Extraction

Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

1. Cellulose RecoveryExpand

   Extraction and filtration can separate compounds based on their solubility properties. In this lab, you'll separate a mixture of cellulose, caffeine, and benzoic acid based on their solubilities in dichloromethane, or DCM, and water.

   Caffeine and benzoic acid are both soluble in DCM, but cellulose is not. Thus, in this part, you'll first dissolve as much of the mixture as possible in DCM and filter out insoluble cellulose. You'll continue working with the solution of benzoic acid and caffeine in the next section, and you'll measure the mass of the recovered cellulose once it dries.

   • Before you start the lab, put on a lab coat, safety glasses, and nitrile gloves. You will perform this lab in a fume hood. Note: DCM is highly volatile, toxic, and an irritant. DCM will go through nitrile gloves within minutes. Change your gloves promptly if you get any DCM on them.
   • Place a 125-mL separatory funnel upright in a ring fixture. Confirm that the stopcock is closed, and then place a glass funnel in the top of the separatory funnel.
   • Obtain a piece of filter paper and weigh it using a top-loading balance. Record the mass of the filter paper in the cellulose row in your lab notebook.

     Table 1: Mass of recovered compounds

     Compound	Empty container (g)	Sample + container (g)	Compound recovered (g)	Recovered mass percentage
     Cellulose
     Caffeine
     Benzoic acid	–	–
     Starting mass of mixture (g)		Recovered mass of mixture (g)		–

     Click Here to download Table 1
   • Return to your fume hood and fold the filter paper in half twice. Place the filter paper point down in the glass funnel and open the paper into a cone.
   • Bring a 100-mL beaker to the balances. Measure ~1.5 g of the mixture, record the mass in your lab notebook, and pour it into the beaker.
   • Label a 50-mL beaker ‘DCM’. Bring this beaker and a watch glass to the solvent fume hood. Pour ~40 mL of DCM into the beaker. Cover the beaker with the watch glass and take it back to your hood.
   • Measure 20 mL of DCM with a graduated cylinder and pour it into the beaker of solids. Stir the mixture with a glass rod for 1 – 2 min and break up any clumps that form. Thoroughly mix the solid to ensure that the soluble substances dissolve completely.
   • Use a Pasteur pipette to wet the filter paper with about 2 mL of DCM to fix it in place.
   • Briefly stir the mixture to ensure that the insoluble material is suspended in solution and then quickly pour it into the funnel.
   • Wash the inside of the beaker with ~10 mL of DCM. Pour the rinse into the funnel and make sure that no solid remains in the beaker. Cover the DCM beaker with a watch glass and put it aside.
   • Once the solution has finished dripping into the separatory funnel, label a large watch glass ‘cellulose’ and remove the glass funnel from the separatory funnel.
   • Remove the filter paper from the funnel, being careful not to tear it or spill the solid, and gently lay the paper on the watch glass. Then, change your gloves.
   • Set the watch glass out of the way in the fume hood to let the filter paper and cellulose dry (1 hour).
2. Acid-base Extraction to Separate Benzoic Acid and CaffeineExpand

   In this part of the experiment, you will separate benzoic acid and caffeine using DCM and water for solvents. This makes a good solvent pair for liquid-liquid extraction because DCM and water are immiscible and have different densities. However, caffeine and benzoic acid are both much less soluble in water than in DCM.

   To separate these compounds, you'll mix the benzoic acid and caffeine solution with sodium hydroxide to convert benzoic acid to sodium benzoate, which is highly water-soluble and virtually insoluble in DCM. The caffeine will be mostly unaffected. When the mixture settles into layers, all sodium benzoate will be in the aqueous layer because it's insoluble in DCM.

   A small amount of caffeine will be in the aqueous layer, but most of it will stay in DCM. You'll keep the aqueous sodium benzoate layer for the last part of the lab, where you will evaporate the solvent from the organic layer to recover solid caffeine.

   Sodium hydroxide is corrosive, so be careful while handling and transporting it. Strong bases can cause ground-glass surfaces like the separatory funnel joint and stopper to fuse together, so always add sodium hydroxide through a long stem glass funnel to keep it from contacting the ground glass.

   • Label two 100-mL beakers as ‘aqueous layer’ and ‘organic layer’ and label a 50-mL beaker ‘1 M NaOH’.
   • Bring the NaOH beaker, a watch glass, and a clean 50-mL graduated cylinder to the hood where bases are kept. Measure 40 mL of 1 M NaOH and pour it into the beaker. Cover the beaker and bring it and the graduated cylinder back to your hood.
   • Place the glass funnel back in the top of the separatory funnel. Measure 20 mL of 1 M NaOH and pour it into the separatory funnel.
   • Spread a thin layer of vacuum grease evenly around the sides of the separatory funnel stopper. Use lab wipes to clean off excess grease and remove any grease on the bottom of the stopper to avoid contaminating the solution.
   • Stopper the separatory funnel, carefully remove it from the stand, and while holding the stopper in place, vigorously shake the funnel vertically for ~ 1 min.
   • Keep holding the stopper in place and carefully invert the funnel by about 150 ° with the stem pointed into the hood. Open the stopcock to vent the built-up DCM vapor. Once the vapor has escaped, close the stopcock. Shake and vent the separatory funnel about 25x in this way.
   • Place the separatory funnel upright in the ring fixture and remove the stopper. Do not disturb it as the organic and aqueous solution separate into layers. Note: Water is less dense than DCM, so the aqueous layer will be on top of the organic layer.
   • Once the layers have separated, place the organic beaker under the separatory funnel. Carefully open the stopcock without jostling the separatory funnel and let the bottom layer drain into the beaker. Close the stopcock as soon as the last of the organic layer passes through it. Note: You'll mix the organic layer with another portion of aqueous base, so it's better to let some of the aqueous layer through here than to leave DCM behind.
   • Set the collected organic layer aside and put the beaker labeled aqueous layer under the separatory funnel. Open the stopcock again to drain the aqueous layer into the beaker. Close the stopcock once the funnel is completely empty.
   • Use a lab wipe to clean off residual grease inside the separatory funnel joint and pour the organic layer back into it using the glass funnel.
   • Add the remaining 20 mL of 1 M NaOH to the separatory funnel.
   • Apply a fresh layer of grease to the stopper and place it in the funnel. Vigorously shake and vent the separatory funnel about 25x. Note: While this will reduce the amount of caffeine that you recover, you'll neutralize any leftover benzoic acid, improving the purity of the organic layer.
   • Once the solutions are mixed well, secure the funnel upright, remove the stopper, and wait for the layers to separate.
   • Place the organic beaker under the funnel and collect as much as of the organic layer as you can without letting the aqueous layer through.
   • Drain the aqueous layer into the corresponding beaker. Set the aqueous layer aside for now and carefully, put the empty separatory funnel out of the way in the hood.
   • Obtain some magnesium sulfate in a weighing boat and use a spatula to add a small amount of it to the organic layer. Gently swirl the beaker so that all the powder contacts the liquid. Note: If the magnesium sulfate turns into translucent clumps, it is picking up water right away and won't be able to absorb more.
   • Continue adding magnesium sulfate until a portion stays white and powdery when you swirl the beaker.
   • Let the magnesium sulfate dry the organic layer for 15 to 20 min.
   • While you wait, measure the mass of a 100-mL Erlenmeyer flask and record it in the caffeine row in your lab notebook. Then, secure the flask in your hood with a 3-prong clamp and place a glass funnel in the flask.
   • Once the organic layer has dried for at least 15 min, obtain another piece of circular filter paper, fold it in quarters, and put it in the funnel.
   • Use a clean Pasteur pipette to wet the filter paper with DCM. Then, pour the organic layer into the funnel and rinse the inside of the beaker with 1 – 2 mL of DCM. Wait for the solution to pass through the filter completely.
   • Remove the funnel and set the flask on a hot plate. Heat the solution to 40 °C to evaporate the DCM, leaving solid caffeine behind. This usually takes about 10 min.
   • While you wait, check on the drying cellulose. Use a glass rod to gently break up clumps to help it dry faster, but be careful not to tear the filter paper.
   • Once the DCM has evaporated completely from the caffeine, turn off the hotplate and let the flask cool. Break up clumps with a glass rod if you see them.
   • Measure the combined mass of the flask and caffeine and record it in your lab notebook. Keep the flask in a safe place during the last part of the lab.
3. Reprotonation of Benzoic AcidExpand

   In the last part of the lab, you'll use hydrochloric acid to reprotonate the benzoate anion. Hydrochloric acid is toxic and corrosive, so be careful with it. Benzoic acid is minimally soluble in water, so most of it will precipitate from solution, allowing you to collect it by filtration.

   • Fill a large beaker 3/4 full of crushed ice and add just enough water to fill the spaces between the ice. Place the beaker of the combined aqueous layers in the bath.
   • Label two 10-mL graduated cylinders ‘3 M HCl’ and ‘deionized water’. Add ~10 mL of deionized water to the corresponding graduated cylinder. Bring the other graduated cylinder to the hood for acids and obtain about 5 mL of 3 M HCl.
   • Obtain two clean Pasteur pipettes, and fill one pipette with 3 M HCl. Stir the HCl into the chilled aqueous solution a few drops at a time. Use pH paper to check if the solution has reached the target pH range of 1 – 3. Keep adding 3 M HCl in this way until you reach the target pH.
   • Let the solution sit undisturbed in the ice bath for about 10 min.
   • Assemble a vacuum filtration setup using a 125-mL filter flask, a Büchner funnel, circular filter paper, and a rubber adapter.
   • Use a clean Pasteur pipette to wet the filter paper with a few drops of deionized water.
   • Once the aqueous solution has been sitting for 10 min, pour it into the Büchner funnel and open the flask to vacuum.
   • Place the graduated cylinder of deionized water in the ice bath and let it cool for 3 – 5 min. Use a spatula to scrape the remaining solid in the beaker into the funnel.
   • Rinse the beaker with ~3 mL of cold water and pour the contents into the funnel. Make sure that all solids are transferred.
   • Leave the vacuum running until water stops dripping from the funnel. Then, close the vacuum and break the seal to the flask. Place the funnel upright in an empty flask to let the benzoic acid dry completely (1 hour).
   • While you wait for the benzoic acid to dry, dispose of the magnesium sulfate in a designated waste container, pour leftover DCM into a halogenated waste container, and place used Pasteur pipettes in the glass waste container.
   • Combine the aqueous filtrate and any leftover HCl in the aqueous beaker. Dilute the mixture with ~10 mL of deionized water and neutralize it to about pH 7 with sodium bicarbonate. Flush the aqueous waste down the drain with running tap water.
   • Wash the rest of your glassware and put away your equipment according to your lab's usual methods, leaving aside the three compounds.
   • Once the benzoic acid is dry, zero a weighing boat on a top-loading balance and transfer the solid to the boat using a clean spatula. Record the mass in your lab notebook.
   • Calculate the masses of cellulose and caffeine by subtracting the masses of the empty vessels from the combined masses of the vessels and compounds.
   • Add together the three masses of the compounds and compare it to the starting mass. Note: If the sum is greater than the starting mass, then a compound wasn't dry when you weighed. Reweigh the compounds and check the masses again.
   • Once you have all three dry masses, dispose of the compounds in the designated waste container and wash your remaining glassware. Lastly, clean the floor of your hood with damp paper towels and throw out any remaining paper waste.
4. ResultsExpand
   • Determine the composition of the unknown mixture based on the masses recovered for each compound. Calculate the mass percentages of cellulose, caffeine, and benzoic acid in the unknown mixture. Note: The actual mass composition of the mixture was 5% cellulose, 47.5% caffeine, and 47.5% benzoic acid.
   • Based on your starting mass, calculate the theoretical initial mass of each compound in your sample, and compare these values to your data.

     Table 2: Percent yield of recovered compounds

     Compound	Starting mass percentage	Theoretical initial mass (g)	Recovered mass (g)	Percent yield
     Cellulose	5
     Caffeine	47.5
     Benzoic acid	47.5
     	Starting mass of mixture (g)		–	–

     Click Here to download Table 2
   • Here are some common sources of error from this experiment.

     Error	Source of Error
     Cellulose mass is very high and the other compound masses are low	Caffeine and benzoic acid might not have dissolved completely at the start of the lab and instead remained on the filter paper labeled for cellulose.
     Benzoic acid mass is low compared to the recovered caffeine	Benzoic acid was not completely converted to sodium benzoate, leaving a mix of caffeine and benzoic acid in DCM.
     Caffeine recovered mass is low	Loss due to the aqueous washes, particularly if the aqueous base was warm
     Low benzoic acid mass recovered	Incomplete protonation of sodium benzoate, solution too warm, or precipitation disturbed