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A separatory funnel, also known as separation funnel, separating funnel, or sometimes referred to simply as sep funnel, is a piece of laboratory glassware used in liquid-liquid extractions to separate the components of a mixture into two immiscible solvent phases of different densities.
A separating funnel has the shape of cone with a hemispherical end, though some models have a cylindrical body and are similar in appearance to dropping funnels without pressure-equalizing tubes. The separatory funnel has a stopper at the top, usually made of PP, and stopcock (tap) at the bottom. Separating funnels used in laboratories are typically made from borosilicate glass and their stopcocks are made from glass or PTFE. Some stopkpocks are the classical horizontal type, while others have a screw-like mechanism made of PP or PTFE.
Plastic (often PP) separatory funnels also exist, though they're rarely used, as they're not easy to properly clean.
A binary liquid, usually water and an immiscible solvent is added. The two liquid layers formed in the funnel are called upper layer and lower layer.
How to operate a separatory funnel
- Place the funnel in a vertical position on its funnel holder (which can be anything from a true funnel holder, O-ring stand or to a circular hole cut in wood)
- Make sure the stopcock is well greased and works
- Close the stopcock
- Add the liquids in the funnel, in their corresponding order
- The funnel is closed using the stopper and removed from its placeholder
- Shake gently the funnel by inverting it a few times, then carefully open the tap to release the pressure, while holding the funnel in a slight angle, with the stopcock section in a superior position, to prevent it from spewing liquid, then close the stopcock
- Repeat this a few times until the liquids have mixed properly
- Place the funnel back in its placeholder and let it sit for a few minutes for the layers to separate
- Carefully drain the first layer in a flask, or the second, depending on which you need in a flask
- Remove the waste layer and then wash the funnel
- After you shook the funnel manually a few times and no pressure builds up, you can place the funnel on a shaker, and turn it on to continue shaking
- For a better extraction you can add some fresh extraction solvent in the funnel to extract further compound from the waste layer
- If a floating emulsion if formed on the top of any layer, gently swirl the funnel to break it; adding small amounts of NaCl can help in breaking the emulsion; if this doesn't work, and the emulsion may affect your process, you can try to remove it using a pipette or a syringe
Separatory funnels can be purchased from lab suppliers or online.
Separatory funnels are commonly used in liquid-liquid extractions.
They can also be used as dropping funnels, most often for open flasks and beakers. They are useful if you want to slowly neutralize a solution.
One issue during separation is the appearance of emulsions which take forever to separate properly. Sometimes a small amount of a saline solution is added to aid separation.
The largest risk when using a separatory funnel is that of pressure build-up. Pressure accumulates during mixing if a gas evolving reaction or physical change occurs. This problem can be easily handled by simply opening the stopper or stopcock routinely while mixing.
Sometimes, if the stopcock isn't greased or washed properly it may freeze. To release it, check the main article on how to solve frozen glassware. It the stopcock is made of PTFE, unscrew the plastic nut and rubber seal, then repeatedly tap the PTFE stopcock knob in the opposite direction to push it away from the glass joint. Inspect the ground glass joint for any cracks before you do this, as there's a risk you might snap the glass joint if you tap it repeatedly.