 |
| Dry ice | Pyridine | −42 |
| Dry ice | m-Xylene | −47 |
| Dry ice | n-Octane | −56 |
| Dry ice | Isopropyl ether | −60 |
| Dry ice | Acetone | −78 |
| Liquid N2 | Ethyl acetate | −84 |
| Liquid N2 | n-Butanol | −89 |
| Liquid N2 | Hexane | −94 |
| Liquid N2 | Acetone | −94 |
| Liquid N2 | Toluene | −95 |
| Liquid N2 | Methanol | −98 |
| Liquid N2 | Cyclohexene | −104 |
| Liquid N2 | Ethanol | −116 |
| Liquid N2 | n-Pentane | −131 |
| Liquid N2 | Isopentane | −160 |
| Liquid N2 | (none) | −196 |
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 § Further reading below).
See also
References
- ^ Lee, Do W.; Jensen, Craig M. (2000). "Dry-Ice Bath Based on Ethylene Glycol Mixtures". J. Chem. Educ. 77 (5): 629. Bibcode:2000JChEd..77..629J. doi:10.1021/ed077p629.
- Methanol/Water mixtures make great cooling baths. Chemtips.wordpress.com. Retrieved on 2015-02-23.
- The ridiculously thorough guide to making a MeOH/Water bath. Chemtips.wordpress.com. Retrieved on 2015-02-23.
- Cooling baths – ChemWiki. Chemwiki.ucdavis.edu. Retrieved on 2013-06-17.
Further reading
- 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
- Carter Research Group. "Cooling Baths". Oregon State University.
- A. J. Meixner; et al. "10.5.2 Different Freezing Mixtures". University of Siegen.