Experimental freezing preparation

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Methods for laboratory cryogenic preparation There are two methods for laboratory cryogenic preparation. One is the physical and chemical method: 1. Use the frozen mixture (ice-salt refrigerant) during the dissolution process of ice to generate low temperature: crushed ice: 0~ -5℃ ； 3 parts ice + 1 part salt: -15~-18℃ ； 3 parts ice + 3 parts crystallized calcium chloride (CaCl2·6H2O): -40℃ ； 3. 4 parts ice + 5 parts crystallized calcium chloride: -40℃~-50℃ ； No matter which freezing mixture is used, the prerequisite is that the ice and salt must be well crushed and mixed evenly. When using two frozen mixtures, the CaCl2·6H2O must be cooled in the refrigerator before it can reach the above temperature. 2. Use sublimation process to generate low temperature: Solid carbon dioxide (dry ice): -78.9℃; solid carbon dioxide + ethanol: -72℃ ； Solid carbon dioxide + ether, chloroform or acetate: -77℃. Since solid carbon dioxide has poor thermal conductivity, it should be mixed with an appropriate liquid, such as propylene, alcohol, etc. Trichlorethylene is particularly suitable because solid carbon dioxide floats on the surface of the trichlorethylene, so the mixture does not foam and overflow. However, it is a common method to use acetone as solvent and mix it with dry ice because dry ice dissolves quickly. 3. Use the evaporation process to generate low temperature: Liquid nitrogen has certain advantages in practical applications. It is a colorless, odorless and tasteless liquid, slightly soluble in water, has poor thermal and electrical conductivity, is slightly lighter than water, and does not produce toxic or irritating gases. At the same time, it does not burn or explode. It combines with sodium, calcium or magnesium to form nitrides (Nitrides). The coldest point is -196°C. Therefore, there are many advantages to using liquid nitrogen: ①, has a low boiling point (-196°C) under atmospheric pressure. If used with an appropriate adjustment and control system, any temperature between minus 37 and 196°C can be obtained. ②, low production cost and easy source. ③, safe and reliable. In fact, although the above method is convenient, it consumes a lot of money, the temperature is unstable, and it is difficult to maintain low temperature for a normal period of time. Nowadays, low-temperature instruments are often used for refrigeration in general laboratories. There are many experimental low-temperature devices on the market now, and the control temperature can be adjusted at will. There are two main categories: One is the compressor principle. The refrigerators, freezers, etc. used in our lives are based on this principle. The disadvantages are large size, slow refrigeration and cooling, loud noise, and the lowest refrigeration temperature is generally above -50°C. The other is water circulation refrigeration of components. This type of instrument is small in size and cools quickly. The refrigeration temperature can reach below -60℃, and no noise is produced during the refrigeration process. The disadvantage is that water circulation is used for refrigeration, which requires a large amount of water. Both of these refrigeration instruments are now available on the market, but in comparison, the second one is more convenient to use. We now have two units in our laboratory. One can cool down to -20°C, and the other can cool down to below -60°C. With them, any low-temperature reaction is possible.