Objectives

This tutorial aims to familiarize you with the following aspects of vacuum sublimation:

• When to perform vacuum sublimation
• The physical basis of vacuum sublimation
• Proper experimental technique

When to Perform Vacuum Sublimation

Sublimation is a separation technique used to purify a solid chemical substance. It is commonly called vacuum sublimation because it is usually carried out at low pressures. In sublimation, a solid is heated in a flask under vacuum. This method of heating usually makes the solid sublime directly into a gas without first melting into a liquid. The gas is then condensed on a cold surface, which causes the solid to re-form. Impurities, which are usually of heavier molecular weight and do not evaporate as easily, are left behind on the bottom of the flask. The solid can be scraped off the cold surface for further reactions or analysis.

A vacuum sublimation apparatus.

Sublimation is the easiest way to effectively separate a solid substance from other solid impurities, as long as those impurities have a vapour pressure (a property related to the boiling point) that is sufficiently different from the desired product.

If the above condition is met, vacuum sublimation should be used to purify any synthesized solid product. If the vapour pressures of the desired product and its impurities are too similar, other preparative separation techniques should be used, such as filtration or even column chromatography after dissolving the solid product in an appropriate solvent system.

The Physical Basis of Vacuum Sublimation

What usually happens to a solid substance when we heat it in the open air? With sufficient energy input, we'd expect the solid to melt to a liquid, and with even more vigorous heating, the liquid will evaporate to a gas. This is the case with most materials. We do not usually expect solids to turn directly into gases, which is exactly what sublimation is.

However, if you've ever observed the behaviour of dry ice, you have seen sublimation in action. Instead of melting to a liquid, the super-cold white chunks of solid carbon dioxide give off "steam" - which is actually the solid CO₂ subliming directly into a gas. Yet CO₂ seems unique among commonly observed substances in this ability to bypass the liquid phase, while sublimation is a highly versatile purification technique. How to get other solids to do the same?

Dry ice - solid carbon dioxide. [http://www.hypertextbook.com]

Before we arrive at the answer, let's briefly explore the concept of phase diagrams. These are very convenient summaries of the physical states of substances at varying temperatures and pressures. By looking at the phase diagram of a substance, we can predict whether it will sublime under open air, and if not, the conditions necessary to force a sublimation.

A phase diagram plots the physical state of a substance as a function of temperature and pressure. Pressure is plotted on the vertical axis, and temperature is plotted on the horizontal axis. Shown below is a sketch of the phase diagram for carbon dioxide (dry ice when solid).

On the diagram above, room temperature and atmospheric pressure are indicated on the axes. It's quite easy to see what happens when solid carbon dioxide is taken out of a freezer and put out onto a lab bench. Simply follow the blue arrow to the right along the temperature axis, while the pressure stays constant at one atmosphere. As it is heated, solid carbon dioxide bypasses the liquid phase and sublimes directly into a gas. Note also that if carbon dioxide were to be heated under a higher pressure (equivalent to shifting the blue arrow upward), it would behave like the substances we see more often, going through a liquid phase before turning into a gas.

Now, let's consider the phase diagram of a more typical solid such as the caffeine that you'll be purifying. A sketch is shown below.

We can see what happens when we make an attempt to heat caffeine at atmospheric pressure. The blue line parallel to the temperature axis at 1 atm of pressure indicates that the caffeine will first melt into a liquid and only then evaporate to a gas. This is unacceptable if sublimation is desired, and this is reason that reduced pressures are used in sublimation. At lower pressures, substances will transition directly from a solid to a gas without passing through a liquid phase: exactly what sublimation sets out to accomplish. The green line shows what happens when caffeine is heated under reduced pressure: the solid sublimes into a gas.

This is the rationale for using a vacuum in sublimations. Also, you will note that substances sublime at lower temperatures as pressures are lowered. Therefore, lowering the pressure also reduces the risk that a substance will be destroyed by intense heat during a sublimation.

Once the substance is sublimed into a gas and its vapours become isolated from the solid impurities in the original sample, the clean vapours must be again converted into a solid. This is done simply by reversing the green arrow shown in the phase diagram above. A cold surface, usually the outside of a test tube that is filled with ice on the inside, is used to recover the solid. Particles in the vapour phase strike the cold surface, cool, and return to the solid phase, completing the purification. In the end, the pure compound has migrated to the cold surface, whereas the impurities are left behind on the starting hot surface.

Question 1

Not all of the sublimed particles strike the cold surface and return to the solid phase. There is a significant likelihood that some will escape the system altogether. How is this possible? Think about the set-up of the sublimation apparatus.

A summary of the physical transformations induced during sublimation.

Proper Experimental Technique

The Flash interactive walk-through below will show you how to properly use a vacuum sublimation apparatus.

Question 2

Is vacuum sublimation, as described, a quantitative technique? Can you get four significant figure accuracy for the yield of a substance by performing the above procedure?

Summary

This tutorial has presented the following topics:

1. The advantages of vacuum sublimation as a preparative technique.
2. The physical properties of substances that allow sublimation to take place.
3. The correct experimental technique for carrying out sublimation.