Principles of a Soxhlet extraction

A flask containing solvent is heated to vaporise the solvent which runs along the sidearm of the Soxhlet until it reaches the condenser. The vapours condense and drip into the thimble in the Soxhlet extractor which should contain the material to be extracted. Ideally, the material to be extracted should be ground as fine as possible to enable the desired compound to easily be removed. Over time, sufficient solvent will have evaporated from the flask and condensed to fill the Soxhlet extractor. Once the Soxhlet is filled, the liquid contents will empty back into the solvent flask along with any compounds which are dissolved in the solvent. Insoluble solids are left behind in the thimble.

The process will continue to cycle, with the solvent evaporating from the flask and refilling the Soxhlet. Over time all the soluble component(s) will be extracted into the round-bottomed flask. The compounds can then be isolated by rotary evaporation.

One of the most common uses for Soxhlet extraction is to obtain compounds from natural products, eg extracting compounds from leaves, berries, seeds or fruit.

Points to consider

• The size of flask and amount of solvent needs to be larger than the capacity of the Soxhlet to avoid boiling the solvent flask dry and to keep the extracted components in solution whilst the Sohlet is running.

• The choice of solvent is crucial for the success of the extraction. The desired component(s) need to be soluble (even if the solubility is limited) and unwanted components must be totally insoluble.

• Crushing, grinding or chopping the solid for extraction greatly reduces the time required for extraction. The solid is held in a thimble which is usually made of cellulose. If you are wanting to quantify the amount of something in the extract, the thimble may also have an internal standard added.

• Soxhlets can be run for as long as necessary, although there is an energy cost in heating them. A Soxhlet extraction might last an hour or could be allowed to proceed for may days.

• The heat source needs to be capable of heating solvent vapours from the flask and up the side arm of the Soxhlet. Isomantles are often used for this purpose.

• In order to cool the solvent vapours in the condenser an efficient condenser is required.