Soxhlet extractors: how can exhaustive and reproducible solid-liquid extraction be performed?

Indispensable for solid–liquid extraction, the Soxhlet extractor continuously concentrates the solvent onto the sample matrix. It delivers exhaustive, reproducible recovery of lipids, organic pollutants and additives. Built from borosilicate glass 3.3, these systems are chosen by volume, thimble size, condenser type (Allihn, Liebig, Dimroth) and the compatibility of ground joints. Used with a flask heater or heating mantle, the Soxhlet remains a dependable, standards-compliant solution that is efficient and straightforward to set up.

What are the different types of Soxhlet extractors and how do they work?

The principle is simple yet ingenious: the solvent heats in the flask, vaporises, condenses and percolates hot through the cartridge containing the sample. When the chamber fills, the siphon triggers and returns the extract to the flask. This repeated cycle delivers an exhaustive extraction without manual solvent replenishment.

Typical kits include a flask (100 to 1000 mL), a Soxhlet body matched to the thimble diameter (often 19, 25, 33, 41 or 60 mm), a condenser (Allihn for higher efficiency, Liebig for modest throughputs, Dimroth for volatile solvents) and standard ground-glass joints (NS 24/29, 29/32, 45/40). Thimbles are available in cellulose, glass fibre or PTFE depending on chemistry and filtration fineness.

Useful variants:
• Conventional Soxhlet with discontinuous cycles, robust and all-purpose.
• Twisselmann (continuous hot extraction), permanent percolation above the thimble to speed up dense matrices.
• Randall/Soxtec (semi-automated) combining hot immersion, rinsing and solvent recovery to cut time and consumption.
• Micro Soxhlet for limited samples and costly solvents.

Selection criteria: sample mass and flask volume, thimble diameter and height, nature of the solvent (hexane, petroleum ether, toluene, acetone, ethanol), condenser type, materials (borosilicate glass 3.3, PTFE joints or stopcocks), compatibility with a heating mantle or flask heater, and the requirements of standardised methods.

At-a-glance comparison of the main set-ups:

Type	Principle	Applications
Conventional Soxhlet	Cycles with siphon; periodic rinsing of the thimble	Fats/lipids, PAHs/PCBs, additives in solid matrices
Twisselmann (continuous)	Condensation above; continuous hot percolation	Dense matrices, polymers, wet soils
Randall/Soxtec	Hot immersion, rinsing, controlled evaporation	Food/feed when high throughput is required
Micro Soxhlet	Miniaturised volumes and thimbles	Scarce samples, expensive/toxic solvents

Quality tip: monitor the number of cycles/hour (typically 4 to 10), the tightness of the joints and good ventilation/fume cupboard use for safe, reproducible extractions.

In which application areas is a Soxhlet extractor used?

• Agri-food: determination of fat content in flours, milk and animal feed using recognised methods. Ideal for exhaustive recoveries and inter-laboratory comparability.
• Environment: extraction of PAHs, PCBs, oils and fats from soils, sludge and sediments prior to GC/GC-MS.
• Pharmaceutical & botanical: recovery of APIs or markers from dry plant matrices; solvent screening.
• Chemicals & polymers: migration of additives, plasticiser content, extraction of polymerisation residues.
• Cosmetics: quantification of oils, waxes and antioxidants in solid ingredients.
• Teaching/R&D: demonstration of solubility equilibria and extraction kinetics with a visual, reliable and cost-effective set-up.

Available Soxhlet extractors and accessories

Soxhlet

Frequently asked questions about Soxhlet extractors

How do you size a Soxhlet and its thimble?
Match the thimble diameter to sample mass (g) and particle size; choose a 250–1000 mL round-bottom flask to ensure a sufficient solvent-to-sample ratio and steady cycles.

Which solvents should you use in Soxhlet?
Hexane, petroleum ether, toluene, acetone, ethanol… depending on target analyte and safety. Aim for a boiling point compatible with the condenser and the matrix; work in a fume hood and away from ignition sources.

Soxhlet vs Soxtec (Randall), which to choose?
Soxtec is faster and uses less solvent thanks to controlled immersion and evaporation. The classic Soxhlet remains preferred when a standardised method requires it.

How long does an extraction take?
Typically 3 to 8 h, with 4–10 cycles per hour depending on solvent and matrix. Stop at constant mass or when the yield plateaus according to your protocol.

How can you reduce the environmental footprint?
Use efficient condensers, recover and recycle the solvent, favour micro-Soxhlet and greener solvents; keep the glassware durable and leak tight.

What safety precautions should you take?
Work in a fume hood, prefer a heating mantle, check joints and clamps, add anti-bumping granules and keep all ignition sources away.

How do you maintain the glassware?
Rinse hot with clean solvent, dry completely, lightly lubricate PTFE joints, brush the condenser and replace any chipped parts.