Video: Hazards when Working with a Schlenk line

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 [E] suction of oil into the nitrogen line

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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.

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