# Cooling bath

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.
Mixed solvent cooling baths (% by volume) :
| % Glycol in EtOH | Temp (°C) | % H$_2$O 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

Traditional cooling bath mixtures :
| 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 N$_2$ | 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 N$_2$ | Ethyl acetate | −84 |
| Liquid N$_2$ | n-Butanol | −89 |
| Liquid N$_2$ | Hexane | −94 |
| Liquid N$_2$ | Acetone | −94 |
| Liquid N$_2$ | Toluene | −95 |
| Liquid N$_2$ | Methanol | −98 |
| Liquid N$_2$ | Cyclohexene | −104 |
| Liquid N$_2$ | Ethanol | −116 |
| Liquid N$_2$ | n-Pentane | −131 |
| Liquid N$_2$ | Isopentane | −160 |
| Liquid N$_2$ | (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).