When an ideal mixture of two miscible liquids is heated to boiling, the solution boils at a temperature between the boiling points of each component. If these liquids have very different boiling points, when the mixture starts to boil, the vapor is rich with the molecules of the more volatile component. This phenomenon is often used to separate mixtures using simple distillation, where a mixture of two miscible liquids is heated and the vapor is then condensed back to liquid and collected.
As the vapor rich with the more volatile component is collected as the distillate, the liquid phase becomes rich with the molecules of the less volatile component. However, this technique requires the solution to be heated at least to the boiling point of the more volatile compound and often beyond that.
In the case of temperature-sensitive organic compounds, this high temperature could lead to the organic molecules decomposing into something else. So, how can we separate these types of compounds? First, let's take a step back.
Recall that the pressure of a vapor in equilibrium with its condensed phase is called vapor pressure. The components of a mixture of liquids each have their own vapor pressure, which we call their partial pressure. We know that a solution boils when the total vapor pressure of the solution is equal to the atmospheric pressure. The total vapor pressure is equal to the sum of the partial pressures of the components.
For a mixture of miscible liquids, meaning that any combination of the liquids forms a homogeneous solution, the partial pressures are calculated from the vapor pressures of the pure compounds multiplied by their mole fractions in the solution. However, for a heterogeneous mixture of immiscible liquids, meaning that the liquids are insoluble in each other, the partial pressures are simply the vapor pressures of the pure compounds.
Since each component of the heterogeneous mixture contributes to the total vapor pressure independently of the other components, the mixture boils when the total vapor pressure, which is the sum of the partial pressures, is equal to the atmospheric pressure. This occurs at a lower temperature than the individual boiling points of each component because the total vapor pressure increases with temperature much faster than you would expect for even the most volatile component.
We can harness this phenomenon to perform steam distillation, which is used to isolate a temperature-sensitive organic compound that decomposes under high heat and is insoluble in water from non-volatile substances. The steam distillation setup is similar to a simple distillation setup with the addition of a water reservoir to replenish water throughout the process.
As the mixture boils, both the water and the organic compound of interest are vaporized. The water and organic compound vapors travel into the condenser, are condensed to liquid, and collected. The immiscible liquids are separated afterward. Only water and non-volatile materials are left in the mixture in the flask.
In this lab, you will set up and perform a steam distillation experiment to extract essential oil from the non-volatile components of an orange peel. You'll then use liquid-liquid extraction to extract the essential oil from water into an organic solvent.
Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA
- Steam Distillation
In this experiment, you will perform a steam distillation to extract an essential oil from an orange peel.
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Before you start the lab, put on the appropriate personal protective equipment, including a lab coat, safety goggles, and gloves. This experiment must be performed in a hood.
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First, you will need to assemble the steam distillation apparatus. Check the apparatus set up by your instructor.
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Set the two stands next to each other in the hood. Place the stir plate on the base of one of the stands and set the heating mantle on top of it. Plug the heating mantle into the temperature controller, but do not turn it on yet.
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Now, obtain a watch glass with 3 dried orange peels from your instructor. Weigh the orange peels on the balance and record the mass of the orange peels in your notebook.
Table 1: Mass of essential oil recovered
Click Here to download Table 1Temperature of steam distillation (°C) Mass of the orange peels (g) Mass of empty flask (g) Mass of flask + essential oils (g) Mass of essential oils (g) Percent mass of essential oil recovered -
Bring your dried peels, funnel, spatula, and the 500-mL round-bottom flask to the instructor's bench and place the peels in the blender. Add about 4 mL of deionized water to the peels. Then, blend them until they are well mixed and smooth.
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Using the funnel, transfer the blended peels to the round-bottom flask. Clamp the flask to the stand above the heating mantle.
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Measure 200 mL of deionized water using your graduated cylinder and transfer it to the round-bottom flask using a funnel. Remove the funnel and add a stir bar to the flask.
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Locate the Claisen adapter and apply a thin layer of grease to the joint. Then, insert the Claisen adapter into the flask.
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Grease the thermometer adapter and insert it into the distilling head.
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Gently insert the thermometer into the distilling head by pushing and twisting through the adapter until the thermometer bulb is below the bend of the distilling head.
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Lightly grease the lower joint of the distilling head, and then insert it into the outer arm of the Claisen adapter.
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Now, obtain the condenser and carefully clamp it to the second stand. It should be oriented nearly horizontally, with the end closest to the distilling head slightly higher.
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Lightly grease the condenser and the side joint of the distilling head before connecting them.
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Next, attach one piece of rubber tubing to the inlet of the condenser, which is the port furthest away from the distilling head. Attach the other end to the tap water faucet in the hood.
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Connect the second piece of rubber tubing to the other port, which is the outlet, and place the end of the tubing in the drain.
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Locate the separatory funnel and make sure that the stopcock is closed by turning it until it is fully horizontal.
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Insert the separatory funnel into the Claisen adapter, and add approximately 100 mL of deionized water to the funnel.
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Pick up the connecting joint and attach it to the end of the condenser.
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Then, attach a 250-mL round-bottom flask to the connecting joint using a plastic clip.
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Finally, check all of the joints to make sure they are tightly connected and then secure each one with a plastic clip.
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Now that the steam distillation apparatus is fully assembled, start the procedure. First, slowly turn the tap water on to fill the condenser with water. Note: Be sure to turn the water on slowly, or the tubing may pop off and cause a flood.
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Next, turn on the magnetic stirrer at the lowest setting and set the temperature controller to the appropriate setting to reach about 90 °C.
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Once bubbles start forming in the mixture and liquid is dripping into the collection flask, adjust the heat to increase or decrease the rate of distillation to about 20 drops/min.
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Record the temperature on the thermometer in your notebook. Now, carefully open the stopcock on the separatory funnel to slowly add water to the flask. Your goal is to add water at the same rate as distillation, about 20 drops/min.
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Allow the distillation to proceed and collect about 50 mL of distillate. Note: Be patient, since this should take about 2 hours. The liquid in the flask will become less cloudy as more oil is collected.
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Once you have collected enough distillate, turn off the heat. Then, shut off the water to the condenser and carefully raise the entire setup.
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Allow it to cool completely before removing the heating mantle and disassembling the glassware.
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Empty the separatory funnel of any remaining water and pour the orange pulp down the drain.
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- Extraction of Essential Oil
Liquid-liquid extraction is performed in a separatory funnel and is used to move a solute to another liquid in which it is more soluble. The two liquids must be immiscible and are usually an aqueous solution and a non-water-soluble organic liquid. Upon mixing, the solute is extracted into the other phase. Once the liquids settle into layers, they can be drained from the funnel one at a time.
In the next part of the experiment, liquid-liquid extraction will be used to extract the essential oil from the water into an organic solvent. You'll then evaporate the organic solvent to recover the essential oil.
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Before starting the experiment, first, make sure that the separatory funnel is rinsed and clean.
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Label the two 250-mL beakers as ‘organic layer’ and ‘aqueous layer’. Then, attach a ring support to one of the stands.
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Place the separatory funnel in the support. Make sure that the stopcock is closed and the separatory funnel is secure. Then, transfer the distillate into the separatory funnel.
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Measure 20 mL of diethyl ether at the instructor's hood and bring it back to your workstation. Pour the diethyl ether into the separatory funnel with the distillate. Note: You should see the immiscible liquids form two layers in the funnel. The top layer is the diethyl ether.
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Apply a thin layer of vacuum grease to the separatory funnel stopper and insert it into the top of the funnel.
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Hold the stopper in place and shake the separatory funnel vigorously for about 1 min.
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Occasionally, vent the funnel by turning it upside down and opening, then closing the stopcock. Be sure to do this inside of the hood and away from your face.
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Once you have finished mixing the contents, vent the funnel again, close the stopcock, and then place the funnel in the ring attached to the stand.
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Allow the aqueous and organic layers to fully separate. Then, place the aqueous layer beaker under the separatory funnel.
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Remove the stopper and slowly open the stopcock to drain the aqueous layer into the beaker. Carefully watch the solution drain and be sure to close the stopcock before the next layer is drained.
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Replace the aqueous layer beaker with the organic layer beaker. Open the stopcock and drain the organic layer into its beaker.
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Now, close the stopcock and return the contents of the aqueous layer beaker to the separatory funnel, add 20 mL of diethyl ether, and repeat the extraction.
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Open the stopcock and drain the aqueous layer into the appropriate beaker, and then drain the organic layer into the organic layer beaker, adding it to the organic layer from the first extraction.
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Now, obtain magnesium sulfate in a weighing boat and scoop a small spatula of it into the organic layer beaker. Swirl the beaker, and then let it sit for about 20 min.
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After 20 min, remove the separatory funnel and ring and mount a regular clamp to the lab stand.
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Weigh an empty 125-mL Erlenmeyer flask and record the mass in your notebook. Then, clamp the flask to the stand and place a conical glass funnel in its neck.
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Fold your filter paper in half and in half again. Then, open the filter paper on one side to create a cone. Place the cone inside the conical funnel.
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Now, filter the magnesium sulfate from the organic layer.
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Next, set your hot plate to 40 °C and place a large beaker containing 400 mL of water on it. Place the Erlenmeyer flask in the warm water bath and allow the organic solvent to evaporate in the hood.
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Once the organic solvent has fully evaporated, remove the Erlenmeyer flask from the water bath and dry the outside with paper towels.
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Weigh the cooled flask and record the mass in your notebook.
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Now that the experiment is complete, be sure to properly clean up your workspace by disposing of all solutions in the non-halogenated waste.
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Wash all of the glassware using detergent and then rinse them with acetone and deionized water.
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- Results
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Determine the mass of the oil extracted from the orange peel by subtracting the mass of the empty flask from the mass of the flask with oil.
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Calculate the percentage of oil in the orange peel using the original mass of the dried peel. Typically, the steam distillation method recovers about 0.9 – 4 wt% of an orange peel’s mass as essential oil.
You distilled the essential oil in this lab at close to 100 °C. The orange oil consists mainly of (+)limonene, which has a boiling point of 176 °C. Thus, the steam distillation technique lets us extract it from the orange peel at a temperature that will not compromise its structure.
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