Video Transcript
When working with a Schlenk line, which
of the following actions is the least problematic? (A) Opening a flask to air before closing
the tap connecting it to the nitrogen line. (B) Raising a Dewar of liquid nitrogen up
to the joint of the cold trap. (C) Holding a flask by the base when
attaching to the Schlenk line. (D) Opening a flask to air before closing
the tap connecting it to the vacuum line. Or (E) opening the tap between the
nitrogen line and a flask under vacuum.
Let’s look at what would happen if we
were to perform each of these actions. Here we have a flask attached to a
Schlenk line. The first action is to open our flask to
air before closing the tap connecting it to the nitrogen line. Having a flask full of air and then
exposing it to the nitrogen line is not really going to do very much. It might mix with the contents and push
some of the air out, but nothing dramatic will happen. So this action is not particularly
problematic.
Let’s look at action (D), raising a Dewar
of liquid nitrogen up to the joint of a cold trap. Ideally, you want the level of liquid
nitrogen to be just below the joint. Raising it higher risks freezing the
grease in the joint and breaking the seal. So this does cause a problem.
(C) says, “Holding a flask by the base
when attaching to the Schlenk line.” This is not a good idea. It can put stress and strain on the arm
of our flask, which can cause it to break. You should hold the glassware near where
you’re attaching the tubing. So (C) also causes a problem.
Next, we have opening a flask to air
before closing the tap connecting it to the vacuum line. This is going to suck air through our
vacuum pump. It also means that air will be drawn
through our cold trap, increasing the risk that we condense liquid oxygen. You should try to avoid drawing air
through the vacuum pump. So this could also cause a problem.
Finally, we have opening the tap between
the nitrogen line and the flask under vacuum. This is going to draw nitrogen gas into
our flask. This is how we fill our reaction vessel
with an inert atmosphere. This step should be done slowly and
carefully as it can cause oil in the oil bubbler to be sucked back into your manifold. So again, this could potentially cause us
a problem. So the least problematic action is
(A).
