How to Purify Sulfur by Recrystallization With Xylene

Intro

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This is a comprehensive guide to the purification of elemental sulfur by recrystallization with xylene. Feel free to check out the above video to see it in action.

Outline

1. Safety precautions and disclaimer
2. The purpose of this experiment
3. Measuring out the reagent
4. Adding Sulfur to Xylene with stirring and heating
5. Reagent to be used
6. Advantages of using Xylene over Toluene
7. Ideal solvent for recrystallization
8. Use of filtration in recrystallization
9. Tips and tricks for filtration
10. Different crystal forms and allotropes of Sulfur
11. Repeat the experiment with a lower temperature to show a different form of crystal
12. Cooling the solution to allow crystallization
13. Calculate the yield
14. Bottle up the product

1. Sulfur powder - 9 grams
2. Xylene - 75mL
3. Conical flask - 250mL
4. Thermometer
5. Hotplate

Safety precautions and disclaimer

All experiments should be carried out with extreme caution, please read till the end to fully understand this experiment before proceeding.

1. Xylene is highly flammable and harmful by inhalation and in contact with skin, it is best to conduct this experiment inside a fumehood or in a well-ventilated area.
2. A hot solvent is involved, and its vapour can easily ignite, so never use an open flame in this experiment or you will risk serious fire hazards.

The purpose of this experiment

Recrystallization is a common purification technique that can purify solids by dissolving impure solids in a hot solvent and then crystallizing out relatively pure solids when the solution is cooled.

Sulfur is one of the solid elements that is very handy to use in this experiment, to showcase the procedure of recrystallization.

Measuring out the reagents

1. To begin the experiment, 9g of sulfur powder is measured out, followed by 75ml of xylene.

This amount corresponds to the solubility of p-xylene at 87°C,
which is the lowest solubility for sulfur among all xylene isomers.

Adding Sulfur to Xylene with stirring and heating

1. Start the setup by adding a stir bar to a conical flask, sulfur powder is then added to a conical flask followed by xylene.
2. Add a thermometer to monitor the temperature. Ideally, it shouldn't touch the bottom as it will provide less accurate readings.
3. The top is covered by aluminium foil to minimize the vapour escape while maintaining an open system. You do not want to use a glass stopper here as heating a closed system will risk explosion.
4. Place the flask on a hotplate, turn on the heating to 150°C and stir, sulfur powder should slowly dissolve into the solvent as the temperature rises, forming a bright yellow solution.

The actual temperature of the solution inside is much lower than the output temperature of my hotplate,
different models may vary.
The temperature of the solution should not exceed 130°C though as it is approaching xylene's
boiling point.

Choice of reagents

Sulfur is commonly available in garden stores as a fertilizer or fungicide, it can also be easily purchased online.

Depending on the location of your purchase, purity varies so you may or may not need to purify it before using it for any chemical reaction.

While sulfur isn't soluble in most organic solvents, it is very soluble in carbon disulfide and somewhat soluble in toluene, benzene and xylenes.

Carbon disulfide and benzene are highly toxic, expensive and also hard to obtain, so it is out of the question. Sulfur is also very soluble in room-temperature carbon disulfide, which makes it a bad candidate for recrystallization anyway.

So the remaining candidates are toluene and xylene, here I pick xylenes for this experiment.

Advantages of using Xylene over Toluene

Firstly, it is cheaper than toluene and readily available in hardware stores, it is available as a relatively pure solvent, the only constituent is xylenes, which is the mix of all three isomers of dimethyl benzene and nothing else.

But more importantly, it has a significantly higher boiling point than toluene, therefore I can utilize a higher temperature to dissolve the sulfur.

Sulfur solubility is similar for both xylene and toluene at the same temperature, however as xylene can be heated significantly above the boiling point of toluene without boiling, more sulfur can be dissolved in the same amount of solvent.

Ideal solvent for recrystallization

The ideal solvent for recrystallization is a solvent in which the solubility of the solute is low when the solution is cold, and high solubility when the solution is hot, giving a high difference in solubility at different temperatures.

In this case for solubility of sulfur is only about 30g per litre of xylene at 30°C, and 170g per litre at 87°C according to literature, the large difference makes xylene one of the best solvents used for sulfur recrystallization.

I have included the links for several papers regarding sulfur solubility in different organic solvents in the references section, feel free to check them out if you want to learn more about them.

Use of filtration in recrystallization

After all sulfur powder is dissolved or no longer dissolves, it would be optional to carry out a hot gravity filtration to filter out any insoluble impurities or excess sulfur.

In my case, the sulfur is pure enough to not leave any insoluble impurities and everything dissolved, so there is no need for filtration.

However many non-lab grade sulfur may contain impurities including clay, calcium sulfate or calcium carbonate etc, so you might need to perform a filtration in case there is anything insoluble in the solution.

A good idea for hot gravity filtration is to warm up the filter paper and funnel as well,
to minimize sulfur powder crushing out during filtration.
Also do not use fritted glass or vacuum filtration, as sulfur powder crushing out during the filtration
will likely clog the filter, making it extremely difficult to clean after.

Different crystal forms and allotropes of Sulfur

The sulfur crystal formed from crystallization can vary in different crystalline forms which correspond to different allotropes, and the formation largely depends on temperature control while dissolving the sulfur.

β-sulfur formation is favoured when sulfur is heated above 95.3°C while dissolving in solution, and cooling down rapidly to minimize the formation of α-sulfur.

Repeat the experiment with a lower temperature to show different forms of crystal

To obtain the alternative form of sulfur, the α-sulfur, which has a rhombic form of crystal,

ideally, the solution should not exceed 95.3°C while dissolving the sulfur, and the solution should be allowed to cool slowly.

Cooling the solution to allow crystallization

After all sulfur crystals dissolve, the remaining solution is allowed to cool.

Rhombic crystals of sulfur will start forming as it cools, you will see the formation of crystals is much shorter in length and look much more like a chunk than a needle.

An ice bath can be used to chill the solution to crushing out more sulfur, but I think it is optional and
will just let it cool to room temperature, as excessive cooling not only increases the yield,
it will also decrease the overall purity of sulfur crystals.
You may also be tempted to use a refrigerator to cool the solution,
but don't do it as most refrigerators aren't solvent-proof or explosion-proof.

Collect the product and calculate the yield

Once the solution reaches room temperature, carefully decant the xylene to a bottle and store it for another recrystallization of sulfur.

Transfer the remaining sulfur crystals to filter paper and allow the xylene to evaporate.

The dried sulfur will be in free-flowing form.

Weight the final mass and calculate the yield.

In my case, I am getting a yield of 80%.

Bottle up the product

Store the dried sulfur powder in an air-tight container, and label it properly.

Self-reminder: Label everything (immediately), it is a good practice.