Frigorific mixture

A frigorific mixture is a mixture of two or more phases in a chemical system that, so long as none of the phases is consumed during equilibration, reaches an equilibrium temperature that is independent of the starting temperature of the phases before they are mixed. The equilibrium temperature is also independent of the quantities of the phases used as long as sufficient amounts of each are present to reach equilibrium without consuming one or more.

Ice

Liquid water and ice, for example, form a frigorific mixture at 0 °C or 32 °F. This mixture was historically used to define 0 °C before 1954. (0 °C is now defined as the triple point of Vienna Standard Mean Ocean Water (VSMOW), a specially purified water.) A mixture of ammonium chloride, water, and ice form a frigorific mixture at about −17.8 °C or 0 °F. This mixture was historically used to define 0 °F.

Explanation

The existence of frigorific mixtures can be viewed as a consequence of the Gibbs phase rule, which describes the relationship at equilibrium between the number of components, the number of coexisting phases, and the number of degrees of freedom permitted by the conditions of heterogeneous equilibrium. Specifically, at constant atmospheric pressure, in a system containing C linearly independent chemical components, if C+1 phases are specified to be present in equilibrium, then the system is fully determined (there are no degrees of freedom). That is, the temperature and the compositions of all phases are determined. Thus, in for example the chemical system H₂O-NaCl, which has two components, the simultaneous presence of the three phases liquid, ice, and hydrohalite can exist only at atmospheric pressure at the unique temperature of –21.2 °C. The approach to equilibrium of a frigorific mixture involves spontaneous temperature change driven by the conversion of latent heat into sensible heat as the phase proportions adjust to accommodate the decrease in thermodynamic potential associated with the approach to equilibrium.

Other examples

Other examples of frigorific mixtures include:^[1]

Uses

The most common use of a frigorific mixture is to melt ice. When ammonium chloride salt is placed on ice when the ambient temperature is greater than −17.8 °C (0 °F), the salt melts some of the ice and the temperature drops to −17.8 °C. Since the mixture is colder than the ambient temperature, heat is absorbed and the temperature rises. This causes the salt to melt more of the ice to drive the temperature down again. The process continues until all of the salt is dissolved in the melted ice. If there is enough salt present, then all of the ice will be melted.

Frigorific mixtures are commonly used in laboratories as a convenient way to generate reference temperatures for calibrating thermometers.

They are also useful for creating low temperatures when mechanical refrigeration is not available, for example to tightly fit two parts of machined metal: one part is soaked in a frigorific mixture, causing it to cool and contract, and then placed into the uncooled second part. As the first part warms, it expands to fit tightly within the second part.

Limitations of acid base slushes

Mixtures relying on the use of acid base slushes are of limited practical value beyond producing melting point references as the enthalpy of dissolution for the melting point depressant is often significantly greater (e.g. ΔH -57.61 kJ/mol for KOH) than the enthalpy of fusion for water itself (ΔH 6.02 kJ/mol); for reference, ΔH for the dissolution of NaCl is 3.88 kJ/mol. ^[3] This results in little to no net cooling capacity at the desired temperatures and an end mixture temperature that is higher than it was to begin with. The values claimed in the table are produced by first precooling and then combining each subsequent mixture with it surrounded by a mixture of the previous temperature increment; the mixtures must be 'stacked' within one another. ^[4][5][6]

Such acid base slushes are corrosive and therefore present handling problems. Additionally, they can not be replenished easily, as the volume of the mixture increases with each addition of refrigerant; the container (be it a bath or cold finger) will eventually need emptying and refilling to prevent it from overflowing. This makes these mixtures largely unsuitable for use in synthetic applications, as there will be no cooling surface present during the emptying of the container.

See also

• Cooling bath

References