Author Topic: Storing Teflon Stopcock-Equipped Glassware  (Read 689 times)

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Rhodium

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Storing Teflon Stopcock-Equipped Glassware
« on: January 27, 2003, 07:24:00 PM »
Aldrich Technical Bulletin AL-111

This piece of glassware is equipped with a Teflon® stopcock. Since the thermal expansion of Teflon® is significantly different from that of glass, special techniques are required when this apparatus is to be used in a coldroom (0-5°C).

The Teflon® plug will contract more than the glass barrel on cooling. Thus, the stopcock will give a good tight seal at room temperature, but eventually will leak when stored in a coldroom. Conversely, the stopcock can be tightened in the coldroom to give a tight seal, but on warming to room temperature, the Teflon® expands, freezing the stopcock.

The best solution to the above problem is to retighten the stopcock in the coldroom after the apparatus has cooled for about 15 minutes. Thereafter, open and close the stopcock only in the coldroom. Do not attempt to turn the stopcock after it has warmed to room temperature.

Teflon® will coldflow slowly with time. Therefore, unattended long-term storage of a tightened stopcock is not recommended. The stopcock should be turned from time to time (at least once every month) to check if a tight seal is still maintained, regardless of where stored.

Storage in a freezer (-20°C or below) presents special problems. If a Teflon® stopcock is tightened in a freezer, then allowed to warm to room temperature unattended, the expanding Teflon® can break the glass barrel. Obviously, this problem can be avoided by using only all-glass stopcocks in a freezer. Alternatively, it is possible to manually loosen the Teflon® plug while continuously turning the stopcock as the apparatus warms. This process can be accelerated by warming the glass barrel with the hand while turning the stopcock.