What is solid-liquid extraction?
Soxhlet extraction (or solid-liquid extraction) is used for the isolation of compounds present in a solid, using a liquid solvent. For example, extraction of active principles (medicinal principles) from plants, in natural products chemistry.
Although, extracting a compound from a solid with a liquid can, in principle, be done by simple extraction and subsequent filtration, however, for this process to be effective it would require many tedious extraction and filtration steps.
Therefore, this equipment was designed to isolate a compound within a solid by means of a continuous extraction using a very efficient technique. With Soxhlet equipment, multiple extractions are performed automatically (continuously), using the same solvent, which is evaporated and condensed, recirculating in the equipment (recycled).
The Soxhlet equipment consist of the following elements:
- Reflux water-cooled condenser.
- Soxhlet cartridge with the solid to be extracted.
- Soxhlet column extraction where the cartridge is inserted with the solid to be extracted, and has a siphon through which the solvent falls after extraction to the flask.
- Round-bottom flask 3 or 4 times the size of the solvent chamber.
The solid to be extracted (seeds, plant leaves, etc.) is placed in a cartridge made of filter paper. There are also cellulose ready-to-use cartridges on the market. Once the solid has been introduced, it is placed inside the extraction column. The flask is filled approximately halfway with solvent, boiling chip is added and heating is begun.
When the solvent (with which the solid is to be extracted inside the Soxhlet cartridge) is heated, it evaporates and rises up the extraction column to the condenser area where it cools and condenses, falls back drop by drop onto the cartridge and extracts the active principle from the solid.
Through the siphon and when a sufficient solvent level is reached, the soluble part of the components to be separated from the solid falls by gravity into the solvent chamber.
When the solvent is heated again, it evaporates pure and again again more compound to be extracted. The process is repeated automatically until the extraction is considered complete.
Video about Solid-liquid extraction
References and notes
 Franz Ritter von Soxhlet (1848-1926) was the German chemist who invented the Soxhlet extractor in 1879, and in 1886 proposed that the pasteurisation process be used for milk and other beverages.