Freezing mixture

A cooling 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 water ice); (2) a liquid 'carrier' (such as liquid water, ethylene glycol, acetone, etc.), which transfers heat between the bath and the vessel; ; and (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 −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.

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