Ground glass joints are manufactured to fit quite well with one another, and yet they are not perfectly airtight. In some situations (e.g. when using reduced pressure inside an apparatus), grease must be applied to each joint to ensure a good seal. Grease is also used whenever the joint may be in contact with a highly basic solution, as basic solutions can form sodium silicates and etch glass.
Grease can be applied with a syringe full of grease (Figure 1.8a), wood splint, or toothpick. Grease should be lightly applied in portions around the male joint, closer to the glass end than the end which will be in contact with reagents (Figure 1.8a). If grease is allowed near the end which will contact the reagents, there is a possibility the reagent will dissolve the grease and become contaminated. The female joint should then be connected, and the joints twisted to spread the grease in a thin layer. The joint should become transparent all the way around the joint, but to a depth of only one-third to one-half of the joint (Figure 1.8b). If the entire joint becomes transparent or if grease is seen spilling out of the joint, too much grease has been used (Figure 1.8c). Excess grease should be wiped off with a KimWipe (one is used in Figure 1.9).
To clean grease from a joint after a process is complete, wipe of the majority of the grease using a paper towel or KimWipe. Then wet a KimWipe with some hydrocarbon solvent and rub the moistened KimWipe onto the joint to dissolve the grease (Figure 1.9). Hydrocarbon solvents (e.g. hexanes) work much better than acetone to dissolve residual grease.