Mini-distillation - setup and running

The following notes describe how to setup and run a distillation using a mini-distillation kit. A mini-distillation kit is suitable for the purification of between about 5 and 20 mL of liquid. The same principles apply to other distillations, modifying the procedure to make it appropriate for the scale required and glassware available.

Notes about grease

As you assemble the glassware consider whether there is a need to use a small amount of grease on joints. Too much grease may result in contaminating your product, but grease may be essential to enable glassware to move (e.g. the pig). For vacuum distillations, all joints will need to be greased.

Clamping the distillation flask

Start by setting up a stirrer-hotplate on a lab jack and clamp the distilling flask with a small stirrer bar above it. Either the 50 mL or one of the 25 mL flasks can take the place of the distilling flask depending on the scale of distillation and should be securely clamped around the neck above the oil bath/hotplate/lab jack.  You should configure the lab jack and hotplate/oil bath in such a way that the lab jack can be fully lowered to remove the oil bath from being in contact with the flask, and that you can raise the oil bath sufficiently to have the oil in contact with as much of the flask as possible. Note: it is usually best to avoid putting the oil bath in place until you are ready to begin the distillation.

Next, add the still-head to the round bottomed flask and into the top of this place the quickfit thermometer and check that the thermometer bulb sits at the head of the glass joint. Remove the thermometer again, and keep it carefully to one side – this will be returned later.

Attaching a condenser

In order to support the condenser a clamp needs to be set up to the side of the still head, approximately half way down where the condenser will be. Set this up in such a way that the side of the clamp arm that moves is on the top, and the fixed side on the bottom. This allows the bottom to support the condenser properly, and the top to be loosely tightened to hold the condenser.

Set up the tubing onto the condenser and clip the condenser onto the side arm from the still head using a Keck clip. The condenser should rest on the clamp you set up previously, and the top of the clamp should be loosely tightened, so that the condenser can’t fall out of the clamp, but should not be tight against the glass wall of the condenser as this is very fragile. The condenser tubing should be attached so that water enters the condenser at the lower end and exits at the uppermost point.

Adding the pig

Set up a second lab jack just below the end of the condenser. Fit the pig onto the end of the condenser and secure with another Keck clip. Onto the outer two arms of the pig fit 10 mL round-bottomed flasks and a pre-weighed 25 mL flask to the middle arm using Keck clips to secure each flask. Ensure that the pig can rotate about the join to the condenser.

Use the second lab jack to support one of the 10 mL flasks in place on the pig. The idea of the pig is that you can rotate around the flasks during the course of the distillation. The first 10 mL flask will collect any material that distils below the desired product (in this case, some residual diethyl ether is likely to be collected) along with the first couple of drops of product. The pig is then rotated so that most of the product is collected in the 25 mL round-bottomed flask (the collection flask), then just before the flask boils dry, the pig is rotated to the final flask and the oil bath immediately lowered. The aim of a good distillation is to end up with pure product, whilst sacrificing only a small quantity of material in the process.

Attaching vacuum

If the distillation is to be carried out under reduced pressure the vacuum attachment on the pig will need to be connected to the vacuum line. Slide a piece of vacuum tubing onto the vacuum attachment and attach the other end to a vacuum line.

Final checks

At this stage you should satisfy yourself that you are confident you know how the distillation will work. Connect up the condenser tubing to the tap and sink and put the oil bath in place. Before continuing you must get your setup checked by a demonstrator.

Running the distillation

Once you have had your setup checked, carefully add your crude product into the distilling flask using a Pasteur pipette to transfer the solution via the neck of the still head where the thermometer goes. Once this is added, reseat the thermometer in the joint. Turn on the water to the condenser. If the distillation is to be carried out under vacuum, slowly open the system to vacuum and ensure there are no leaks.

Raise the oil bath around the round-bottomed flask. The oil level should be raised as deep as possible around the flask, without the base of the oil bath hitting the bottom of the flask, or the top of the oil bath hitting the clamps. Also place a 300 °C thermometer into the oil bath so that you can monitor the temperature of the oil during the distillation. Turn on the stirring and begin heating the solution gently.

Lower boiling impurities

The thermometer in the oil bath should gradually rise during the course of the distillation, but the vapour thermometer in the still head will only show an increase in temperature if there is vapour in the still head. This will only occur when something is boiling off from your distillation flask. There is a lag between the oil bath temperature and the actual temperature in the distillation, as the glassware will lose heat to the environment and take a while to heat up. If you are distilling reactions or heavily contaminated materials, you may see lower boiling compounds distil across (especially reaction solvents), when all of a given component has distilled, the vapour thermometer will drop back down to room temperature, as there will no longer be any vapour to keep the thermometer warm.

Collecting the desired fraction

As you continue to heat the oil bath, you will slowly reach the boiling point of the product and start to see vapour condensing in the still head again. Allow a few drops of the product to collect in the 10 mL round bottomed flask and take a note of the vapour temperature at this point (this is that start of the distillation temperature range or ‘boiling point’ of your product), then rotate the pig so that the bulk of your product collects in the middle 25 mL round-bottomed flask. Watch the level steadily reduce in the distilling flask and collect into the collection flask.

Finishing a distillation

Just before the distillation flask runs dry, note down the final distillation temperature range of your product, rotate to the third flask, lower the oil bath and switch off the heating on the stirrer hotplate. Allow the glassware to cool down and then carefully remove the product flask and weigh this.

If the neck of the collection flask is greased, you will need to carefully pipette out the product into another vessel to weigh, taking care not to contaminate any of the product with grease.

Once the glassware is entirely cool, dismantle and wash up glassware.