Frigorific mixture
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A frigorific mixture is a mixture of two or more phases in a chemical system that, so long as none of the phases are completely consumed during equilibration, reaches an equilibrium
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
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 once used to define 0 °C. That temperature is now defined as the
triple point In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the sub ...
of Water with well-defined isotope ratios. A mixture of ammonium chloride, water, and ice form a frigorific mixture at about −17.8 °C or 0 °F. This mixture was once used to define 0 °F.


Explanation

The existence of frigorific mixtures can be viewed as a consequence of the Gibbs
phase rule In thermodynamics, the phase rule is a general principle governing "pVT" systems, whose thermodynamic states are completely described by the variables pressure (), volume () and temperature (), in thermodynamic equilibrium. If is the number of d ...
, 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 linearly independent chemical components, if +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 Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be underst ...
into
sensible heat Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic vari ...
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: {, class="wikitable" ! Materials !! Parts (w/w) !! Equilibrium temperature , - , Ammonium chloride (NH4Cl) , 5 , rowspan="3" , −12 °C / 10 °F / 261 K , - ,
Potassium nitrate Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and nitr ...
(KNO3) , 5 , - ,
Water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
, 16 , - , Ammonium chloride (NH4Cl) , 5 , rowspan="2" , −15.5 °C / 4 °F / 257.5 K , - , Water , 16 , - , Ammonium nitrate (NH4NO3) , 1 , rowspan="2" , −15.5 °C / 4 °F / 257.5 K , - , Water , 1 , - , Sodium sulfate (Na2SO4) , 3 , rowspan="2" , −16 °C / 3 °F / 257 K , - , Dilute
Nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
(HNO3) , 2 , - , Sodium sulfate (Na2SO4) , 8 , rowspan="2" , −18 °C / 0 °F / 255 K , - ,
Hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...
(HCl) , 5 , - , Snow/ice , 1 , rowspan="2" , −18 °C / 0 °F / 255 K , - , Common salt (NaCl) , 1 , - , Snow/ice , 1 , rowspan="2" , −26 °C / −15 °F / 247 K , - ,
Potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...
, Crystallized (KOH) , 1 , - , Snow/ice , 1 , rowspan="2" , −51 °C / −60 °F / 222 K , - , Sulphuric acid, dilute (H2SO4) , 1 , - , Snow/ice , 2 , rowspan="2" , −55 °C / −67 °F / 218 K , - ,
Calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...
(CaCl2) , 3 , - , Sulphuric acid, dilute (H2SO4) , 10 , rowspan="2" , −68 °C / −90 °F / 205 K , - , Snow/ice , 8


Uses

A frigorific mixture may be used to obtain a liquid medium that has a reproducible temperature below ambient temperature. Such mixtures were used to calibrate
thermometer A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermometer ...
s. In chemistry a cooling bath may be used to control the temperature of a strongly exothermic reaction. A frigorific mixture may be used as an alternative to mechanical
refrigeration The term refrigeration refers to the process of removing heat from an enclosed space or substance for the purpose of lowering the temperature.International Dictionary of Refrigeration, http://dictionary.iifiir.org/search.phpASHRAE Terminology, ht ...
. For example to fit two machined metal parts together, one part is placed in a frigorific mixture, causing it to contract so that may be easily inserted into the uncooled second part; on warming the two parts are held together tightly.


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. 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. Walker, R. and Wall, M. (1795). Observations on the Best Methods of Producing Artificial Cold. By Mr. Richard Walker. Communicated by Martin Wall, M. D. F. R. S. Philosophical Transactions of the Royal Society of London, 85(0), pp.270-289. 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 A cold finger is a piece of laboratory equipment that is used to generate a localized cold surface. It is named for its resemblance to a finger and is a type of cold trap. The device usually consists of a chamber that a coolant fluid (cold tap ...
) 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

Thermodynamics Physical chemistry Chemical thermodynamics