Video Transcript
When working with a Schlenk line,
which of the following is the least likely hazard? A) Condensation of liquid
oxygen. B) Breakage of a flask during
attachment. C) Implosion of the vacuum
line. D) Implosion of the nitrogen
line. Or E) suction of oil into the
nitrogen line.
The easiest way to answer this
question is by looking at a Schlenk line setup. So, let’s make a little more space
and get a diagram of a Schlenk line. Here, we have a diagram of a
Schlenk line setup. Let’s start by labeling all of the
important parts.
First, we have our dewar
manifold. This has gas down one side and
vacuum down the other. This gas is usually something like
nitrogen, but it could be other inert gases. The gas comes in on the left-hand
side through a tap down the manifold and then out through the oil bubbler. The oil bubbler creates a one-way
valve. This means that the nitrogen flow
can leave the system, but air doesn’t get back in.
Down the other half of the
manifold, we have a vacuum line. The vacuum pump, attached on the
right-hand side here, will evacuate the front of the manifold. We can then attach our reaction
vessel to the end of the tubing on the Schlenk line. Then, by turning the three-way tap,
we can expose our reaction vessel EVA to the gas, the vacuum, or nothing at all.
The last key feature to mention at
this point is the cold trap. While the Schlenk line is in use,
the cold trap will sit inside a dewar of liquid nitrogen. This means that any gases evolved
in your reaction vessel will be condensed in the cold trap and will not reach the
vacuum pump. This protects the vacuum pump.
So, now that we’ve identified
several of the key features of a Schlenk line, let’s go back to our question. This question is asking us which of
the following answers is the least likely hazard. As you know, Schlenk lines are
incredibly useful in lots of areas of chemistry, particularly in organic
chemistry. But they do come with some
associated hazards. Here, we’ve been given five
potential hazards, but only one of them is the least likely. Let’s look at each one in turn.
Answer A is the condensation of
liquid oxygen. When used correctly a Schlenk line
is unlikely to condense liquid oxygen, but it can happen if there’s an air leak in
any of the system. If air gets into the vacuum line,
perhaps with a leak in your flask, or because one of the joints has a leak, this
means we risk condensing oxygen. As the air travels through the
vacuum line and into the cold trap, the boiling point of oxygen is higher than that
of liquid nitrogen. And this means that the oxygen can
easily condense. If this happens, you’ll notice a
pale blue liquid inside your cold trap.
When using a Schlenk line, you
should always know what to do in case of liquid oxygen condensation. If the oxygen is allowed to
vaporize, there’ll be an enormous increase in pressure, which could cause an
explosion. To prevent the condensation of
liquid oxygen, you should never open your Schlenk line to air while the vacuum pump
is on if there is also a liquid nitrogen dewar on the cold trap. So, while answer A is thankfully
fairly uncommon, it does happen. So, it’s not a correct answer to
our question.
Let’s look at B, breakage of a
flask during attachment. In order to attach a reaction flask
to your Schlenk line, you’re going to need to insert the opening of the flask into
some rubber tubing. Remember that this flask is going
to be exposed to vacuum, so this tubing is going to be thick walled. This could make it quite stiff and
difficult to attach to flasks. There is always the risk that when
forcing glassware into rubber tubing, you’re going to cause a crack.
There’s a delicate balance between
the force required to push the flask into the tubing and too much force, which could
create a break. This risk is even higher if the
glassware is not in good condition. For example, it may already have a
crack or a chip. So, Answer B is definitely a
potential hazard of working with a Schlenk line. And this means it is not the
correct answer. Remember that it’s good practice to
check all glassware for any kind of cracks when you’re attaching it to equipment,
particularly equipment which uses vacuum.
Now, let’s look at C, implosion of
the vacuum line. Remember that an implosion is the
opposite of an explosion. When glassware implodes, it
collapses violently in on itself. In order for this to occur, there
needs to be a force pulling the glassware inwards. A strong vacuum is an ideal source
for this kind of force. Just like for flasks, if there’s
any form of damage to the vacuum line manifold, it can easily implode on itself
under the force of the vacuum pump. Again, it’s good practice to always
check your glassware carefully for any kind of crack or damage. So, the implosion of the vacuum
line could be a hazard if there’s any kind of damage to the glassware, so this is
not our correct answer.
Now, let’s look at D, implosion of
the nitrogen line. Remember that for glassware to
implode, it needs a force to draw the glass inwards. When the gas line is open, for
example, we have nitrogen gas flowing. This gas will flow down the back
side of the manifold and out of the oil bubbler. The oil bubbler ensures that there
is not an increase in pressure as the gas enters the manifold. Of course, a buildup of pressure
and gas inside the system would cause an explosion rather than an implosion. As you can see there’s no real
source of a force pulling inwards along the gas line. This means that it’s very unlikely
that the nitrogen line will be able to implode. So, this could definitely be an
answer to our question.
Let’s check the last answer just to
be safe, E, suction of oil into the nitrogen line. The oil inside the oil bubbler is
there to create a one-way valve, allowing the gas from the gas line to bubble out
but stopping air from getting back in. If the flow rate of the gas is too
high, it can blow oil out of the oil bubbler. This obviously makes quite a mess,
but can also mean that your reaction is exposed to air.
What this potential answer is
talking about is the opposite suction of the oil into the nitrogen line. To consider whether this is likely,
let’s look at pressure. Outside the system, we have
atmospheric pressure. If we increase the pressure in the
gas line, it blows the oil out. However, if we decrease the
pressure inside the gas line, this could potentially cause the oil to suck back
in. This lowered pressure in the gas
line could be due to a temperature change, or it could be due to vacuum. Either way, the oil gets sucked
back into the manifold and creates an even bigger mess. So, the last answer is a potential
hazard when working with a Schlenk line.
So, of these five answers, the
least likely hazard is D, implosion of the nitrogen line. While all these hazards of using a
Schlenk line might seem scary, they’re actually amazing pieces of equipment that can
allow you to do all kinds of chemistry.
