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.
Cooling baths are generally one of two types: a cold fluid — but most commonly the term refers to a mixture of 3 components: a cooling agent ; a liquid "carrier", which transfers heat between the bath and the vessel; 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 EtOHTemp % H2O in MeOHTemp
0%−780%−97.6
10%−7614%−128
20%−7220%N/A
30%−6630%−72
40%−6040%−64
50%−5250%−47
60%−4160%−36
70%−3270%−20
80%−2880%−12.5
90%−2190%−5.5
100%−17100%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.
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 agentOrganic solvent or saltTemp
Dry icep-xylene+13
Dry iceDioxane+12
Dry iceCyclohexane+6
Dry iceBenzene+5
Dry iceFormamide+2
IceSalts 0 to −40
Liquid N2Cycloheptane−12
Dry iceBenzyl alcohol−15
Dry iceTetrachloroethylene−22
Dry iceCarbon tetrachloride−23
Dry ice1,3-Dichlorobenzene−25
Dry iceo-Xylene−29
Dry icem-Toluidine−32
Dry iceAcetonitrile−41
Dry icePyridine−42
Dry icem-Xylene−47
Dry icen-Octane−56
Dry iceIsopropyl ether−60
Dry iceAcetone−78
Liquid N2Ethyl acetate−84
Liquid N2n-Butanol−89
Liquid N2Hexane−94
Liquid N2Acetone−94
Liquid N2Toluene−95
Liquid N2Methanol−98
Liquid N2Cyclohexene−104
Liquid N2Ethanol−116
Liquid N2n-Pentane−131
Liquid N2Isopentane−160
Liquid N2−196

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:
  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.