Cooling bath

A typical experimental setup for an aldol reaction. Both flasks are submerged in a dry ice/acetone cooling bath (−78 °C) the temperature of which is being monitored by a thermocouple (the wire on the left).

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)[1]
% Glycol in EtOH Temp (°C) % H2O 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,[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]

Traditional cooling baths

Traditional cooling bath mixtures[4]
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 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

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.[citation needed]

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[citation needed] 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 (see § Further reading below).

See also

References

  1. ^ a b c Lee, Do W.; Jensen, Craig M. (2000). "Dry-Ice Bath Based on Ethylene Glycol Mixtures". J. Chem. Educ. 77 (5): 629. Bibcode:2000JChEd..77..629J. doi:10.1021/ed077p629.
  2. ^ Methanol/Water mixtures make great cooling baths. Chemtips.wordpress.com. Retrieved on 2015-02-23.
  3. ^ The ridiculously thorough guide to making a MeOH/Water bath. Chemtips.wordpress.com. Retrieved on 2015-02-23.
  4. ^ Cooling baths – ChemWiki. Chemwiki.ucdavis.edu. Retrieved on 2013-06-17.

Further reading