A separatory funnel is laboratory glassware used for liquid-liquid extraction, a technique designed to isolate and purify compounds. This method separates the components of a mixture based on their differing solubilities within two immiscible liquid phases. The funnel allows a dissolved substance, or solute, to transfer from one liquid to the other. This process is highly effective for purifying reaction mixtures and relies on the fact that chemical substances prefer to dissolve in one liquid phase over the other.
Anatomy of the Apparatus
The separatory funnel features a conical or pear-shaped glass body that serves as a reservoir for the liquids. Its sloping sides help users clearly observe the boundary line, or interface, between the two liquid layers. The top opening is sealed with a ground glass or Teflon stopper, which prevents the contents from spilling or evaporating during mixing.
The funnel tapers down to a narrow stem containing the stopcock, a small valve that controls the flow of liquid. Stopcocks, typically made of glass or polytetrafluoroethylene (PTFE, or Teflon), allow the user to precisely drain the lower liquid layer drop by drop. The stopcock is the final control point for achieving a clean separation.
The Science of Separation
The fundamental principle governing the funnel’s function is immiscibility: the two liquids, or solvents, placed inside must not dissolve into each other. They remain separate and form distinct layers, allowing for the transfer of a target compound between them. This transfer is driven by the solute’s preferential solubility in one solvent over the other, a phenomenon called partitioning.
Once the two immiscible layers form, their arrangement is determined by their respective densities. The liquid with the lower density rests on top, while the denser liquid sinks and forms the bottom layer. For example, when water (density about 1.0 g/mL) is mixed with a non-halogenated organic solvent like diethyl ether, water typically forms the bottom layer because the ether is less dense. However, a halogenated organic solvent like dichloromethane (density about 1.3 g/mL) would form the bottom layer instead.
Step-by-Step Extraction Process
The extraction process begins by securely supporting the separatory funnel in a ring stand and ensuring the stopcock is fully closed. The mixture to be separated, along with the extracting solvent, is carefully added to the funnel through the top opening, taking care not to fill the funnel more than three-quarters full to allow for proper mixing. The stopper is then placed firmly in the neck of the funnel.
The contents are mixed by picking up the funnel, securing the stopper with one hand, and gently inverting and shaking it for a period, typically around 20 to 30 seconds. This shaking is necessary to temporarily disperse the two liquids into tiny droplets, which vastly increases the surface area where the target compound can partition from one layer to the other.
During the shaking process, internal pressure can build up due to the vaporization of the solvents or the release of gases from chemical reactions, such as when using bicarbonate solutions. To prevent the stopper from popping out, the pressure must be safely released through a procedure called venting. This is done by inverting the funnel and slowly opening the stopcock, pointing the stem away from people and equipment.
After mixing and venting, the funnel is returned to the ring stand and the stopper is removed to allow for atmospheric pressure equalization, which is necessary for the liquid to drain later. The mixture must then be left undisturbed for several minutes to allow the two liquid layers to fully separate and settle into a sharp interface. The separation is complete when no swirling or droplets are visible within either layer.
The final step is draining the layers, which is accomplished by opening the stopcock to slowly release the bottom layer into a clean collection flask. The user carefully watches the interface as it approaches the stopcock, closing the valve just before the upper layer begins to enter the stem. The top layer is then poured out through the top neck of the funnel into a separate container, ensuring the two separated phases never mix again.