Lesson Video: Schlenk Line Techniques | Nagwa Lesson Video: Schlenk Line Techniques | Nagwa

Lesson Video: Schlenk Line Techniques Chemistry

In this video, we will learn how to use a Schlenk line or vacuum manifold, assessing the hazards of certain methods and common errors.

17:48

Video Transcript

In this video, we will learn about Schlenk line techniques. Schlenk lines were developed by German chemist Wilhelm Schlenk. His work on air- and moisture-sensitive compounds prompted the development of the Schlenk line.

Let’s begin by looking at what makes up a Schlenk line. To the left of our apparatus, we can see the dual manifold. This carries inert gas down one side and vacuum down the other. The inert gas is commonly nitrogen. But for reactions which are sensitive to nitrogen, argon can also be used, though of course this is more expensive.

On the front of the dual manifold, there are multiple arms with three-way taps. Attached to each of these is a piece of tubing which can connect to the reaction vessel. On the right of our inner gas line, we have tubing leading to an oil bubbler. This provides a vent for the reaction flask and acts as a one-way valve, allowing nitrogen out but nothing back in. The oil bubbler can also be used to monitor the flow rate of the gas line.

To the right of the vacuum part of the manifold, we have a cold trap. This uses liquid nitrogen to condense any gases drawn through the vacuum line before they reach the pump. This protects the vacuum pump, which is attached to the right of the cold trap. These are the main components of a Schlenk line, but each one is unique. For example, some may have two cold traps or fewer connection points or it could be built the opposite way round.

Notice that there are taps installed between each feature, which allows for different sections to be safely isolated. Let’s label these taps now. Let’s look now at how to turn the Schlenk line on. It is hugely important that a Schlenk line is turned on and off in the correct order. Mistakes in this process could affect your reactions or even cause a safety risk. It’s important to take your time.

The first step is to check your equipment. Look for any damage or cracks and check that all taps are properly greased and move freely. If they are sticky or need regreasing, do this now. Next, we’ll set up the gas line. Open tap A and ensure that taps B to D and tap F are closed. Next, open the gas supply. For the purposes of this, we’ll assume it’s nitrogen. If you’re using a cylinder, ensure that the contents of this cylinder are sufficient for the whole reaction. Then adjust your gas flow rate. Ideally, you want one bubble per second coming out of the oil bubbler.

Now we need to leave the gas line to purge for at least 20 minutes. You can increase the gas flow rate to make this quicker. But if you do so, remember to decrease the flow rate again once complete. And be careful not to blow the oil out of the oil bubbler.

While this is purging, we can set up the vacuum side. First, we will need to prepare the cold trap. You need to check that it’s empty, clean, and dry. Grease the trap well and remember to clip the trap in place. Never rely on vacuum to hold equipment together. Next, we can position an empty Dewar flask underneath the trap. You want to position this flask so that the top is approximately one to two centimeters below the bottom of the trap’s joint.

Now we are able to open taps E and F and turn on the vacuum pump. Now we need to leave the vacuum pump to warm up and evacuate the whole of the vacuum side of our manifold. This should take between three and five minutes. Once we’re happy that the vacuum side of our manifold has been evacuated, we can fill our Dewar with liquid nitrogen. Never carry out this step if there is any evidence of air leaks or any of your taps are open to the air. Cover the top of your Dewar flask with a towel or foil and remember not to wear nitrile gloves when handling liquid nitrogen.

You are now ready to begin working with the line. Let’s look at how to attach a flask. There are a variety of flasks which are suitable for use with a Schlenk line, the most common of which is probably the Schlenk tube. However, round-bottomed flasks with side-arm attachments are also very useful. To connect tubing to glassware, gentle pressure is needed and a wiggling or rocking motion. Avoid a twisting motion. Wiggling provides gentle force, while twisting puts a strain on the side arm. To remove the flask, you can apply gentle pressure on the glassware and wiggle the tubing off with the other hand. Again, try to avoid twisting movements.

Now that we’ve attached a flask, we need to look at what to do next. Before we can begin work, we need to perform the evacuation/back-fill cycle on our reaction flask. This is very important as we’re going to be working with air-sensitive chemistry.

The first step is to evacuate our reaction vessel. To do this, we open the flask to vacuum. In this diagram, it means turning tap B to the vacuum line. As you do this, you will hear a noise change from the vacuum pump. How long it takes to carry out this step depends on the size of your reaction vessel, but it’s usually roughly 10 to 30 seconds. Once the flask is fully evacuated, the vacuum pump noise will go back to normal.

Evacuating our flask has removed any reactive atmosphere. Now we can replace it with an inert atmosphere. To do this, we change tap B over to the gas line side. This step needs to be done very carefully and gently. If you open the tap to the gas line too quickly, you will end up sucking oil from the oil bubbler back into the manifold. As you carefully open your vessel to the gas line, keep an eye on the oil bubbler. If the oil begins to rise up the inner tube, close the tap and reopen it more slowly. Keep doing this until the bubbler flow rate is back to normal and you can fully open the tap.

Now you will need to repeat this whole process again at least twice. Evacuate the vessel using the vacuum line and then refill with inert gas. Let’s take a closer look at the three-way taps to see how these work. Three-way taps can be in one of three positions.

In one position, the tap can open the flask to the vacuum line but not to the gas line. This evacuates our flask. In another orientation, our flask can be opened only to the gas line. Finally, the tap could also isolate the flask completely. Make sure you are familiar with how these work. Also, remember that larger vessels take longer to back-fill, and this can be trickier to do without sucking back oil. If you do suck oil back in, allow the gas line to purge for a few more minutes before continuing. Ensuring that all of our taps move freely is key when using the Schlenk line.

Let’s have a look at how to correctly grease them. First, be careful not to overgrease your joints as this can contaminate the reaction. Apply two thin strips of grease on opposite sides of the male joint. Then insert it into the female part. Once inserted, gently rotate to evenly distribute the grease. Be careful to only rotate in one direction as rotating in both directions will break the seal. You should create a clear, continuous film between the two surfaces.

Once we finished working with our Schlenk line, we need to know how to correctly shut it down. Shutting down the line is essentially the reverse of setting the line up. First, ensure that all of your flasks which are attached to the line are under nitrogen and the taps are either closed or open to the nitrogen line. Never leave reaction flasks under vacuum. Next, switch off the vacuum pump and immediately remove the liquid nitrogen Dewar. Once the liquid nitrogen has been removed, it is safe to vent the vacuum line by opening tap F. Next, you are able to turn off the gas line.

If you need your reaction vessel to remain open to the nitrogen line, you can leave this on. But just reduce the flow rate. Next, remove the cold trap, being mindful of the temperature. This can cause freezer burns. Finally, allow any solvent caught in the cold trap to thaw and dispose of it correctly. If you’re going to leave your equipment for a while, write down what the solvent was so that you remember.

Now that we’ve learned how to correctly use our Schlenk line, let’s have a look at some of the common problems. One of the first problems you might encounter when using a Schlenk line is incorrect Dewar flask position or fill level. The liquid nitrogen level should be just below the joint. If the liquid nitrogen is above the joint, it can freeze the grease, breaking the seal and making it hard to remove. If the level is too low, you risk incomplete condensation, which could damage your vacuum pump.

Another potential hazard, though thankfully less common, is condensation of liquid oxygen in your cold trap. This will result in a pale-blue liquid forming inside your cold trap. This is usually a result of an air leak along your vacuum line or from opening the vacuum line to air while the Dewar was in place. The hazard here is that as the liquid oxygen warms up, it will rapidly expand and can easily cause an explosion. It is also highly reactive. If this occurs, remove the liquid nitrogen and immediately vent the vacuum line. Never remove the liquid nitrogen without venting the line. Then you’ll need to leave your apparatus in the fume hood with the sash pulled down for 20 to 30 minutes. And warn colleagues to keep their distance. Check to see if it’s all gone and then work can resume. Remember also to keep your liquid nitrogen level constantly topped up. It should never run empty.

Explosions are another potential hazard, usually caused by gas buildup with no vent, for example, by heating a closed system or an out-of-control reaction. Always ensure that vessels are open to the nitrogen line while heating and that there is a gas bubbler to prevent the pressure buildup.

Finally, we have implosions. Star cracks or damage to any of the components which are exposed to the vacuum can lead to an implosion. This is why checking your glassware before beginning work is so important.

Now let’s look at a couple of potential questions.

When shutting down a vacuum line, in which order should the following actions be performed? (1) Turn off the vacuum pump. (2) Remove the Dewar of liquid nitrogen. (3) Vent the vacuum line.

The first step in shutting down a vacuum line is always to turn off the vacuum pump. If you were to vent the vacuum line with the pump still on, you’re simply going to draw air through the pump. Where still if you have the Dewar of liquid nitrogen still in place, you’re going to draw air through your cold trap. Drawing air through a cold trap is a great way to condense liquid oxygen, which is a real hazard, so we want to avoid that.

For that reason, the next step is going to be to remove the liquid nitrogen. This must be done before you vent the vacuum line to the atmosphere. This means that venting the vacuum line is our last step. So our answer is (1), (2), and then (3).

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).

Let’s summarize what we’ve discovered about Schlenk line techniques. Schlenk lines allow you to put reaction vessels under inner atmosphere or vacuum. We’ve also seen that turning on and shutting down our Schlenk line has a strict order of actions. When turning on our Schlenk line, we first set up the gas line and then the vacuum line. To shut it down, we do the reverse.

After attaching a new flask, the evacuation/back-fill cycle should be carried out at least three times. And finally, Schlenk lines can be hazardous. For example, there’s a risk of explosions, implosions, condensing liquid oxygen, burns, or breakages. Luckily, when handled carefully, Schlenk lines are wonderful pieces of equipment in order to carry out air-sensitive or moisture-sensitive reactions.

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