Separating funnels: how can a clean, controlled liquid-liquid separation be performed?

Essential on the lab bench, the separating funnel enables precise liquid–liquid separation and washing of extracts with fine control of the phase interface. Pear-shaped or cylindrical, a leak-tight stopcock and stopper ensure safe handling from mixing to decanting. Ideal for purifying an organic extract, removing polar impurities, acid–base extractions or brine washes.

Which types of separating funnel are available and how do you choose the right one?

Two main families coexist: the glass separatory funnel (borosilicate 3.3) and the plastic separatory funnel (PP or PTFE). Glass provides excellent inertness to most organic solvents, dimensional stability and easy cleaning; it accepts PTFE stopcocks without grease, or ground-glass with grease. PP is lightweight, shatterproof and economical but limited with certain solvents; PTFE is chemically inert and non-adsorptive, ideal for trace work and sensitive analytes.

The shape matters: pear-shaped (Squibb type) for a narrow interface and a clean phase break; cylindrical for an easy-to-read level and good stability on a stand. Choose the stopcock to match the fluid: wide bore for viscous phases, fine tip for dropwise dosing. Common volumes range from 50 mL to 5 L; fill to ~2/3 to leave space for shaking and degassing. Versions with a safety coating (plastic-coated glass) reduce the risk of shards.

On sustainability, reusable glass cuts waste; plastics avoid breakage in routine use and in teaching labs. In all cases, the most impactful factor remains solvent use: right-size, favour protocols that minimise emulsions and always work in a fume cupboard.

See the comparison table below to guide your choice:

Type	Principle	Applications
Borosilicate glass 3.3	Inert, easy to clean, compatible with most solvents. PTFE stopcock or ground glass.	Organic extractions, multiple washes, regulated environments, high precision.
Plastic PP	Lightweight, shatterproof, economical. Limited with aromatic/chlorinated solvents.	Teaching, routine aqueous/alcohol work, field use.
Plastic PTFE	Chemical inertness, non-adsorptive, no greasing.	Trace/sensitive analyses, aggressive solvents, demanding QC.

In which fields is a separating funnel used?

• Life sciences: extraction of metabolites, lipids, alkaloids; acid/base washes prior to HPLC/GC.
• Organic chemistry and synthesis: purification of intermediates, neutralisation and washing of organic phases before evaporation.
• Environment: liquid–liquid extraction (LLE) of water samples (PAHs, chlorinated solvents, pesticides), pre-SPE fractionation, effluent compliance testing.
• Food industry: extraction of flavours, additives and non-polar contaminants; defatting of extracts.
• Pharmaceutical: multiple washes to remove inorganic residues, counter-ions and solvents, in QC and R&D.
• Cosmetics: purification of essential oils, fragrances, non-polar actives.
• Teaching/practicals: learning partition equilibria; safety (ventilation, secure mounting on a stand).

Available types of separating funnels

Frequently asked questions about separating funnels

Glass or plastic: which to choose?
Glass is highly inert and easy to clean; PP is lightweight but limited with certain solvents; PTFE is near-universal, ideal for trace analysis, but pricier.

PTFE stopcock or ground-glass stopcock?
PTFE is self-lubricating, leak-tight without grease and low-maintenance. Ground-glass requires careful greasing and can seize if poorly maintained.

How to avoid emulsions?
Shake gently, vent frequently, add salt (brine) to “break” the emulsion, let it settle, and use a wide-bore tap for viscous phases.

What volume of separating funnel to choose?
Select a capacity 1.5 to 3× the total volume of the phases to keep 30–40% headspace for mixing. 50–250 mL for micro-scale, 250–1000 mL for routine work.

Cleaning tips?
Rinse with a compatible solvent, then a neutral detergent. Avoid aggressive solvents on PP. PTFE tolerates almost everything. No abrasives, to preserve sealing surfaces.

Safety with internal pressure?
Work in a fume cupboard, hold both stopper and tap, vent frequently (especially with volatile solvents or bicarbonate). Do not fill beyond 2/3.

Which accessories are useful?
Clamp/stand, filling funnel, PTFE joints and tap kits, spill tray. Plastic-coated glass is recommended to reduce shattering.