



I got a request for some more details on the methanol/water bath from a few weeks back. Enjoy!

Step 1: Pick a Dewar and Measure the Solvents

The desired ratios are in the last post. 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.

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).

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.

Step 4: Add Powderized Dry Ice Until Bath Begins to Freeze

This requires about a 5:1 liquid:dry ice ratio. A large amount of 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.

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 (within 10 °C or so).

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. My record with a 500 mL Dewar is one hour at -20 °C without further dry ice addition.

Note: Cooling the bath with liquid nitrogen works similarly to the above steps, but requires active stirring after coolant addition. LN2 has a tendency to freeze the top layer of the bath, and this must be broken up and stirred into the liquid fraction.

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 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.