Balloons & Septa

Introduction to Balloons & Septa

Balloons & Septa offer a convenient way to carry out reactions under alternative atmospheres. Most commonly, they are used to allow reactions to be carried out under an inert atmosphere (usually nitrogen or argon) in order to minimise the amount of oxygen or water vapour present, which would hamper the progress of the reaction.

This could be due to:

Reactions are also carried out using balloons in order to introduce a reactant in the gas phase. The most common include the use of hydrogen (for example in hydrogenations) or oxygen (where greater than atmospheric levels are required).

Balloons are usually attached to reaction flasks using needles which are inserted into glassware via ground glass joints which have been fitted with a rubber septum.

Balloons

Balloons are used to allow reagents (especially solvents) to be stored or reactions to be performed under alternative atmospheres. Balloons offer a simple and inexpensive way to introduce an alternative atmosphere, especially the use of inert gases. The use of a balloon allows reactions to be undertaken which would prove difficult under a regular atmosphere, but without the added complexities of a more rigorous method, such as a Schlenk line.

An image of the correct setup for a balloon used to purge a flask. A balloon filled with nitrogen, attached to a syringe with a needle on the end. The needle is sheathed.

Construction

Balloons used in a lab usually comprise two balloons (one inside the other) attached to a syringe barrel (usually with rubber tubing and tape to secure). The syringe barrel allows the balloon to be easily connected to a needle, which can be used to introduce the balloon into a flask through a septum.

An image of a flask that is being purged with nitrogen. The flask has a suba seal in the neck, and a balloon attached to a syringe and needle has pierced the suba seal. There is also a second needle in the suba seal, so the air can flow out of the flask.

Gases

Various gases can be used such as nitrogen or argon to provide inert atmospheres, or gases such as hydrogen or oxygen where the gas is introduced as a reagent. Nitrogen is the most common inert gas used, partly as it is cheaper than argon, although argon's higher density offers advantages in that the gas tends to form a better protective layer in a flask.

An image of a balloon being purged with nitrogen. The balloon is attached to a syringe and is being filled up through a tube on a nitrogen tap. The balloon is currently empty.

Septa

Rubber septa are used to create an airtight seal in a reaction flask, through which needles can be injected. There are various sizes of septa available which fit the various ground glass joints available in laboratory glassware. The septa are often referred to by the tradename 'Suba Seal'. Septa are often reused in a laboratory environment but do eventually wear out and can be a source of leaks, particularly when they become worn from multiple needle punctures.

An image of a flask with a suba seal secured in the neck, showing the correct way to form airtight conditions for the glassware.