University of Mary Washington Extraction of Unknown Chemistry Lab Report

ExtractionBackground
Extraction is a technique that separates compounds (usually solids) based on solubility. Depending on the phases involved,
extractions are either liquid-solid or liquid-liquid. If you have ever brewed a cup of tea or boiled bones for soup, you have
performed a liquid-solid extraction. Compounds that are soluble in hot water (the liquid) are extracted from tea leaves or
bones (the solid). Flavoring extracts are also made by liquid-solid extraction; vanilla extract is made by soaking vanilla
beans in alcohol.
In organic laboratory, liquid-liquid extraction is most commonly used. Liquid-liquid extraction requires two immiscible
liquids known as the organic phase and the aqueous phase. The aqueous phase is water-based and can be an acidic, basic,
neutral, or a saturated salt solution. The organic phase is an organic solvent, usually diethyl ether or dichloromethane,
which has minimal solubility in water. For instance, ethanol would be a poor extraction solvent because it forms a solution
with water. Organic extraction solvents do not mix with water, they form distinct layers, much like oil and water. The
denser liquid is the bottom layer. Compounds can be separated based on which liquid they are more soluble in.
Extraction works based on solubility and solubility is based on the principle of “like dissolves like”. Neutral organic
compounds will dissolve in the relatively non-polar organic phase; highly polar and ionic compounds will dissolve in the
aqueous phase. Certain neutral organic compounds can be converted to ions with acid-base chemistry, which changes
them from organic soluble to water-soluble. Acid-base extraction takes advantage of this change in solubility to separate
compounds.
To demonstrate the principles of extraction we will consider the separation of four compounds: aniline, propanoic acid,
phenol, and naphthalene. Aniline, acetic acid and phenol can be converted to ions under relatively mild conditions
therefore they can be extracted into the aqueous phase. Naphthalene cannot be converted to an ion so it will always
remain in the organic phase. By a sequence of extractions the four compounds can be separated.
NH2
+
acid
+
base
NH3
weak base
pKa = 4.63
(of ammonium ion)
weak acid
pKa = 4.8
weaker acid
pKa = 9.9
not acidic or basic
pKa = 43
Aniline
O
OH
Propanoic acid
O
O
OH
O
+
base
Phenol
Naphthalene
We will begin by dissolving a mixture of these four compounds in an organic solvent then bring them in contact with an
aqueous phase. To protonate aniline and make it water soluble, it must come in contact with an acid that is stronger
than its conjugate acid, that is, an acid with a pKa at least 1 unit lower than 4.63. HCl (pKa = -7), is acidic enough to
protonate aniline and draw it into the aqueous phase. After the two phases are brought in contact, the aqueous layer
that contains the protonated aniline can be drawn off, thus separating it from the rest of the compounds.
organic phase
organic phase
OH
O
NH2
OH
O
OH
OH
aqueous phase: 1.0 M HCl
aqueous phase: 1.0 M HCl
NH3
Next, we will bring the organic layer in contact with 1.0 M NaHCO3 (H2CO3 pKa = 6.35). Note that propionic acid is a stronger
acid than H2CO3 (the conjugate acid of NaHCO3) but phenol is not. This means that NaHCO3 is strong enough to
deprotonate propionic acid but not phenol therefore only propionic acid will be ionized and drawn into the aqueous phase.
The aqueous phase is separated, leaving phenol and naphthalene in the organic phase.
organic phase
organic phase
OH
OH
O
OH
aqueous phase: 1.0 M NaHCO3
aqueous phase: 1.0 M NaHCO3
O
O
Finally, we will bring the organic layer in contact with 3.0 M NaOH (H2O pKa = 15.74), a base that is strong enough to
deprotonate phenol. This brings phenol into the aqueous layer leaving only naphthalene in the organic phase.
organic phase
organic phase
OH
aqueous phase: 3.0 M NaOH
aqueous phase: 3.0 M NaOH
O
Once the layers are separated, all four compounds will have been isolated. Addition of acid or base will bring the aqueous
ionic species back to their original neutral form.
organic phase
3.0 M NaOH
O
1.0 M NaHCO3
add acid

1.0 M HCl
add acid

O
O
NH3
add base

Practical Aspects of Extraction
glass stopper
The Separatory Funnel
Separating two the two liquid layers in an extraction requires a specialized piece of
equipment called a separatory funnel. A separatory funnel (often shortened to “sep
funnel”), seen in the drawing on the right, has a ground glass joint with a fitted stopper
on the top and a stopcock on the bottom which leads to a narrow spout. The stopper
is never greased. The stopcock, which is made of Teflon, should be stored loose and
tightened before using – it should feel snug but still turn easily. When the stopcock is
horizontal, the valve is closed, when vertical it is open. When the liquids are ready to
be separated, the sep funnel is placed in a ring support (not a clamp), the glass stopper
is removed, the stopcock is opened and the bottom layer drains into a clean flask. If
the flow stops soon after you begin, you have probably forgotten to remove the
stopper and the pressure difference is preventing flow. When the interface between
the two liquids gets to the top of the stopcock, the stopcock is closed leaving the top
layer in the sep funnel and the bottom layer in a flask thus separating the two liquids.
ground glass joint
stopcock
Separatory Funnel
Making Contact Between the Layers
In order for the extraction to take place, the two phases must come in contact with one another. Vigorous shaking is the
best way to get contact. During extraction, the funnel is inverted and vigorously shaken. This allows the solids being
separated to migrate into the layer they are most soluble in. During the shaking process it is important to securely hold
the glass stopper in with your hand as pressure can build up inside the sep funnel; heat transferred from your hands or
from an exothermic heat of dissolution will vaporize some of the low boiling organic solvent and produce pressure. It is a
good idea to pause from shaking a few times, point the spout into the hood (not at someone’s face!) and open the stopcock
to release pressure. This is especially important when your extraction liquids produce gas. (For instance, shaking acids
with sodium bicarbonate, NaHCO3, will produce carbon dioxide gas.)
Getting the Most out of an Extraction
The dissolving process is often not an all-or-nothing proposition. For example, 90% of a solid might be extracted into the
aqueous layer while 10% remains in the organic layer. Since not everything can be extracted in one go, extraction solvents
are always added two or three times. Chemists will shake with an extraction solvent, separate the layers, then shake with
a fresh portion of extraction solvent, separate the layers and combine the two common solvents. If 90% of a solid is
dissolved each time, two extraction cycles should extract 99% of the material.
Dealing with Emulsions
Ideally, as soon as you stop shaking, the liquids will separate into two clearly defined layers. In practice, shaking can lead
to an emulsion – a stable or semi-stable mixture of immiscible liquids. If you do not see two layers or if the interface of
the two liquids is fuzzy, you have an emulsion. Sometimes emulsions will settle out given time, other times, they may
need help. One remedy is to add a few milliliters of saturated NaCl solution to the funnel and swirl (never shake again if
you have an emulsion) – NaCl makes the aqueous layer more polar, making an emulsion with an organic layer less
favorable. If an emulsion does not go away on its own within a few minutes, consult your instructor.
Another situation where students only get one layer is when they accidentally add two aqueous layers to the funnel, rather
than having an aqueous and an organic layer. Label beakers to keep track of which beaker contains what and don’t throw
out any washes until the end of the lab when you are sure you have what you need!
Recovering Solids
When the extraction process is over, the chemicals need to be recovered from the solvents. The aqueous solutions contain
ionic forms of the compounds. Neutralizing the solution with acid or base will convert the ions back to neutral organic
compounds. Once neutralized, the compounds will no longer be soluble in water and will precipitate. In this lab you will
extract with weak base (NaHCO3) and strong base (NaOH). Both solutions should be chilled then neutralized with 6.0M
HCl. The resulting precipitates can be isolated with vacuum filtration.
During extraction organic solvents pick up trace amounts of water that need to be removed before recovering the solid.
To “dry” the solvent (remove traces of water) a drying agent is used, in our case, anhydrous MgSO 4. Add a small spatula
tip (the volume of a pea) of MgSO4 to the Erlenmeyer that holds the diethyl ether solution and swirl gently. When MgSO4
absorbs water it clumps together and looks sticky. Continue adding magnesium sulfate in small portions and swirling until
the solid no longer sticks to itself or the sides of the flask. When the recently added magnesium sulfate is loose and sandy
when swirled, it indicates that there is no more water to react with and you can stop adding. Do not over-add drying
agents – product will stick to its surface and may decrease your yield. Ask the instructor if you are unsure how much to
add.
Once dry, you will cold gravity filter your diethyl ether solution into a pre-weighed round bottom flask to remove the
drying agent. Hot filtration and vacuum filtration are never used with low boiling organic solvents. The solvent will be
removed with a rotary evaporator (known as a “rotovap”) – an instrument which quickly performs simple distillations
under reduced pressure.
Extracting verses Washing
The terminology used in extractions differs based on our purpose. If we are shaking an organic layer with bicarbonate to
remove trace acid impurities, we say that we are “washing” the organic layer with bicarbonate. If we are shaking with
bicarbonate to dissolve a desired carboxylic acid product in the aqueous layer, we say we are “extracting” with
bicarbonate. We say we washed with a solvent if we will later discard it. We say we extracted with a solvent if we will use
its contents.
Procedure
In this laboratory exercise, you will separate up to three solids using acid-base extraction. Your unknown will contain two
or three of the compounds below. Use the knowledge of their structures and the example given in this handout to make
sense of the procedure below. It is helpful to draw a flow chart of the procedure in your notebook. Figure out which
compounds will be in each layer at each stage of the extraction. Determine which acid-base reactions will take place
based on the pKa data below.
Benzoic Acid
2-Naphthol
Biphenyl
pKa = 4.2
pKa = 9.5
pKa = 42
Extraction
You will be given a vial that contains either 2 or 3 of the chemicals listed above. Dissolve about 0.8 g of the unknown in 30
mL of diethyl ether in an Erlenmeyer flask. Once dissolved, pour the solution into the separatory funnel. If the unknown
doesn’t dissolve completely, continue anyway.
Extract the organic layer with 7 mL of 1.1 M NaHCO3. Drain and reserve the NaHCO3 layer. Extract the organic layer a
second time with a fresh 7 mL of NaHCO3. Combine the two bicarbonate extracts and set aside.
Extract the organic layer 7 mL of 3.0 M NaOH. Drain and reserve the NaOH layer. Extract the organic layer a second time
with a fresh 7 mL of NaOH. Combine the two extracts and set aside. Pour the organic layer into a separate Erlenmeyer
flask.
Recovery of solids
Remove any traces of water from the diethyl ether layer by adding small portions of anhydrous MgSO 4 until it no longer
sticks to itself. Cold gravity filter the solution into a wide beaker. Heat the beaker on a hot plate to dryness.
Reclaim benzoic acid and 2-naphthol by cooling the NaHCO3 and NaOH extracts in an ice bath then acidifying each
extraction with 6M HCl. Neutralizing NaHCO3 will produce large amounts of carbon dioxide. Calculate the approximate
volume of 6M HCl needed to neutralize each solution before lab. Add this amount and enough extra to complete
precipitation. The pH may be checked by seeing if it turns blue litmus paper red. Cool the mixtures in an ice-water bath,
vacuum filter each separately.
Characterization
Determine which solids were in your unknown. Record the weight of any solids recovered and take their melting point.
Solids recovered from aqueous solvents must dry until the next lab period before their meting point is measured.
Chemicals: diethyl ether, 1.1 M sodium bicarbonate, 3.0 M sodium hydroxide, 6M HCl, benzoic acid, 2-naphthol, biphenyl,
magnesium sulfate.
Waste
– Filtered aqueous layers go in the labeled waste jug for this lab.
– Solids go in the disposal jar for this lab.
Common Mistakes
 Trying to drain the funnel without removing the stopper. If you notice no liquid flowing, this is the problem
 Greasing the stopper
 Allowing pressure to build up by not venting
 Venting in someone else’s face rather than in the hood
 Mixing up layers or prematurely discarding layers
 Adding liquid to a sep funnel when the stopcock is open
Extraction Report Guidelines and Rubric
General Guidelines:
● Reports must be typed.
● The report must be in your own words. Plagiarized reports will earn a zero. Review what
constitutes plagiarism here if needed.
● Upload to Canvas and make sure that the file can be displayed – some file types may
need to be converted to pdfs
Your report must include the following sections:
Title: Extraction of Unknown …[write the number/letter you were given]
Name/Course/Semester/Year: Ex: Emil Fischer, CHM245.001A, Spring 1871
Experimental: Briefly describe what you did in the laboratory in your own words in past tense.
● Use of “I” or passive voice are both fine.
● Be sure you are writing what you actually did in the laboratory if it differed from the
procedure. Include volumes or masses for all chemicals used. For instance, write the
amount you massed, not what is in the procedure.
Results Don’t interpret or analyze in this section, just state the data.
● Insert table with three lines – one for each solvent in the extraction. For each write
○ the appearance, mass, and melting range of the solid (include units)
○ or note that a solid was not obtained.
Discussion: Discuss each extraction solvent (hydrogen carbonate, hydroxide, ether) in a
separate paragraph. For each solvent
● Make a claim – the presence/absence of a solid in the layer indicates presence/absence
of which molecule?
● Explain how you know the molecule is present/not present based on acidity and
solubility principles. pKa should be part of your discussion. For aqueous solvents,
explain the role of HCl in recovery.
● Compare the melting point to the literature values if a solid was present. Discuss if this
supports or conflicts with what you determined based on acidity.
Conclusion: State which solids you determined to be in your unknown. Summarize the main
support for this conclusion in one sentence.
Rubric
Title and name
1
Experimental
Includes actual masses and
volumes used.
2
Results
Melting points, masses and
appearances included with
units.
2
Discussion
Acidity, solubility, pKa
thoroughly discussed and
correctly interpreted for each
solvent.
6
Melting points compared to
literature values
3
States correct solids present
1
Conclusion
Total
15