Cooling bath
In organic chemistry, a cooling bath 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 organic liquids after distillation, to remove solvents using a rotary evaporator, or to perform a chemical reaction below room temperature (see: kinetic control). The cooling agents used in these baths include dry ice, liquid nitrogen, and water ice.
Contents |
[edit] Method of maintaining temperature
| Cooling agent | Organic solvent or salt | T (°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: above) | 0 to -20 |
| Dry ice | 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 | Cyclohexane | -104 |
| Liquid N2 | Ethanol | -116 |
| Liquid N2 | n-Pentane | -131 |
| Liquid N2 | Isopentane | -160 |
| Liquid N2 | (none) | -196 |
[edit] 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 ratio of salt to ice influences the temperature:
- -10°C can be achieved with a 1 to 2.5 ratio of calcium chloride hexahydrate to ice.
- -20°C can be achieved with a 1 to 3 ratio of sodium chloride to ice.
- -40°C can be achieved with a 1 to 0.8 ratio of calcium chloride hexahydrate to ice.
[edit] 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.
[edit] 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).
[edit] 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).
[edit] Types
The simplest and cheapest cooling bath is an ice/water mixture, which maintains a temperature of 0 °C. For lower temperatures, three main types of cooling baths are typical:
[edit] Ice
A slurry of ice and an inorganic salt such as sodium chloride or calcium chloride can provide temperatures down to about −40 °C. The temperature depends on the amount and type of salt used, based on the freezing point depression effect.
[edit] Dry ice
A slurry of dry ice and a suitable organic solvent, such as acetone, can provide temperatures down to about −100 °C (with diethyl ether). Temperatures in the range of -12 °C to -78 °C can be conveniently generated with ethylene glycol / ethanol / dry ice mixtures.[2]
[edit] See also
[edit] References
- ^ http://chemwiki.ucdavis.edu/VV_Lab_Techniques/Cooling_baths
- ^ Lee, Do W.; Jensen, Craig M. (2000). "Dry-Ice Bath Based on Ethylene Glycol Mixtures". J. Chem. Ed. 77: 629. doi:10.1021/ed077p629. http://jchemed.chem.wisc.edu/Journal/issues/2000/May/abs629.html
- Jonathan M. Percy, Christopher J. Moody, Laurence M. Harwood (1998). Experimental Organic Chemistry: standard and microscale. Blackwell Publishing. ISBN 978-0632048199.
- Wilfred Louis Florio Armarego, Christina Li Lin Chai (2003). Purification of Laboratory Chemicals (5th ed.). Butterworth-Heinemann. ISBN 978-0750675710.
[edit] External links
- Carter Research Group. "Cooling Baths". Oregon State University. http://www.chemistry.oregonstate.edu/carter/Baths.html.
- A.J. Meixner, et al.. "10.5.2 Different Freezing Mixtures". University of Siegen. http://www2.uni-siegen.de/~pci/versuche/english/v105-2.html.
