Using a Separating Funnel

Students learn how a separating (separatory) funnel is used to separate immiscible liquids by density and solubility differences. The demonstration covers correct setup, safe shaking and venting, identifying layers, draining/pouring layers, troubleshooting emulsions, and drying an organic layer.

1. Clamp the separatory funnel securely by the neck; ensure the stopcock is closed and place a labeled receiving flask beneath the stem.
2. Add the first liquid phase to the funnel, then add the second immiscible phase (commonly, organic solvent and water). Insert the stopper firmly.
3. Support the stopper with one hand and the stopcock with the other; invert the funnel and gently swirl to mix.
4. Vent immediately by opening the stopcock while the funnel is inverted and pointed away from people and equipment; close and repeat a mix–vent cycle several times.
5. Return the funnel to the ring stand and allow the layers to separate fully (wait until a sharp interface forms).
6. Identify which layer is which: compare known densities, or add a drop of water—if the drop joins the lower layer, the lower layer is aqueous.
7. Drain the bottom layer slowly into a labeled flask, stopping just before the interface reaches the stopcock.
8. Remove the funnel from the clamp and pour the top layer out of the top into a second labeled flask to avoid contaminating it with residual bottom-layer hold-up in the stopcock channel.
9. For improved recovery, perform multiple small-volume extractions rather than a single large one; combine like layers as appropriate.
10. Dry the organic layer if needed by adding a suitable anhydrous drying agent (e.g., magnesium sulfate or calcium chloride), swirl, wait a few minutes, and decant or filter off the dry solution.

Using a Separating Funnel - Cormac Quigley:

How to Use a Separatory Funnel - Chemistry in a Nutshell:

• Demonstrate an extraction with colored solutes (e.g., iodine in an organic solvent, then extract into aqueous iodide) to visualize partitioning.
• Show “salting out” by adding sodium chloride to the aqueous phase to reduce emulsion formation and shift distribution.
• Run a microscale version in a centrifuge tube with a Pasteur pipette as a mini-separator for rapid trials.
• Quantify extraction efficiency by measuring color intensity or absorbance of each layer and comparing single vs. multiple extractions.

• Wear splash goggles, lab coat, and appropriate chemical-resistant gloves; conduct work in a fume hood when using volatile or hazardous solvents.
• Always vent the funnel away from people—pressure can build rapidly with volatile solvents or acid/base reactions.
• Keep a firm grip on the stopper and stopcock while shaking; do not clamp the funnel while shaking.
• Check that the stopcock is closed before filling and that all joints are secure to prevent spills.
• Dispose of waste properly (e.g., segregate halogenated and non-halogenated organic waste; never pour organic solvents down the drain).

• Why is venting necessary during shaking? (Mixing can generate vapor pressure or gases; venting prevents dangerous pressure buildup.)
• How can you confirm which layer is the aqueous layer? (Compare densities or add a drop of water to see which layer it joins.)
• Why are several small extractions more effective than one large extraction with the same total volume? (Partitioning is governed by the distribution coefficient; multiple contacts extract a greater fraction overall.)
• What causes emulsions and how can you break them? (Fine dispersions form from vigorous mixing or surfactants; allow time, add salt, gently swirl, or use a small amount of a mutually soluble solvent.)
• Why dry the organic layer before evaporation? (Dissolved water carries impurities and can reduce yield or purity; drying agents remove residual moisture.)