A cooling bath or ice bath, in laboratory chemistry practice, is a liquid mixture which is used to maintain low temperatures, typically between 13 ðC and âÂÂ196 ðC. 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) â 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.
Mixing solvents creates cooling baths with variable freezing points. Temperatures between approximately âÂÂ78 ðC and âÂÂ17 ðC can be maintained by placing coolant into a mixture of ethylene glycol and ethanol, while mixtures of methanol and water span the âÂÂ128 ðC to 0 ðC temperature range. Dry ice sublimes at âÂÂ78 ðC, 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 âÂÂ78 ðC (âÂÂ98 ðC and âÂÂ114 ðC 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.
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A bath of ice and water will maintain a temperature 0 ðC, since the melting point of water is 0 ðC. 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 âÂÂ78 ðC, a mixture such as acetone/dry ice will maintain âÂÂ78 ðC. Also, the solution will not freeze because acetone requires a temperature of about âÂÂ93 ðC 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).