Sealing solids Glass taper joints can be sealed hermetically with
PTFE sealing rings (high vacuum tight, air leakage rate 10−6 mBar × L/sec and below),
o-rings (optionally encapsulated o-rings), or PTFE sleeves, sometimes used instead of
grease that can dissolve into contamination.
PTFE tape, PTFE resin string, and wax are other alternatives that are finding widespread use, but require a little care when winding onto the joint to ensure a good seal is produced.
Grease is used to lubricate glass stopcocks and joints. Some laboratories fill them into
syringes for easy application. Two typical examples: Left –
Krytox, a fluoroether-based grease; Right – a silicone-based high vacuum grease by
Dow Corning. A thin layer of
grease made for this application can be applied to the ground glass surfaces to be connected, and the inner joint is inserted into the outer joint such that the ground glass surfaces of each are next to each other to make the connection. In addition to making a leak-tight connection, the grease lets two joints be later separated more easily. A potential drawback of such grease is that if used on
laboratory glassware for a long time in high-temperature applications (such as for
continuous distillation), the grease may eventually contaminate the chemicals. Also,
reagents may react with the grease, especially under
vacuum. For these reasons, it is advisable to apply a light ring of grease at the fat end of the taper and not its tip, to keep it from going inside the glassware. If the grease smears over the entire taper surface on mating, too much was used. Using greases specifically designed for this purpose is also a good idea, as these are often better at sealing under vacuum, thicker, and so less likely to flow out of the taper, become fluidic at higher temperatures than
Vaseline (a common substitute), and are more chemically inert than other substitutes.
Cleaning Ground glass joints are translucent when clean and free of debris. Solvents, reaction mixtures, and old grease appear as transparent spots. Grease can be removed by wiping with an appropriate solvent;
ethers,
methylene chloride,
ethyl acetate, or
hexanes work well for
silicone- and
hydrocarbon-based greases.
Fluoroether-based greases are quite impervious to organic solvents. Most chemists simply wipe them off as much as possible. Some fluorinated solvents can remove fluoroether greases, but are costlier than laboratory solvents. ==Testing==