Stopcock

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Glass (above) and PTFE (below) stopcocks with rubber and plastic seal and their respective screw caps.

A stopcock is a form of valve used to control the flow of a liquid or gas.

General

A stopcock consists of an elongated conical item with a handle and a ground glass joint, always male. Most models are full glass and have a two or three way bored holes and are full glass. Other models, are hollow and have two bored holes. Stopcocks are made of borosilicate glass or PTFE.

Stopcocks are often parts of laboratory glassware such as (automatic) burettes, chromatography column, Dean-Stark apparatus, dropping funnels, gas collecting tube, gas syringe, Imhoff cone, Schlenk flasks/lines, separatory funnels, various adapters.

Availability

Stopcocks can be bought from lab suppliers or online.

You can also scavenge them from broken glassware, like burettes, dropping and separatory funnels.

Maintenance

Make sure stopcocks are always well greased, to prevent them from freezing. Teflon stopcocks do not normally require greasing, but it's a good idea to grease them, especially if you're handling burettes, when titrating exact volumes.

References

Relevant Sciencemadness threads