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How to Make a -30C Solution

In organic chemistry, you often need to cool a reaction mixture down to very low temperatures to ensure the desired result. The most common way to do so is by means of a cooling bath in a Dewar flask. Be forewarned, however, that the inside glass surface of a Dewar flask is very fragile, and since it contains a vacuum between the two glass layers, if shattered, it will implode. Only ever use Dewar flasks that have a protective metal casing on the outside, and be careful when you work with them. Also be aware that the extremely cold temperature of dry ice can injure you if it comes into direct contact with your skin or hands.

Things You'll Need

Gloves
Goggles
Lab coat
Fume hood
2-propanol
Dewar flask
Thermocouple
Crushed dry ice
Spatula
Tongs

Instructions

- 1
  Put your Dewar flask in the fume hood and add pure 2-propanol (isopropyl alcohol) until you reach the desired level. Do not overfill the flask--remember that you'll need to add some dry ice in just a minute.
- 2
  Insert one of the leads on the thermocouple into the solution so that you can monitor its temperature.
- 3
  Using the spatula or a pair of tongs, add crushed dry ice to the isopropanol. Be aware that when you do so, the isopropanol will foam for a little bit, so don't add a lot of dry ice all at once or you'll cause your flask to foam over. Wait for the foaming to stop before you add more.
- 4
  Monitor the temperature and continue to add dry ice slowly until you reach -30 C.
- 5
  Add dry ice as needed to maintain the temperature.

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