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
Step 3: Mix Solvents, Fill Dewar 1/3 Full
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
Step 6: Set and Maintain the Desired Temperature with 1-2 Pieces of Dry Ice
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.
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.