Cold mixture
As freezing mixture are mixtures designated substances that - for a certain time after mixing - for removing heat can be used.
The effect of the cold mixture uses three effects:
- The process of dissolving substance A in substance B can be endothermic .
- By adding an additive , the melting point of a substance can be lowered significantly. The physical causes of this effect are explained in the article lowering the melting point .
- The cooling substance is subject to a phase transition , the achievable temperature is then the phase transition temperature (see below: dry ice, liquid nitrogen).
In contrast to refrigerants , most refrigerant mixtures can be stored separately without a pressure vessel . For this reason, refrigeration mixtures are often used when low temperatures are to be generated by simple means and kept briefly without a refrigeration machine (e.g. in the laboratory).
When refrigerants and processes for the mechanical production of cold were still unknown, i.e. before 1860, cold mixtures were used to produce cold artificially.
Types of refrigeration mixtures
| composition | Min. Temperature (° C) |
|---|---|
| 100 g water + 100 g ice | 0 |
| 100 g water + 75 g NaNO 3 | −5.3 |
| 100 g water + 140 g KI | −12 |
| 100 g water + 133 g NH 4 SCN | −18 |
| 100 g ice + 33 g NaCl | −21 |
| 100 g ice + 143 g CaCl 2 · 6 H 2 O | −50 |
| Ethanol + CO 2 (solid) | −72 |
| Diethylether + CO 2 (solid) | −77 |
| Acetone + N 2 (liquid) | −94 |
With salts
Cold mixtures of salts with water use the endothermic heat effects when substances are dissolved for cooling . The change in temperature during dissolution results from the sum of the energy required to dissolve the ionic lattice of the salts and the hydration energy that is released during dissolution: If the lattice energy is greater than the hydration energy, the mixture withdraws the total missing energy from the environment , the solution cools down. Temperatures down to −18 ° C can be reached with cold mixtures of salts in water.
Lower temperatures (down to −50 ° C) are reached with cold mixtures of water ice with salts. Mixing with ice adds to the endothermic dissolving of the salt in the melt water, the cooling due to the previous melting of the ice .
See also brine (refrigeration technology) .
With dry ice
Dry ice (i.e. frozen CO 2 ) sublimes and removes the necessary heat of sublimation from the solution, so that the cold mixture cools down. Cold mixtures with dry ice can reach temperatures down to −78 ° C, the sublimation temperature of dry ice: in ethanol down to −72 ° C, in diethyl ether down to −77 ° C and in chloroform down to −77 ° C.
With liquid nitrogen
Freezing mixtures with liquid nitrogen (N 2 ) can theoretically reach temperatures of −196 ° C, the boiling point of liquid nitrogen. The practically usable temperature depends on the melting point of the solvent used : as soon as part of the solvent freezes, a constant temperature is reached. The temperature can be kept constant by topping up with liquid nitrogen.
Historical
Ole Rømer (1644–1710) used the freezing point of a brine as a fixed point on the temperature scale he developed in 1701 . Daniel Fahrenheit then used the lowest temperature that he could produce with a mixture of ice, water and ammonium chloride (= ammonium chloride) or sea salt as the zero point of his scale proposed in 1708 : −17.8 ° C. The discovery of the freezing point depression by Charles Blagden (1748-1820) was known that these increased linearly with the salt concentration. In the second half of the 18th century, cold mixtures were made from snow and acids, such as dilute nitric acid . The pharmacist Richard Walker reported in 1788 on various cold mixtures. It was also able to solidify mercury (melting point −38.83 ° C) in summer . Tobias Lowitz (1757–1804) achieved temperatures of down to −50 ° C with the help of mixtures of calcium chloride containing water of crystallization (“salty lime”) and snow. In 1796 he reported on his experiments in the winter of 1792/1793: with one of his cold mixtures he was able to solidify twelve pounds of mercury in a heated room, with calcium chloride and snow 35 pounds of mercury. In 1819 an extensive list of refrigeration mixtures was known.
See also
literature
- Heinz GO Becker, Werner Berger and Günter Domschke: Organikum. 22nd edition, Wiley-VCH, Weinheim 2004, ISBN 978-3-527-31148-4 . P. 16
Web links
Individual evidence
- ↑ Ferdinand Fischer: About the production of ice. In: Polytechnisches Journal . 224, 1877, pp. 165-174.
- ↑ a b c Entry on cold mixtures. In: Römpp Online . Georg Thieme Verlag, accessed on January 15, 2015.
- ↑ a b c Abraham Rees: Freezing . In: The Cyclopaedia; Or, Universal Dictionary of Arts, Sciences and Literature . tape 15 . Longman, Hurst, Rees, Orme & Brown, London 1819, Freezing Mixtures (available online at biodiversitylibrary.org [accessed June 15, 2018]).
- ^ Richard Walker: Experiments on the Production of Artificial Cold . By Mr. Richard Walker, Apothecary to the Radcliffe Infirmary at Oxford. In a letter to Henry Cavendish, Esq. FRS and AS In: Philosophical Transactions of the Royal Society of London . tape 78 , January 1, 1788, ISSN 0261-0523 , p. 395–402 , doi : 10.1098 / rstl.1788.0027 ( royalsocietypublishing.org [accessed April 30, 2017]).
- ^ Claus Priesner: Lowitz, Tobias. In: New German Biography (NDB). Volume 15, Duncker & Humblot, Berlin 1987, ISBN 3-428-00196-6 , pp. 259-261 ( digitized version ).
- ↑ Tobias Lowitz : Experiments on the production of artificial cold . In: Lorenz von Crell (Hrsg.): Chemical annals for the friends of natural science, Arzney knowledge, household art and manufactories . tape 1 , no. 1 . CG Fleckeisen, 1796, p. 529-539 ( limited preview in Google Book Search).