Step by step process for purging a flask

In order to set up a reaction for the introduction of an inert atmosphere, you must first purge a round bottomed flask to flush out the air and replace it with your inert gas. The following instructions relate to purging a round bottom flask with stirrer bar with nitrogen, prior to a reaction.

Sealing a flask

1. Obtain a round-bottomed flask which is suitable for the reaction. Depending on the reagents and reaction being undertaken, this flask may have been oven dried prior to use.

2. Add contents to the flask which can be added before the introduction of the inert atmosphere. For example a solid reagent or stirrer bar.

3. Obtain a rubber septum (SubaSeal™) of the correct size for the neck of the flask. The colour of the septum corresponds to its size.

4. Place the septum into the neck of the round bottom flask and twist it in whilst pushing it down. Once it is inserted, fold the excess rubber around the neck of the round bottom flask to create a secondary seal. Newer septa, or septa which have swollen from use with solvents may be tricky to insert.

Introducing the nitrogen atmosphere

1. Obtain a lab balloon and short needle.

2. Inflate the balloon with nitrogen gas. You may wish to partially inflate and deflate the ballon a few times to purge the balloon of any air, before filling it to a reasonable size.

3. Grasp the balloon above the rubber tubing between thumb and finger in order to retain the gas in the balloon and fit the needle (still in its protective sheath).

4. Carefully remove the needle sheath and pierce the septum. The septum will only pierce within the circle, and only if inserted straight. The sides of the septum contain much thicker rubber which will prevent the needle from entering the flask.

5. Introduce a second short needle as an 'exit'. The pressure of the balloon will cause nitrogen to be pushed from the balloon into the flask and displace the atmosphere out of the exit needle.

6. The size of the flask, balloon and how rigorous an atmosphere is required will determine how long to allow the flask to purge. A larger flask will require a greater volume of gas to ensure the flask is purged.

7. The purge ballon is expected to deflate during the purge. You may need to obtain another balloon, fill this with nitrogen, and attach to the purge needle. You can then remove the other balloon, leaving the needle in place.

Maintaining a nitrogen atmosphere

1. Replace the balloon just before finishing the purge, then remove the purge needle and dispose into a sharps bin.

2. The balloon should maintain its size during the reaction if you have no leaks. If the balloon appears to be decreasing during the reaction, then:

• Inflate a new balloon

• Quickly swap the balloon attached to the purge needle (without removing the needle)

• Investigate for the presence of a leak