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A stopcock is a form of valve used to control the flow of a liquid or gas in a lab item.
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 always 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.
Stopcocks can be bought from lab suppliers or online.
Make sure stopcocks are always well greased, to prevent them from freezing. Teflon stopcocks do not normally require greasing, but it's still a good idea to grease them, especially if you're handling burettes, where precision is crucial when titrating exact volumes.