SOXHLET EXTRACTION

Soluble solids can be extracted from insoluble impurities by repeatedly leaching the impure solid using water or an organic solvent.
The soxhlet extractor is an efficient apparatus for such continuous extractions.

By leaching with refluxing solvent, the apparatus permits the extraction of delicate substances such as natural products which frequently require prolong extraction and are often heat sensitive.
The soxhlet extraction apparatus is in 3 distinct parts.
A heated round bottom flask containing extracting solvent
The soxhlet extractor itself which holds the material to be extracted in a thick porous paper thimble.

An efficient usually double surfaced reflux condenser
The heart of the apparatus is the extractor
This has a by-pass tube to allow vapor to reach the condenser, a sample chamber and a Siphon to return condensate from the sample chamber to the flask.

Material to be extracted us placed in a paper thimble which just fit in the wide part of the soxhlet glassware.
The thimble should be no more than 2/3 full.
Loosely packed glass or cotton wool placed on top of the solid serves to prevent any solid splashing out.

Then slide the prepared thimble into the soxhlet.
Its top must be at least 1cm above the top of the siphon tube
This prevents any solid being washed out into the siphon.
Next, fill around bottom flask of capacity 3 or 4 times that of the soxhlet 2/3 full to with the extracting solvent.

Securely clamp the flask resting either in a stem bath if flammable solvent is used or a heating mantle if a low flammability solvent is used.
Add a few anti-bumping granules
Then assemble the apparatus.
Remember that water will condense on apparatus heated over a steam bath so the joints must be greased to prevent water sipping inside the apparatus.

Connect the cold water supply to the bottom of the condenser and the overflow tube to waste.
Turn on the cooling water and heat the flask in this case in the heating mantle.
Adjust the rate of heating so that the solvent refluxes at a steady rate.
Vapor from the boiling solvent enters the wide tube on the right of the soxhlet and when it condenses drips into the paper thimble.
Since the sample chamber is sealed at its bottom end, the solvent passing through the thimble cannot reenter the flask until its level reaches the top of the siphon tube.
At this point, the siphon operates discharging the extract solution into the round bottom flask.

This cycle of events repeats every few minutes and it is continued for as long as it is necessary to complete the extraction.
Completion is judged by the disappearance of color from the fresh extract or if the extract is colorless simply by try and error.
If necessary, the extraction should be run for several hours.
Finally turn the heater off and allow the apparatus to cool.
Product can be recovered from solution by filtration and solvent removal.

SOLVENT EXTRACTION

When an organic compound has to be isolated from an aqueous mixture, solvent extraction using a separating funnel is often a quick and easy technique.

Non polar organic materials are usually soluble in organic solvents but not in water whilst
Non polar ionic materials are soluble in water and not usually in organic solvents.

This is the principle of like dissolves like
Separation can often be done by allowing the components of the mixture to distribute themselves between a water layer and a non miscible organic layer.

The piece of equipment used to mix up and separate phases is known as a separating funnel.
Funnels come in a variety of shapes and sizes but all have a stopper for the neck and a tap in the stem
The isolation of organic product from an aqueous reaction mixture containing unwanted ionic material is  a typical separation.
Having first ensure that the tap is closed, pour the mixture into a separating funnel of at least twice its volume.

Since the solvent used is likely to be flammable or toxic, the work should be carried out in a well ventilated fume cupboard.
The chosen solvent must be immiscible with water, have a low boiling point and preferably be of low toxicity
Diethyl ether is a popular choice.

When using such highly flammable solvents, ensure that there are no sources of ignition present.
The mixture should be extracted first with about one third of its volume of solvent then with two subsequent portions of about a quarter of its volume.
A slightly larger of volume is used for the first extraction because ether is not totally immiscible with water.
Less dense solvents like ether which floats on top of the aqueous layer, are handled in the following way:

Stopper the funnel and taking care to hold the stopper and tap in place, invert and gently shake the funnel.
Then release any built up pressure by briefly opening the tap.
Continue with progressively more vigorous shaking, always taking care to point the stem away from nearby colleagues when opening the tap.
After shaking for a few minutes, release the pressure.
Support the funnel in the ring and allow the layers to separate.
Once the separation is achieved, a series of transfers are needed to remove the upper solute rich organic layer and replace it with fresh extracting solvent.

Remove the stopper and run the lower aqueous layer into a suitable clean vessel such as a conical flask.
If the tap is opened without removing the stopper, irregular flow and some remixing can occur.
If this happens, close the tap and allow the layers to settle.
Then remove the stopper and open the tap once again.
Drainage should now be smoother.
Reduce the run off rate as the interface between the layers approaches the tap.

Close the tap just before the interface reaches it. Then swirl the funnel to dislodge any aqueous phase from the sides.
A soon as the aqueous layer has passed through, close the tap.
Pour the ether layer into a clean, dry conical flask.
Before continuing with the two subsequent extraction of the aqueous phase, securely stopper the flask.

Sequential extraction of the aqueous layer with two further small portions of solvent are recommended because this procedure will always achieve a more efficient separation than a single extraction with the same total volume of solvent.
Add the second and third ether layers to the flask containing the first extract.

Return the combined organic extracts to the separating funnel and wash with a similar volume of cold water to remove any water soluble impurities.
Since small amount of water are always left in the organic layer, it is common to wash this layer with saturated salt solution
With its high affinity for water, the brine removes dissolved water from the organic layer and so act as a preliminary drying agent.
Combine the aqueous washings with the aqueous layer and label the flask.

Do not discard this until the final product has been recovered successfully.
When the extracting solvent is more dense and form the lower layer, the transfer procedure is not necessary
The lower solute rich layer is simply run off and fresh portion of solvent added to the separating funnel to further extract the less dense phase.

Either way, dry the combined organic extract by adding a desiccant such as anhydrous magnesium sulfate.
The first few grams of the desiccant may form dense hydrated lumps which stick to the bottom of the flask.
Add further desiccant until free powder is suspended when the flask is swirled and settles only slowly to leave a clear solution.
Then stopper the flask and periodically agitate it over a period of 5 to 10mins to ensure the extract is thoroughly dried.
Remove the desiccant by filtering the mixture through a funnel lightly plugged with cotton wool.

Or under suction through a filter paper in a hush funnel.
Finally strip off the solvent in a rotary evaporator and purify the residue by recrystalisation for a solid
or distillation for a liquid.
Sometimes agitation of the two phases will produce an emulsion.
Filtering the mixture will often break the emulsion but there are no hard and fast rules for these situations. It a case for experimentation
The addition of cold saturated Brine is frequently successful in breaking emulsions.

This is because emulsion often form when the solute dissolved in the immiscible liquid has made their densities very similar or where a surfactant is present
The Brine increases the density and ionic nature of the aqueous phase.
This makes the density of the two phases less similar and helps to reduce the emulsifying effect of surfactants.
Another strategy is to add more solvent thereby decreasing the density of the organic phase by reducing its solute concentration.