A separating funnel is a piece of laboratory glassware that every analytical chemist, pharmaceutical scientist, and organic chemist uses regularly — yet it is rarely given the detailed consideration it deserves when equipping a laboratory. Understanding what a separating funnel is, how liquid-liquid extraction actually works, which funnel configuration is appropriate for a given application, and what specification choices matter most will help Irish laboratory professionals select the right equipment and use it correctly for reliable, reproducible extraction results. This guide covers everything from first principles.
A separating funnel — also referred to as a separatory funnel, separation funnel, or simply a sep funnel — is a specialised piece of laboratory glassware designed specifically for liquid-liquid extraction: the physical separation of two immiscible liquid phases that have been mixed together and then allowed to separate under gravity based on their different densities.
The defining structural features of a separating funnel are:
Separating funnels are manufactured from borosilicate 3.3 glass to ISO standards and are available in a wide range of capacities — from 50mL for small analytical-scale extractions to 2000mL for preparative-scale work. The shape of the funnel body — Squibb (cylindrical) or pear (conical) — determines suitability for different extraction scales and applications, as discussed below.
Liquid-liquid extraction (LLE) is a separation technique that exploits the difference in solubility of a compound between two immiscible liquid phases — typically an aqueous phase and an organic solvent phase — to selectively transfer the compound of interest from one phase to the other. The technique is based on the partition coefficient of the compound: its equilibrium distribution between the two phases at a given temperature and pH.
When a mixture of compounds is dissolved in one solvent phase and a second, immiscible solvent is added, each compound distributes itself between the two phases in proportion to its partition coefficient. Compounds with high solubility in the organic phase preferentially partition into the organic layer. Compounds with high solubility in the aqueous phase remain in the water layer. By draining and collecting the two phases separately, the compound of interest is isolated — either enriched in the organic phase (for subsequent evaporation and concentration) or retained in the aqueous phase (with impurities removed into the organic layer).
The practical power of liquid-liquid extraction lies in its simplicity, versatility, and efficiency. Unlike chromatographic separation techniques, LLE requires no stationary phase, no column, and no complex instrumentation — just two immiscible solvents, a separating funnel, and an understanding of the chemistry of the system being separated.
Understanding the correct technique for using a separating funnel is important for achieving clean, reproducible separations and avoiding the most common practical problems:
Check that the PTFE stopcock is correctly seated and closed. Place the separating funnel in a ring stand with a ring clamp at an appropriate height for your collection vessel. Verify the stopcock is leak-free by adding a small volume of solvent before loading the main sample.
Add the sample solution and the extraction solvent through the top of the funnel. The combined volume of both phases should not exceed approximately 75% of the funnel capacity — sufficient headspace is essential for safe mixing. Insert the ground glass stopper and ensure it is secure.
Invert the funnel gently and open the stopcock immediately to release any pressure build-up — particularly important when using volatile organic solvents such as diethyl ether or dichloromethane, which can generate significant vapour pressure during mixing. Hold the funnel inverted with one hand on the stopper and one hand on the stopcock body. Swirl and invert gently several times, venting after every one or two inversions. More vigorous shaking increases contact between the phases and improves extraction efficiency — but excessive agitation with certain solvent systems can generate a stable emulsion that is very difficult to break.
Return the funnel to the upright position in the ring stand and remove the stopper to allow the funnel to breathe. Allow the two phases to separate completely — this typically takes 1–5 minutes depending on the solvent system and the nature of the sample. The denser phase sinks to the bottom and the less dense phase rises to the top. For most organic solvent / water systems, the aqueous phase is denser and sits below the organic layer — but this is reversed for halogenated solvents such as dichloromethane and chloroform, which are denser than water and sink below the aqueous phase.
Open the stopcock and drain the lower phase slowly into a collection vessel, watching the phase boundary carefully. Close the stopcock precisely at the moment the interface reaches the stopcock outlet — leaving the minimum volume of lower phase behind while avoiding carry-over of the upper phase. Remove the upper phase by pouring it out through the top opening of the funnel. Repeat the extraction with fresh solvent if multiple extraction steps are required.
Separating funnels are used across a wide range of analytical chemistry, pharmaceutical, and research laboratory applications. The most common in Irish laboratory environments include:
Liquid-liquid extraction using separating funnels is a core technique in pharmaceutical analytical sample preparation — used to extract active pharmaceutical ingredients (APIs) or their metabolites from biological matrices such as plasma, urine, or tissue homogenate prior to chromatographic analysis. The selectivity of the extraction can be tuned by adjusting the pH of the aqueous phase to control the ionisation state of the analyte — exploiting the fact that ionised species remain in the aqueous phase while neutral species partition into the organic solvent.
In pharmaceutical research and development and academic organic chemistry, separating funnels are used for workup of synthetic reactions — extracting the desired product from the reaction mixture by partitioning it into an appropriate organic solvent, followed by sequential washes with aqueous base (to remove acidic impurities), aqueous acid (to remove basic impurities), and brine (to remove residual water). This extraction workup is a routine step in virtually every preparative organic chemistry procedure.
Liquid-liquid extraction using separating funnels is used for sample cleanup prior to chromatographic analysis — removing matrix interferences, concentrating trace analytes from large-volume aqueous samples, and transferring analytes into a solvent compatible with the analytical method. Environmental samples, food extracts, and biological fluids are routinely processed by liquid-liquid extraction before HPLC, GC, or spectrophotometric analysis.
The octanol-water partition coefficient (logP) is a fundamental physicochemical parameter in pharmaceutical research — used to predict the absorption, distribution, metabolism, and excretion (ADME) properties of drug candidates. Experimental logP determination is performed using a separating funnel to equilibrate the compound between n-octanol and water phases and measure its concentration in each phase.
Organic extracts from synthesis reactions or extraction procedures are routinely washed in a separating funnel with aqueous solutions to remove unwanted water-soluble components — including inorganic salts, acids, bases, and polar reagents — before evaporation and product isolation.
Environmental monitoring laboratories use liquid-liquid extraction with separating funnels to extract organic pollutants from water samples prior to analysis by GC-MS or HPLC — a standard approach for the analysis of pesticides, hydrocarbons, and other organic contaminants in wastewater, river water, and environmental matrices.
The stopcock of a separating funnel is its most critical component — and the choice between PTFE and glass stopcocks has significant practical consequences for everyday laboratory use.
Traditional glass stopcocks require the application of stopcock grease — a viscous silicone or hydrocarbon-based lubricant — to maintain an airtight seal and prevent the glass plug from seizing within the glass barrel. Stopcock grease has several practical disadvantages:
PTFE stopcocks eliminate all of these issues. PTFE (polytetrafluoroethylene) provides an inherently self-lubricating surface that requires no grease, is chemically resistant to virtually all organic solvents and aqueous reagents used in laboratory LLE, and maintains a reliable seal without the maintenance requirements of greased glass stopcocks. Both the Glassco Squibb shape separating funnels and Glassco pear shape separating funnels available from Varen Scientific are fitted with PTFE stopcocks as standard — making them the appropriate and practical choice for pharmaceutical and analytical laboratory use in Ireland.
Laboratory separating funnels are available in two principal body shapes — Squibb (cylindrical) and pear (conical/ISO shape) — each with specific advantages for different extraction applications.
The Squibb separating funnel has a cylindrical body that maximises the usable volume per unit height — providing the greatest capacity for a given funnel size. The cylindrical body is well suited to large-volume preparative extractions where the primary requirement is to hold the largest possible combined volume of the two phases. The Glassco Squibb shape separating funnel with PTFE stopcock is available in capacities from 50mL to 2000mL with NS 19/26 joint on 50–100mL sizes and NS 29/32 on 250–2000mL.
The pear shape separating funnel has a conical body that narrows progressively toward the stopcock outlet — concentrating the lower phase in a smaller volume near the drainage point. This narrowing makes the phase boundary more visible and easier to identify precisely, and allows the lower phase to drain completely and cleanly through the stopcock with less residual lower-phase volume remaining in the funnel. For analytical-scale extractions where complete and precise phase drainage is important — and where the volume of either phase may be small — the pear shape is the preferred configuration. The Glassco pear shape separating funnel with PTFE stopcock is available in the same 50mL to 2000mL capacity range.
| Feature | Squibb Shape | Pear Shape |
|---|---|---|
| Body profile | Cylindrical | Conical — narrows to base |
| Volume per height | Maximum capacity | Moderate |
| Phase boundary visibility | Good | Excellent — phases concentrate at narrow base |
| Lower phase drainage | Good | Excellent — complete and precise |
| Best for | Large-volume preparative extraction | Analytical-scale precise extraction |
| Available capacities | 50mL – 2000mL | 50mL – 2000mL |
In summary, a separating funnel is needed in any of the following laboratory situations:
Varen Scientific supplies Glassco borosilicate 3.3 separating funnels with PTFE stopcocks to pharmaceutical, analytical, and research laboratories across Ireland. Both Squibb and pear shape configurations are available in capacities from 50mL to 2000mL, supplied from Glassco's European warehouse in the Netherlands for fast delivery across Ireland.
For the full range of Glassco borosilicate laboratory glassware — including laboratory funnels, volumetric flasks, and beakers — visit our laboratory glassware section.
For further technical information on liquid-liquid extraction principles and method development, refer to guidance published by the Health Products Regulatory Authority (HPRA) on pharmaceutical analytical methods. To request a quote or discuss your laboratory glassware requirements, contact our team or use the Request a Quote button on any product page.
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