A cooling bath, in laboratory chemistry (often but not always organic 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 water ice); (2) a liquid 'carrier' (such as liquid water, ethylene glycol, acetone, etc.), which transfers heat between the bath and the vessel; ; and (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.
|Cooling agent||Ethylene glycol||Ethanol||Temp (°C)|
Ethylene glycol and ethanol baths
Temperatures between approximately −78 °C and −17 °C can be maintained by placing dry ice into a mixture of ethylene glycol and ethanol. The bath's temperature can be set by varying the relative amounts of ethylene glycol and ethanol. Dry ice sublimes at −78 °C.
If a bath is made with only ethanol, then it will maintain −78 °C until all the dry ice has sublimed. The bath will not freeze because ethanol's freezing point is −114 °C.
In addition, a cooling bath can be made with both ethanol and ethylene glycol. Since ethylene glycol freezes at −12.9 °C, then the "freezing point" of this mixture will increase to above −78 °C. Instead of freezing solid, however, the solution becomes thicker and gel-like once the dry ice has performed enough cooling. If a 60/40 mixture of ethanol/ethylene glycol is used, then a thick gel will form around the dry ice pieces around −60 °C, helping to maintain the temperature at approximately −60 °C.
Relative to traditional cooling baths, ethylene glycol mixtures have the advantage of never freezing solid. 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)|
|Ice||Salts (see: above)||0 to −20|
|Dry ice||Benzyl alcohol||−15|
|Dry ice||Carbon tetrachloride||−23|
|Dry ice||Isopropyl ether||−60|
|Liquid N2||Ethyl acetate||−84|
Water and ice baths
A bath of ice and water will maintain a temperature 0 °C since the freezing 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 to 2.5 ratio by weight of calcium chloride hexahydrate to ice.
- −20 °C can be achieved with a 1 to 3 ratio by weight of sodium chloride to ice.
- −40 °C can be achieved with a 1 to 0.8 ratio by weight of calcium chloride hexahydrate 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. Therefore, other liquids with a lower freezing point (pentane: −95 °C) can also be used to maintain the bath at −78 °C.
Dry ice baths above −77 °C
In order to maintain temperatures above −77 °C, a solvent with a freezing point above −77 °C must be used. When dry ice is added to acetonitrile then the bath will begin cooling. Once the temperature reaches −41 °C, the acetonitrile will freeze. Therefore, dry ice must be added slowly to avoid freezing the entire mixture. In these cases, a bath temperature of −55 °C can be achieved by choosing a solvent with a similar freezing point (n-octane freezes at −56 °C).
Liquid nitrogen baths above −196 °C
Liquid nitrogen baths follow the same idea as dry ice baths. A temperature of −115 °C can be maintained by slowly adding liquid nitrogen to the organic solvent (ethanol) until it begins to freeze (ethanol freezes at −116 °C).
In water and ice-based baths, tap water is commonly used due to ease of access and the higher costs of using ultrapure water. However, tap water and ice derived from tap water can be a contaminant to biological and chemical samples. This has created a host of insulated devices aimed at creating a similar cooling or freezing effect as ice baths without the use of water or ice.
The American Chemical Society notes that the ideal organic solvents to use in a cooling baths 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).
- 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.