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Chem 2123
Experiment 2
Separation and Purification Techniques:
Acid/Base Extraction, Distillation, and Recrystallization
Purpose :
To separate, purify and identify components of an unknown mixture of compounds using
extraction, distillation and recrystallization techniques. Chapters in Mohring to read – 4, 5, 6, 7, 10, 11, 12,
13, 15
Equipment: Büchner funnel, separatory funnel, stemless funnel, small filter paper, large filter paper.
Website link to videos of interest!! - http://bcs.whfreeman.com/mohrig3e/ under standard taper and
Williamson resources look at extraction,
Chemicals: 10% NaHCO3, 1.25 M NaOH, 6 M HCl, activated carbon, MgSO4
Mixture Components:
Liquids
Methyl tert-butyl ether (MTBE)
Toluene
Unknown Solids
Benzoin/Benzil
Dimethylterephthalate
Benzoic Acid
4-t-Butylphenol
2-Chlorobenzoic acid
2-Naphthol
o-Toluic acid
Triphenylmethane
Triphenylmethanol
Notes:
1.
2.
3.
Boiling Point (°C)
54-56
110-111
Boiling Point (°C)
-
Melting Point (°C)
Melting Point (°C)
132-137/94-96
141-142
122-123
98-101
141-143
122-123
104-105
92-94
162-163
Do not leave containers of MTBE or MgSO4 open.
Work in the hood!!!
Cover the MTBE when walking your beaker or graduated cylinder to your hood
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Procedure:
Week One
Obtain an unknown mixture from the TA and determine the total mass, ~1.6 grams. The mixture will contain
0.8 g each of two unknown solids. Dissolve the solid in a mixture starting with 15 mL of toluene. Then add
~10 mL MTBE. Note which seemed to dissolve more of the unknown. Use additional amounts of whichever
solvent seems to dissolve your sample to complete transfer and dissolution. Don’t use more than 50 mL of
total solvent. It shouldn’t take ~5 minutes to dissolve the sample. Record the amount of each solvent
you added.
Transfer the mixture to a separatory funnel. Add about 10 mL of water and note which is the organic layer
and which is aqueous. Add 10 mL of 10% aqueous sodium bicarbonate (NaHCO3), stopper the funnel and mix
the contents taking care to vent the solution frequently*. Once you’ve vented a couple times, shake the funnel
more vigorously and allow the layers to separate. Collect the aqueous layer in a labeled Erlenmeyer flask
Repeat the process using a second 10 mL of water and 10 mL of sodium bicarbonate solution and collect the
aqueous layer in the same Erlenmeyer.
*When sodium bicarbonate reacts with acid, it forms carbonic acid, which readily forms CO2 gas. Just as a
shaken can of soda sprays liquid all over the place, the carbon dioxide in your separatory funnel will take any
liquid that gets in its way along for the ride. Make sure there’s no liquid trapped at the neck of the stopcock
when you vent, and vent carefully.
Add 10 mL of 1.25 M aqueous sodium hydroxide (NaOH) to the funnel, stopper, and carefully mix the
contents as before. Remember to vent the solution in the funnel during mixing. Draw off the lower aqueous
layer into a new Erlenmeyer flask. Repeat with a second 10 mL of NaOH solution, and add this to the same
flask.
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Add 10 mL of distilled water to the organic layer in the funnel, stopper and carefully mix as before. Drain the
water layer into a waste beaker and work up the organic layers as described in next section.
Collect the organic layer from the funnel and add a small amount (size of half a grape) of anhydrous MgSO4.
Swirl the flask and, if the MgSO4 doesn’t flow freely, add a little more. After drying for ~ 5-10 min, filter the
solution through a gravity filter into a clean, dry 50-mL roundbottom flask. Cap the flask, label it with your
name and date, and store it in the flammable cabinet to use next week.
Calculate the approximate amount of acid required using M1V1 = M2V2 and review number with TA
before proceeding. Do this calculation before lab!!! We want you to do this calculation so you know about
how much acid you need so that when you go to get the acid out of the container, you take an appropriate
amount and don’t take too much.
Acidify the contents of the other flasks (labeled #1 and #2) by addition of 6 M hydrochloric acid (HCl),
swirling the solution gently and test the pH with litmus paper. Slow addition is particularly important for the
NaHCO3…dropwise is recommended.
Note which flask produced solids in your notebook, this is key information for interpreting the results.
Cool any flasks with precipitates in an ice water bath. Isolate the solids by vacuum filtration with a Büchner
funnel. Wash the solid with a small amount of ice water and disconnect the vacuum.
Check the filtrate solution to see if any more solids form by adding a little 6M HCl, to see if any more
precipitate forms. If so, you should also collect this by vacuum filtration.
Collect any solid on a pre-weighed watch glass and air dry for at least a week. If the solid is clumpy, you
should break it up into small pieces with a spatula after drying for a few days, and then let it dry a few days
more.
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Extraction Flow Chart: What do I do with each flask?
Weigh unknown sample
Add 15mL of MTBE and Add 10mL of Toluene
Transfer to Separatory funnel
Add more solvent to transfer to sep. funnel
Add 10mL of DI-H2O to sep. funnel, Add 10mL of NaHCO3
Shake and vent carefully!
Aqueous layer
Organic layer
Shake and vent carefully!
Keep and label as Flask 1:
DI-H2O /NaHCO3 layers
Add 10mL of DI-H2O to sep. funnel, Add 10mL of NaHCO3
Shake and vent carefully!
Aqueous layer
Organic
layer and vent carefully!
Shake
Add 10mL of NaOH
Shake and vent carefully!
Add to Flask 1: DI-H2O
/NaHCO3 flask
Aqueous layer
Keep and label as Flask 2:
NaOH layer
Organic layer
Add 10mL of NaOH
Shake and vent carefully!
Aqueous layer
Organic layer
Add to Flask 2: NaOH layer
Add 10mL of DI-H2O
Shake and vent carefully!
Aqueous layer
Discard this water layer
Take Flask 1 and precipitate
SLOWLY with HCl, swirly
gently, and testing for neutral pH.
Take Flask 2 and precipitate
SLOWLY with HCl, swirly gently,
and testing for neutral pH.
Only ONE
of these
Flasks
(1 or 2)
should
form a
precipitate!
Organic layer
Drain into an Erlenmeyer
labeled as Organic Layer:
Add pea-sized amounts of
MgSO4, swirl flask, let sit for
10 minutes, then gravity filter
into a 50mL round bottom
flask.
Save this for next week!
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Week 2
Determine the mass of the recovered solid from week 1 and calculate the percent recovery. Also
determine the melting point of the dry solid. You may want to do a mixed melting point with the
known compound to confirm identity. Or you could do an IR to confirm identity.
Add a stir bar to the 50 mL round bottom containing your filtered organic layer. Assemble a simple
distillation apparatus (dry glassware) as shown below, using this flask as your still pot. Pay particular
attention to the location of the thermometer bulb, because if it’s too high or too low, your temperature
readings may be significantly off. When complete, ask a TA or the instructor to check your setup. (It’s a good
idea to pre-weigh the receiving flask!)
Heat the round bottom with a heating mantle and use a stir plate to stir. The exact heat setting will depend on
your particular mantle and heating controller, but ~3 is probably a good place to start. If nothing’s distilling
after 20 min, gradually adjust the setting to a higher level. As the material distills, more heat will be required
to keep it going. Therefore, you will have to increase the heat setting to 5-6 or more. Remember, there is a
lag time for the electrical heat!!
When liquid starts to drop in your collection flask, observe the thermometer and take an initial reading once
the temperature levels off. Continue collecting distillate until the temperature rises to about 100 °C. At that
point, switch the collection flask to a pre-weighed,100-mL round bottom or Erlenmeyer flask and continue
collecting until there’s less than ~1 mL of liquid left in the still pot or the temperature rises by another 15 °C.
Record the temperature range and amount of each liquid, mL or grams, collected in your notebook.
Determine the mass of MTBE and toluene that you distilled and using its density, calculate the volume
and percent recovery based on the original amounts you added in week 1 to dissolve the solids.
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Week 2 Prelab Questions (Due on the day Week 2 is performed in the lab)
1. In your Extraction from the previous week, you had two layers in the separatory funnel. One of these
layers was an aqueous layer while the other was an organic layer. What property determines which
layer is which in the separatory funnel?
2. If you do not know which layer is organic and which is aqueous in the separatory funnel, what test can
you perform to figure this out?
3. Sodium bicarbonate and sodium hydroxide are two different types of bases. Explain what type of base
they are and what type of acids they would be associated with in an acid/base extraction.
4. For a distillation, explain the importance of the thermometer placement.
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Week 3 – Recrystallization
After allowing the 50-mL round bottom to cool, a solid should form. Determine an appropriate solvent for
recrystallization of the impure solid provided. The three solvents to choose from are methanol/xylenes,
ethanol, and heptane.
Add a small amount of the solid (~10-20 mg) to three test tubes. To each test tube add a different solvent (~ 12 mL) and note the solid’s solubility at room temperature using a stirring rod for mixing. Heat any samples
that do not dissolve using a hot water bath and note whether the solid dissolves. If the solid dissolves on
heating, cool the test tube in an ice water bath and note whether solids reform. All of the solid may not
dissolve, but that is not important at this stage. The key question is which solvent provides crystals after
cooling in ice bath. Be cautious, the water is hot enough to melt some of the samples and they will then
solidify on top of water not form crystals in the water phase.
Once you have picked a recrystallization solvent, add ~10 mL of hot recrystallization solvent to the round
bottom flask. Use this liquid to transfer the solid in the rb flask to an Erlenmeyer flask and boil.
If some of the solids haven’t dissolved after 5 min of boiling, add additional solvent by pipet in ~2-10 mL
increments. Once the mixture returns to a boil, note whether more solids dissolve. If not much dissolves with
2 mL add more!!!! Repeat as necessary until solids dissolve. Since you will be performing a hot filtration,
you will need to add a small excess of solvent so that recrystallization doesn’t occur during filtration.
Add an extra 30% of the total volume (these solvents evaporate easily, so premature crystallization is
likely…you can always boil off the excess solvent after the hot filtration). This is an excellent time to add
your activated carbon while it is cool!
You will need to use activated carbon to adsorb any colored impurities. Remove the flask from the heat and
let it cool to just below boiling. Add a small amount of activated carbon (no more than 0.1 g, size of 2 peas)
and return the flask to the heat ing mantle. DO NOT add activated carbon to a boiling solution. It WILL
boil over!! Have some hot solvent ready, ~5-10 mL, to rinse your filter paper to recover any solids that
drop out.
Place a 125-mL Erlenmeyer and a stemless funnel in the oven for ~5 min and prepare a fluted filter. Perform a
hot, gravity filtration using the fluted filter, preheated funnel and Erlenmeyer flask. If needed, boil off the
excess solvent you added.
Allow the solution to cool to near room temperature. Complete the crystallization by cooling the flask in an
ice water bath for 10-15 min. You should also cool some of your recrystallization solvent in an ice bath during
this time for use later.
Collect the crystals on a Büchner funnel by vacuum filtration and wash the filter cake with small portions of
the cold recrystallization solvent. Spread the crystals on a pre-weighed watchglass using a spatula and allow
them to dry for one week.
Determine the weight and melting point (or you could take an IR) of the purified product. Calculate the
percent recovery.
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Questions to answer for lab report! (Make a section for these in your lab report and
answer them. They will be graded as a homework assignment.)
1. What is the solubility of most organic compounds in water?
2. What effect does reaction with NaOH or NaHCO3 have on organic compounds to affect their solubility in
water. What form are they in the water phase?
3. What does the addition of HCl do to cause the organic compound to become less soluble in water.
4. Is the MTBE you distilled pure MTBE or a mixture of MTBE/toluene, explain your answer.
5. How does the extreme volatility of MTBE during the extractions affect your recovery?
6. Describe how temperature affects solubility of a compound in a good recrystallization solvent versus poor
recrystallization solvent?
7. Why do the impurities stay in solution?
8. Where is your major yield loss in recrystallization, refer to answer to question 6 above?
Lab Report Outline
Include theory of extraction, distillation, and recrystallization. (1/2 page for each).
Correctly identity both compounds that were in your unknown mixture
% recovery of both compounds
% recovery of your MTBE and toluene from distillation
It would probably be a good idea to start working on your lab report throughout this three week lab
instead of waiting for the last week to start it.