How to Perform a Liquid-Liquid Extraction
Phillip Church
If you are a college student and major in any type of chemistry or life science, you will
have to take the dreaded organic chemistry lab. While many people say this is the hardest class in
college, most of the processes and procedures are not that hard to perform. The most
fundamental and prevalent of these procedures is a liquid-liquid chemical extraction of an acid.
This process is one in which the organic and aqueous components are separated by immiscible
layers, usually to isolate a single substance that sticks with either the organic or aqueous layer,
depending on its solubility and polarity.
The main reason to perform this task is to isolate an acid from the rest of the solution.
The people who have to worry about this fairly simple task are any college student in an organic
chemistry lab, as this is an important technique no matter where the class is taken. This process
should be implemented when a purification based on polarity and solubility is necessary. Safety
is always paramount during one of these experiments, therefore goggles and gloves should
always be worn in order to prevent damage to these extremities. Information on chemicals
should also be researched before they are handled so that caution can be exercised to the
appropriate degree. Some acids can burn skin or even, in the case of hydrofluoric acid, sink right
through skin and eat away at bone material. For this reason, it is always a necessity that the
appropriate chemicals are researched before any experiment takes place. The task should not take
more than 20 minutes for someone who has never performed it before.
Necesssary Equipment
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Chemical solution
Separatory funnel
Waste beaker
Ring stand
Goggles
Gloves
Procedural Preparations
First, one must adequately set up the apparatus in order to ensure that the process goes smoothly.
1. Fit the separatory funnel into the ring stand so that the stopcock faces downwards and the
glass stopper faces upwards.
a. Note that the stopcock is CLOSED in a horizontal position, and OPEN in the vertical
position. It is easy to leave the stopcock open and have the solution pour all over the
desk, which is never a good thing, so always be sure to close the stopcock when it is
applicable.
2. Pour the liquid to be extracted into the separatory funnel while making sure that the
stopcock is closed, and replace the glass stopper after.
a. A visual is included below for reference, without the glass stopper.
Courtesy of colorado.edu
Separating the Layers
At this point, the solution is prepared in the funnel and needs to be separated into organic and
aqueous layers.
1. Remove the funnel from the ring stand.
2. Press a finger firmly over the glass stopper and hold the funnel upside down so that the
stem and stopcock are facing upwards.
3. Swirl the solution vigorously for 30 seconds in order to separate the two layers fully,
being sure to vent the funnel liberally by turning the stopcock to the open position to
allow gas to escape.
a. WARNING! If pressure is not vented from the funnel, gas can build up and cause the
stopcock to open and hazardous chemicals to blow out. Be sure to vent the funnel
often.
4. Replace the funnel on the ring stand, the layers should be separated.
a. This part of the procedure is slightly complex and may be hard to visualize.
Therefore, a figure is included here.
Image courtesy of uwimona.edu; Images (from left to right) depict steps 2, 3, and 4.
Washing the Solution
It is important to note, at this point, that the layers settle according to their respective densities.
For example, the aqueous layer solvent is usually water (density of 1.00 g/mL). If the organic
solvent has a greater density than water, the organic layer will settle to the bottom of the funnel.
For the sake of reduced explanation, this instruction set will assume that the organic solvent is at
the bottom of the funnel after separation.
1. Add sodium bicarbonate (NaHCO3) to the solution to extract the acidic component in the
aqueous phase.
2. Open the stopcock to drain the organic layer into the waste beaker.
a. Note: Do not pour the organic waste down the sink, as it can damage the pipes. An
organic waste disposal will be set up at a designated position in the lab.
3. Rinse with NaHCO3 once more to ensure all organic layer has been removed.
4. Rinse with hydrochloric acid (HCl) in order to convert the base created by the NaHCO3
back into the acid desired.
5. Pour the remaining solution onto anhydrous sodium sulfate, which siphons off excess
water, leaving behind only the desired acid product.
a. A flow chart has been included in order to give a resource that can be glanced over
for the gist of the process.
Courtesy of tdmu.edu; The procedure described here is only the left portion of this flow chart.
Conclusion
The process has now been completed. The sample that was obtained can be analyzed
using different techniques in order to determine the molecular formula and weight of the
substance. If problems arise, reference the images and notes provided, as they can help
tremendously with clarification.
Useful Terms
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Miscibility: the ability for two solutions to mix together to form a continuous solution,
where miscible solutions can mix and immiscible ones cannot
Polarity: whether or not a chemical has two distinct poles of positive and negative charge.
Polar mixes with polar, while nonpolar mixes with nonpolar
Solubility: the ability for a solid, liquid, or gas to dissolve in some solvent