G.Patton

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Introduction

I want to show in this topic simple rules of handling, step-by-step and video manual of dry-ice dewar bath with acetone preparing. Hope that this thread helps you to be more confident and safe in low-temperature synthesis procedures. The Liquid nitrogen (N2) laboratory handling is the second part of cooling baths agents topic.

General
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A cooling bath, in laboratory chemistry (often but not always organic chemistry) practice, is a liquid mixture which is used to maintain low temperatures, typically between 13 °C and −196 °C. These low temperatures are used to collect liquids after distillation, to remove solvents using a rotary evaporator, or to perform a chemical reaction below room temperature (see:kinetic control).

Cooling baths are generally one of two types: (a) a cold fluid (particularly liquid nitrogen, water, or even air) — but most commonly the term refers to (b) a mixture of 3 components: (1) a cooling agent (such as dry ice or water ice); (2) a liquid 'carrier' (such as liquid water, ethylene glycerol acetone, etc.), which transfers heat between the bath and the vessel; and (3) an additive to depress the melting-point of the solid/liquid system.

A familiar example of this is the use of an ice/rock-salt mixture to freeze ice cream. Adding salt lowers the freezing temperature of water, lowering the minimum temperature attainable with only ice.

Reducing solution of any solvent by reducing the temperature, this is basic chemistry and physics. It applies to any solute in a solvent, many substances dissolved in another affect its freezing point as in antifreeze, or salt in water, and will often raise its boiling and freeze points, well, unless you have an endothermic reaction like a cold pack which uses ammonia nitrate or urea in water, or exothermic like a Meal Ready to Eat pack. We desire to reduce the temperature to lower solubility of our solvent and effect crystallization, and to be able to recycle our solvent over and over. Of particular importance is that the freezing points of naphtha and acetone are below the temperature of dry ice, -109.3 °F or -78.5 °C.

Acetone dry ice baths are a common laboratory technique as the acetone does not freeze at the temperature of dry ice 109.3 °F or -78.5 °C and the acetone conducts the heat energy away from your solvent in a sealed container (beware acetone readily attacks many plastics, so glass or metal is advised)if you have an open container water from humidity can condense and dissolve your crystals after return to room temp and that can require protracted drying. This is often used to prevent boil off (at room temperature) of low boiling point substances (like ether/ammonia that have been distilled). Using dry ice in water is not totally ineffective nor dry ice alone, but neither conducts heat away nearly so fast.

All users of Carbon Dioxide, Solid or Dry Ice must review this topic before use. Dry ice is the solid form of carbon dioxide that is available in flakes, pellets or block form and is non‐combustible. It is most often used for rapid cooling of materials or shipping biological samples. It poses unique hazards to those who may work with or around it - those hazards are addressed below.

HAZARDS

Carbon Dioxide, Solid or Dry Ice is NOT considered a Hazardous Substance by the 2012 OSHA Hazard Communication Standard (29 CFR 1910.1200). The precautions associated with handling this material are as follows:

Contact Hazard: At -109 °F (-79 °C), skin contact with dry ice can lead to severe frostbite; skin cells freeze and become damaged rapidly.
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Asphyxiation Hazard: Dry ice will sublime (change from solid to gas) at any temperature above -79 °C. This releases potentially substantial volumes of CO2 (1 kg solid = ~550 liters gas), which can displace oxygen quickly in the air around the dry ice, causing dizziness, headaches, difficulty breathing, loss of consciousness and death. This is especially of concern in nonventilated or confined spaces.
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Overpressurization Hazard: Due to the rapid emission of large volumes of CO2 gas, any dry ice that is stored in a closed container can pressurize the container. Given enough time at normal room temperature, such a container may violently rupture if the gas is not able to escape.
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Handling rules

1. Know the dangers of handling dry ice. Handling dry ice is dangerous due to its extremely cold temperatures that can lead to frostbite and tissue injury. The carbon dioxide vapors in an unventilated area are also toxic. If prolonged contact with dry ice occurs, do not rub the affected area. Remove clothing that is not frozen to the skin and place the affected area in a warm water bath. Avoid direct dry heat.
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2. Protect yourself by dressing in a long-sleeved shirt, long pants, and closed-toed shoes. Proper protection is essential while handling dry ice. The best protection is to cover all of the surfaces of your body that could be exposed. Gloves and goggles are extremely important to protect your hands and eyes from injury.
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3. Pick up the dry ice with tongs. Never handle dry ice directly with your bare hands. If possible, use metal tongs when transferring chunks of dry ice to new locations. If you do not have tongs available, wear an oven mitt or towel while handling the dry ice. Metal tongs with serrated edges work best.
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4. Use a chisel to break off smaller pieces from the block. If you purchased a block of dry ice and need smaller pieces, use caution while chiseling them off. Break off pieces of the ice by setting a chisel to the desired point, and tapping it lightly with a mallet. Always wear eye protection while chiseling to prevent chips from flying into your eyes.
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5. Use the dry ice in a well-ventilated area. Dry ice is frozen carbon dioxide. As it warms, it sublimates (turns directly from a solid to gas, skipping the liquid phase) into its gaseous form. Exposure to large amounts of gaseous carbon dioxide is hazardous to your health and can cause you to lose consciousness or suffocate. Working in a room with good ventilation or an open window can prevent a dangerous buildup of gas and keep you safe. Symptoms of excessive carbon dioxide inhalation include dizziness, headache, and increased heart rate.
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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. Too much gas in an airtight container can lead to an explosion.
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7. Melt the ice when you are finished with it by pouring warm water over it. The warmer the dry ice gets, the faster it sublimates. To dispose of it, you can either leave it open to the air in a warm area or pour warm water over it until it is gone. Do not leave children unattended around dry ice. Don't try to dump the dry ice down a sink drain or toilet, or you could damage the pipes. Don't dispose of dry ice in the trash. Don't let the dry ice evaporate in a small area without proper ventilation. The build-up of carbon dioxide can lead to unconsciousness and even suffocation.
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8. Wear protective gloves when handling dry ice. Dry ice is freezing and should never be handled directly. Protective, insulated or leather gloves should be worn while handling the ice. An oven mitt or towel is also sufficient to protect your skin. Prolonged direct contact to dry ice can freeze your skin cells and cause an injury similar to a burn. If possible, use tongs to transfer the dry ice to different locations.
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9. Use certified food-grade dry ice. Dry ice can be obtained in a pellet or block form. During production, the dry ice is in contact with many potential contaminants that can be hazardous to your health. For many applications, these contaminants are not hazardous, as the items that are being kept cool will not be consumed. As dry ice warms up, it turns directly to the gas carbon dioxide. Carbon dioxide is what gives the fizz to a soda, and is perfectly safe to consume in these small amounts.
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10. Use containers with loose lids that are not airtight. Frozen carbon dioxide sublimates into a gas, skipping the liquid phase. Dry ice must be contained in a cooler, fridge, or freezer that allows this gas to escape. Storing dry ice in an airtight container can lead to a hazardous buildup of gas and can be an explosion hazard.

Dry ice must be stored in a Styrofoam chest, insulated cooler, or a special cooler designed for the storage of dry ice. The cooler must then be located in a well-ventilated place, such as the open lab. NEVER store coolers in closets, cabinets, refrigerators, or walk in coolers/cold rooms.

Dry ice will sublimate about five to 4.5 kg every 24 hours (blocks last longer) in a typical storage cooler. Plan on purchasing dry ice as close as possible to the time needed.
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First Aid

SKIN CONTACT
  • In case of cold burns (frost-bite):
  • DO NOT apply hot water or radiant heat.
  • Move the exposed person into a warm area before thawing the affected part; if feet were exposed, carry the exposed person, if possible.
  • Bathe the affected area immediately in luke-warm water (not more than 35 deg C) for 10 to 15 minutes, immersing if possible, and without rubbing the exposed area. Remove any exposed clothing as well as any jewelry.
  • Seek medical attention.
EYE CONTACT
  • Using eyewash, flush eyes while holding eyelids open;
  • Seek medical attention.
INHALATION
  • Move from contaminated area.
  • Lay the exposed person down. Keep warm and rested.
  • Prostheses such as false teeth, which may block the airway, should be removed, where possible, prior to initiating first aid procedures.
  • Seek medical attention.
INGESTION
  • If conscious, immediately give a tepid glass of water.
  • Never give anything by mouth to an unconscious person. Seek medical attention.

Step-by-step manual

Step 1: Pick a Dewar and Measure the Solvents
For this example, I’ll be making a -20 °C bath, which requires a 30:70 ratio of MeOH to water. This Dewar holds 150 mL, so I need 100 mL of liquid. Large Dewars are more wasteful, but maintain their temperature far better.
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Step 2: Crush Dry Ice
You can use large chunks of dry ice, but the powder variety cools much faster. I fill the pictured plastic ice buckets, then crush the dry ice with the bottom of the hammer (not the claw or face).
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Step 3: Mix Solvents, Fill Dewar 1/3 Full
Transferring the solvent from the graduated cylinder to an erlenmeyer flask ensures good mixing. Pour half your solvent into the Dewar.
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Step 4: Add Powdered Dry Ice Until Bath Begins to Freeze
This requires about a 5:1 liquid:dry ice ratio. Numerous bubbles and fog will evolve at the start, so add the dry ice slowly. The ice should remain after 10 seconds of stirring with a spatula, but the solution shouldn’t freeze solid.
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Step 5: Add Remaining Solvent
This will melt the ice and evaporate any remaining dry ice, leaving you with a bath that is approximately your desired temperature (control temperature by laboratory grade thermometer up to -78.5 °C).
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Step 6: Set and Maintain the Desired Temperature with 1-2 Pieces of Dry Ice
Chunks of dry ice about 1.5 cm x 1 cm work best. After about five minutes, you should see a fuzzy blob of ice form around the dry ice, indicating that you are at the desired temperature. When the bath starts to warm this blob, will float to the surface, and it’s time to add another piece of dry ice.
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Experimental:

To a 50 mL mixture of methanol/water (30/70) in a 150 mL Dewar flask is added approximately 10 g of crushed dry ice. The solution is allowed to bubble for thirty seconds, during which time a large volume of CO2 gas was released, and approximately 40% of the solution froze. When gas evolution slowed, a second 50 mL solution of methanol/water was added. A dry ice pellet (cylindrical, 1 cm x .5 cm x .5 cm) was then added, and the temperature was verified via an ethanol/dye thermometer. The Dewar was then used to cool a 4 mL vial for an organic reaction, and the solution remained at -20 °C for approximately 15 min without intervention.

 
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banister

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Nice write up. Going to use this to expedite harm reduction and safety issues at the lab. The boys and gals are going to love it quick simplicity. Maybe even make a quick pop quiz in order to certify everyone has thoroughly understood everything.
 
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