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.
| 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 |
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]
| 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 |
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:
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 that the ideal organic solvents to use in a cooling bath have the following characteristics:
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 below).