8.8: Step-by-Step Procedures For Extractions (Microscale)

Microscale Extractions

Microscale work involves the manipulation of less than \(300 \: \text{mg}\) of compound, and usually involves solvent volumes of \(5 \: \text{mL}\) or less. A separatory funnel would be impractical when working with such small quantities, and conical vials (Figure 4.35), test tubes, or centrifuge tubes are typically used instead.

Figure 4.35: Progress of the extraction of methyl red from the acidic aqueous layer (bottom) into the organic layer (top). The inversions were done very slowly in order to see the extractions stepwise. With even gentle mixing the methyl red (and thus color) extracts rapidly.

The pictures in this section show the extraction of \(2 \: \text{mL}\) of a mildly acidic aqueous solution containing a single drop of methyl red solution into \(2 \: \text{mL}\) of ethyl acetate. The color (methyl red), is extracted from the aqueous layer (bottom) into the ethyl acetate layer (top).

Figure 4.36: a) Adding solvent to the conical vial by pipette, and using a beaker to support the vial, b) Supporting the vial with a cork ring, c+d) Attaching the Teflon-lined cap to the vial, e) Manual mixing of layers (using a different system).

Mix the Solutions (for microscale extraction)

1. Pour the contents to be extracted into a conical vial, or a glass tube with a screwtop lid. As these containers are prone to tip, use a beaker (Figure 4.36a) or inverted cork ring (Figure 4.36b) for support.
2. Add the extractive solvent by pipette (Figure 4.36a). 
3. Gently mix the two solutions using the following method:
   1. Secure a cap firmly on the vial (Figure 4.36c+d) then invert and shake the tube for 10-20 seconds (Figure 4.35). Conical vials and centrifuge tubes tend to be less airtight than separatory funnels, so there should be no need to vent the system during shaking unless \(\ce{NaHCO_3}\) or \(\ce{Na_2CO_3}\) solutions are used.

Figure 4.37: a+b) Withdrawal of the bottom layer, with pipette into the tip of the vial, c) Removal of a residual drop of the bottom layer by allowing the layers to separate inside a Pasteur pipette, d) Separated layers.

Separate the Layers (for microscale extraction)

4. Separate the layers with a Pasteur pipette. The design of conical vials and centrifuge tubes allows for efficient separation of the layers through withdrawal of the bottom layer by pipette. This means that even if the top layer is to be reserved, the bottom layer still needs to be removed first.
   1. Hold the conical vial or tapered tube in the same hand as a container for the bottom layer (label it). Withdraw the majority of the bottom layer by Pasteur pipette, and dispense into the container (Figure 4.37a).
   2. When withdrawing, always place the pipette tip to the point of the conical vial or tapered tube (Figure 4.37b).
   3. It may be difficult to remove the very last drop of bottom layer from the point of the vial. To do so, withdraw the entirety of the bottom layer and a small amount of the top layer into the pipette. Allow the layers to separate inside the pipette (Figure 4.37c), then delicately expel the bottom layer from the pipette into the container. Return the rest of the top layer to the conical vial.
5. If the bottom layer is the desired layer, and another extraction is to be done, add fresh organic solvent to the top layer still in the conical vial and repeat the extraction and separation.
6. If the top layer is the desired layer, remove it from the conical vial using a fresh pipette into a clean container. If another extraction is to be done, return the bottom layer to the conical vial, add fresh solvent and repeat the extraction and separation.