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In organic chemistry, a cooling bath is an experimental technique that provides and maintains a low-temperature environment by using a coolant to cool a liquid mixture. The temperature range achievable with a cooling bath is typically from 13°C to -196°C. It is commonly used in reactions and experimental procedures that need to be carried out at temperatures below room temperature, as these reactions and processes are usually exothermic or involve thermally unstable intermediates or products. The coolants used in the cooling bath include dry ice, liquid nitrogen, and crushed ice. Methods for generating and maintaining low temperatures There are mainly three methods for generating and maintaining low temperatures: ice-salt baths, dry ice-solvent baths, and liquid nitrogen slush baths. The liquid nitrogen slush bath is prepared by carefully adding liquid nitrogen to a continuously stirred organic solvent to create an ice-cream-like substance; stirring with a glass rod helps prevent the liquid nitrogen slush bath from solidifying in certain areas. The temperature range that can be achieved with a liquid nitrogen slush bath is from 13 to -196°C. Generally, when a Dewar flask is used to achieve good insulation, a liquid nitrogen slush bath can last for several hours. However, if the reaction requires a low temperature to be maintained for a longer period of time, such as during an overnight reaction, it is necessary to use mechanical cooling methods like refrigerators, circulating condensers, or freezers to sustain that low temperature over an extended period. The liquid nitrogen slush bath is particularly suitable for degassing reaction solvents and for the condensation and collection of fractions during vacuum distillation. An ice-salt cooling bath: At atmospheric pressure, the temperature of an ice-water mixture is 0°C. A cooling bath composed of ice and saltwater, obtained by mixing a concentrated salt solution with crushed ice, can generate and maintain a temperature below 0°C. Changing the concentration of the salt solution allows adjustment of the steady-state temperature that can be maintained by the cooling bath. Different types of salts enable different minimum temperatures for the cooling bath, and the temperature range that can be achieved and maintained in practice is usually between 0 and -51°C (see the table below for details). However, when the concentration of the salt solution reaches a certain level, the ice-salt cooling bath, which was originally in a state of a mixture of ice and water, will completely solidify into granular ice-salt particles due to reaching the eutectic temperature. This reduces the heat-conducting surface area of the experimental apparatus submerged in it, thereby hindering the timely dispersion of heat. Dry ice solvent cooling bath: Dry ice, which is solid carbon dioxide, is available in granular and rod form for purchase; it can form mixtures with various solvents that provide an excellent cooling effect. The method for preparing and maintaining a dry ice solvent bath is simple and reliable. Generally, granular dry ice is carefully added one by one to the desired solvent until dry ice blocks coated with the frozen solvent appear; at this point, the temperature of the cooling bath has reached its steady state temperature. Thereafter, it is sufficient to add more dry ice blocks at regular intervals and stir the mixture to maintain the temperature ； Moreover, the temperature reproducibility of the dry ice solvent bath is good, with the variation in the steady-state temperature being able to be kept within ±1°C. The purity of the solvent has a significant impact on the dry ice solvent bath. A typical example is a dry ice-acetonitrile bath prepared using analytically pure acetonitrile, whose steady-state temperature is -42°C. However, when 0-3% acrylonitrile is mixed into the analytically pure acetonitrile, the steady-state temperature of the cooling bath drops from -42°C to -51°C. On the other hand, a dry ice-acetonitrile bath prepared using technical-grade acetonitrile maintains a steady-state temperature of -42°C, and this temperature remains consistent. When preparing a dry ice solvent bath, it is possible to use either a single pure solvent or a mixed solvent obtained by combining two mutually soluble solvents. With mixed solvents, the steady-state temperature of the desired dry ice solvent bath can be adjusted by changing the ratio of the two solvents. For example, pure o-xylene has a melting point of only -26°C; therefore, a cooling bath prepared by mixing it with dry ice has a steady-state temperature of -26°C and a viscous, sludgy consistency. In contrast, a dry ice solvent bath made using a mixture of o-xylene and m-xylene not only has lower viscosity but also allows for approximately linear adjustment of the cooling bath’s temperature by varying the volume ratio of o-xylene to m-xylene, with the temperature range spanning from -26°C to -72°C℃ ； Another mixed solvent system that can be used is one composed of ethylene glycol and ethanol; by adjusting the ratio of ethylene glycol to ethanol, the temperature can be adjusted in a nearly linear manner. The temperature range of the resulting cooling bath is from -12°C to -78°C, and it is possible to maintain a constant temperature reliably for up to 5 hours by periodically adding some dry ice. Table of cooling temperatures for various commonly used cooling baths in the laboratory. Cooling baths in degrees Celsius: 1,3-dimethylbenzene/dry ice – 12; 1,4-dioxane/dry ice – 6; cyclohexane/dry ice – 5; benzene/dry ice – 2; formamide/dry ice – 0; crushed ice – from -5 to -20. Ice-salt bath – -10.5; ethylene glycol/dry ice – -12; cycloheptane/dry ice – -15; benzyl alcohol/dry ice – -22; tetrachloroethylene/dry ice – -22.8; carbon tetrachloride/dry ice – -25; 1,3-dichlorobenzene/dry ice – -29; o-xylene/dry ice – -32; m-toluidine/dry ice – -41; acetonitrile/dry ice (acetonitrile should not be completely frozen) – -42; pyridine/dry ice – -47; m-xylene/dry ice – -52; diethylene glycol diethyl ether/dry ice – -56; n-octane/dry ice – -60; isopropyl ether/dry ice – -60; chloroethane/dry ice – -68; 85% ethanol/dry ice – -72; ethanol/dry ice – -77; chloroform/dry ice – -77; ethyl acetate/dry ice – -78; propane/dry ice – -83; propylamine/liquid nitrogen – -83.6; ethyl acetate/liquid nitrogen – -89; n-butanol/liquid nitrogen – -94; hexane/liquid nitrogen – -94.6; propane/liquid nitrogen – -95.1; toluene/liquid nitrogen – -97; methanol/liquid nitrogen – -108; isooctane/liquid nitrogen – -110; methyl tert-butyl ether/liquid nitrogen – -116; ethanol/liquid nitrogen – -116; ether/liquid nitrogen – -131; n-pentane/liquid nitrogen – -160; isopentane/liquid nitrogen – -196; liquid nitrogen