It is Summer, the temperature is rising, and the Hogs wish for a COOLing bath! But in the lab, cooling baths are slightly different. Imagine that you have a very reactive or sensitive reagent or that you want to slow down a procedure in order to selectively synthesize one specific product. If that is the case, you need to decrease the temperature of your reaction. When the Hogs need to run a reaction below room temperature, they use cooling baths. These are often made from mixture of a cryogenic component, such as liquid nitrogen, ice or dry ice (CO2) and a solvent or a salt. Salt-ice COOLing mixtures For temperatures from zero down to – 55 °C an ice/water bath with addition of salts is the easiest setup and most safety in every undergraduate teaching laboratory. The final temperature will depend on how finely crushed the ice is and how well the mixed components are. The salt lowers the freezing point of the mixture, The choice of containers is very flexible. From Tupperware containers and beakers to used styrofoam ice-cream boxes. However, baths colder than – 10 °C should be made in an insulating container, otherwise it will not maintain the temperature for a long period of time. Hog tip: Mix crushed ice with a near equal volume of cold-water, so the water will fill the empty air spaces between the ice and the reaction flask. Dry-ice COOLing baths Dry ice is the solid form of carbon dioxide, CO2. It is a dense, snow like substance with a surface temperature of - 78.5 °C that sublimates (passes directly into the vapor without melting) into carbon dioxide gas. As a penny pincher Hog, he likes dry ice, because it is a fairly inexpensive cryogenic agent. Nevertheless, he spent money in a Dewar. In order to avoid Hog Jr. spending time adjusting more often the bath temperature, the ideally bath container for a dry ice bath is a Dewar. The solvent helps transferring the temperature of the cooling component to the reaction flask. For an efficient heat transfer between dry ice and the solvent, the solvent is required to have a melting point below - 78 °C. Solvents with higher melting point can also be used and will result in a higher bath temperature. However, you must add dry ice slowly to avoid freezing the entire mixture. Liquid Nitrogen baths When we need temperatures below - 78 °C we use liquid nitrogen baths. Liquid nitrogen N2is a cryogenic fluid with a melting point of - 196 °C. Hog paws are freezing! When handling liquid nitrogen only a Dewar should be used. As liquid nitrogen is a liquid, the use of a solvent for uniform heat transfer is not necessary. However, if you need a temperature higher than - 196 °C, there is a variety of organic solvents that could be used to make mixtures with liquid nitrogen at different temperatures. Safety: Always use cryogenic protective gloves and safety goggles. The gloves will protect your skin, as cryogens can rapidly the burn skin and cause frostbites.
Hog tip: If you open your reaction flask while it is immersed in the cooling bath, air will quickly flow inside. If your reaction is moisture sensitive, it will not be so COOLing at all. Penny pincher Hog tip: Solvents used for cooling baths can be kept, stored and reused for another bath. References:
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