Balloons & Septa offer a convinient 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. The use of an inert atmosphere allows reactions to be carried out where the presence of oxygen, water vapour, or both would hamper the progress of the reaction. This might be due to competing reactions (eg aerial oxidation from the atmosphere), or quenching of reagents (eg the reaction of materials with water). 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 rigourous method, such as a Schlenk line.

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.

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.

Septa

Rubber septa are used to create an air tight 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.