Miscellaneous: Sulphur purification

Sulphur is a chemical element which has many different applications. Among other things, it is known to be a useful catalyst in various types of reactions. In such cases, the sulphur should be of high purity. Common sulphur, which can be bought in certain types of stores, can't be used since it contains various impurities. In the case of this purification experiment, we used sulphur containing pellets. Such pellets are primarily used in viticulture for the disinfection of wine barrels.

Along with the source of sulphur, we needed to prepare a solvent, an electrical hot plate, distilled water and some basic laboratory glassware. In theory, it would be best to use carbon disulphide as the solvent, since sulphur is highly soluble in this compound. However, since carbon disulphide is a fairly hazardous substance, it is simpler, cheaper and safer to use some of the other solvents which also dissolve sulphur. Some of these are benzene, toluene, xylene, cyclohexane, trichloroethylene and tetrachloroethylene. In this case, trichloroethylene has been used. At temperatures above 80 °C, 100 grams of trichloroethylene dissolve approximately 9 grams of sulphur. Most of the sulphur crystallizes upon cooling.

First, the sulphur pellets were crushed until a fine powder was obtained. This powder was then added to a larger amount of water, which was heated until the boiling point was reached (above image, to the left). In this step, adhesives and other compounds soluble in water, are being dissolved, while the sulphur stays undissolved. After several minutes, the hot solution is filtered. This way the still impure sulphur is separated from the rest of the impurities which dissolved in water (above image, to the right). Afterwards, the retentate was dried.

The dried retentate was added to 80 mL (117 g) of trichloroethylene. The temperature of the solution was then brought up to its boiling point. After that, the solution was filtered, whereby the solution of sulphur in trichloroethylene becomes separated from the rest of the impurities. On the image above, to the right, one can see the solution after filtration. Some of the sulphur already crystallized at that time. After the solution has cooled down to room temperature, it was further cooled to -23 °C. At that temperature, most of the sulphur crystallized.

After cooling, the solution was filtered once again. That time, the retentate was sulphur of very high purity, which was dried afterwards. The result can be seen on the image above. The remaining trichloroethylene can be used again for the purification of additional amounts of sulphur.