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 Soxlet 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 Soxlet 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 Soxlet 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 nescessary, although there is an energy cost in heating them. A Soxlet 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.

Glassware setup

The exact setup will vary depening what teh Soxlet is required to do, but in general the follwoing steps apply:

1. Select an appropriate sized round-bottomed flask, Soxhlet, condenser and heat source.

2. Securely clamp the round bottomed flask containing solvent and stirrer bar or anti-bumping granules over the heat source. It is usually desirable to be able to remove the heat source at the end of the reaction, for example using a lab jack.

3. Fit the Soxhlet extractor to the round bottom flask and securely clamp in place around the top ground-glass joint.

4. Add the thimble containing the solid to the Soxhlet extractor. If there is an internal standard to add then this should be added to the thimble. Note that the thimble is porous, so if any liquids (eg standards which are liquids or solutions), ensure the thimble is in the extractor before adding these to avoid losing any.

5. Add the condenser to the top of the Soxhlet extractor.

6. Turn on water to the condenser and start the extraction.

7. Once the extraction is complete, turn off/remove the heat and allow to cool.

8. The glassware is dismantled in the reverse order to addition (condenser removed first, then the Soxhlet and finally the round-bottomed flask).

9. Liquid from the extractor can be added to the round bottomed flask which contains the extracted material.

Isolation is usually acheived by rotary evaporation, but will depend on what further steps are required and the volatility of any compounds isolated. Note stirrer bars or anti-bumping granules may need to be removed.