Cooling baths are essential in laboratory chemistry, particularly organic chemistry, as they allow for the maintenance of ultra low temperatures ranging from 13°C to -196°C. These low temperatures are necessary for various tasks such as collecting liquids after distillation, removing solvents using a rotary evaporator, or performing a chemical reaction below room temperature for kinetic control.
Chilling baths tend to be one of two varieties: (a) a frigid liquid (particularly liquid nitrogen, water, or even air) — but most often the phrase alludes to (b) a combination of 3 components: (1) a cooling agent (for example dry ice or water ice); (2) a liquid ‘transporter’ (such as liquid water, ethylene glycerol acetone, etc.), which shifts heat among the bath and the vessel; and (3) an additive to reduce the melting-point of the solid/liquid system.
For instance, a common example of this is the use of an ice/rock-salt mixture to freeze ice cream. By adding salt, the freezing temperature of water is lowered, allowing for the attainment of lower temperatures compared to using ice alone.
Decreasing the temperature of any solvent is an essential concept in chemical science and physics. This idea applies to any solute that is dissolved in a solvent, as seen in antifreeze or salt in water, which will usually increase its boiling and freezing points. There are exceptions, like cold packs, which use ammonia nitrate or urea in water, and Meal Ready to Eat packs, that are endothermic or exothermic. The aim is to reduce the temperature to lessen the solubility of the solvent, to cause crystallization, and to make sure the solvent can be reused. Notably, the freezing points of naphtha and acetone are lower than the temperature of dry ice, which is -109.3 °F or -78.5 °C.
To conduct a dry ice bath, acetone is commonly used as it does not freeze at the temperature of dry ice. This mixture conducts heat energy away from the solvent in a sealed container, preventing boil-off at room temperature. However, it is important to note that acetone can readily attack many plastics, making it advisable to use glass or metal instead.
Before using carbon dioxide, solid or dry ice, it is essential to review this topic as it poses unique hazards to those who work with or around it. Dry ice is the solid form of carbon dioxide and is available in flakes, pellets, or block form. It is non-combustible and is often used for rapid cooling of materials or shipping biological samples.
Carbon Dioxide, Solid, or Dry Ice is not categorized as a hazardous substance according to the 2012 OSHA Hazard Communication Standard (29 CFR 1910.1200). However, there are certain precautions that need to be taken into consideration when handling this material. These are:
Skin Contact Risk: Dry ice at -109 °F (-79 °C) can lead to severe frostbite upon contact with the skin. The skin cells freeze quickly, leading to significant damage.
Asphyxiation Risk: Dry ice can sublime (turn from solid to gas) at temperatures above -79 °C. This process releases substantial amounts of CO2 (1 kg solid = ~550 liters gas), which can rapidly displace oxygen in the surrounding air, causing dizziness, headaches, difficulty breathing, loss of consciousness, and even death. This risk is particularly significant in enclosed spaces or areas with limited ventilation.
Overpressurization Risk: Closed containers that hold dry ice can become pressurized due to the rapid release of large amounts of CO2 gas. If the gas cannot escape, the container may rupture violently after some time at normal room temperature.
Safe Handling of Dry Ice
1. Understand the hazards of handling dry ice. Dry ice can cause frostbite and tissue injury due to its extremely low temperatures, and the carbon dioxide vapors it emits in unventilated areas can be toxic. If prolonged contact occurs, do not rub the affected area. Instead, remove any clothing that is not frozen to the skin and place the affected area in warm water. Avoid direct dry heat.
2. Protect yourself with appropriate clothing and gear. Dress in a long-sleeved shirt, long pants, and closed-toe shoes, and wear gloves and goggles to protect your hands and eyes from injury.
3. Use tongs or an oven mitt to handle dry ice. Never handle dry ice directly with your bare hands. If possible, use metal tongs to transfer chunks of dry ice to new locations. If tongs are not available, wear an oven mitt or towel while handling dry ice.
4. Use caution when breaking off smaller pieces. If you need smaller pieces of dry ice, use a chisel and mallet to break them off. Always wear eye protection to prevent chips from flying into your eyes.
5. Work in a well-ventilated area. Dry ice is frozen carbon dioxide, and exposure to large amounts of gaseous carbon dioxide is hazardous to your health. Make sure you are working in a room with good ventilation or an open window to prevent a dangerous buildup of gas.
6. Store dry ice in an insulated container that is not airtight. Dry ice sublimates relatively quickly, but its shelf life can be extended by storing it in an insulated container such as a Styrofoam cooler. Make sure the container is not airtight to prevent the buildup of carbon dioxide gas.
7. Dispose of dry ice safely. To dispose of dry ice, melt it by pouring warm water over it. Do not dispose of dry ice in the trash or down a sink drain or toilet, and do not let it evaporate in a small area without proper ventilation.
8. Wear protective gloves when handling dry ice. Dry ice is freezing and should never be handled directly. Wear protective, insulated, or leather gloves while handling dry ice. Prolonged direct contact with dry ice can freeze skin cells and cause an injury similar to a burn.
9. Use certified food-grade dry ice. When using dry ice for food or beverage applications, use only certified food-grade dry ice to avoid potential contamination.
10. Use containers with loose lids that are not airtight. Dry ice must be contained in a cooler, fridge, or freezer that allows gas to escape. Storing dry ice in an airtight container can lead to a hazardous buildup of gas and be an explosion hazard.
Dry ice ought to be held in a Styrofoam bin, an insulated container, or a distinctive container intended for the capacity of dry ice. The holder should then be put in a very much ventilated spot, for example, the open lab. NEVER put away holders in storerooms, cupboards, ice chests, or stroll in coolers/cold rooms.
Dry ice will vanish at a rate of 5 to 4.5 kg each 24 hours (blocks last more) in a typical storage cooler. Intend to purchase dry ice as near as could be expected under the circumstances to the time required.
In case of cold burns (frostbite):
DO NOT utilize hot water or radiant heat.
Move the exposed individual into a heated room prior to thawing the affected part; if feet were open, transport the exposed person, if practicable.
Immerse the influenced area promptly in mild water (not over 35 deg C) for 10 to 15 minutes, submerging if achievable, and without scrubbing the exposed area. Eliminate any exposed clothing as well as any jewelry.
Look for medical care.
Using eyewash, cleanse eyes while keeping eyelids open;
Search for medical attention.
Relocate from contaminated region.
Lay the exposed individual down. Keep warm and rested.
Artificial devices such as false teeth, which may hinder the air passage, should be eliminated, where feasible, before initiating first aid measures.
Look for medical care.
If conscious, promptly give a tepid cup of water.
Under no circumstances give anything by mouth to an unconscious individual. Look for medical attention.