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Introduction
Alums are a class of compounds that
are usually composed of a monovalent
metals, a trivalent metal and sulfate.
The most common alum is probably potassium aluminum sulfate which is
also called potash alum. When these
compounds are synthesized and crystallized, the crystal structure often
includes a significant amount of water in proportion to the alum. Thus, the alum that we will synthesize in
this lab exercise is KAlSO4·12H2O.
Alums are used in water purification,
leather tanning, mordant dyeing, as astringents, and in baking powder. Alum occurs in nature as the mineral kalunite. Sodium aluminum sulfate, or soda alum, NaAl(SO4)2·12H2O,
is also used in baking powder. Ammonium aluminum sulfate, or ammonia alum, NH4Al(SO4)2·12H2O,
is used in tanning, in dyeing and fireproofing textiles, in vegetable glues
and porcelain cements, and in water purification. Chromium potassium sulfate,
or chrome alum, KCr(SO4)2·12H2O, is used as
a mordant in dyeing, in tanning, and in photographic fixing baths to harden
gelatin films and plates.
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Materials
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Aluminum foil
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Sulfuric Acid, 3M
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Potassium Hydroxide. 3M
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Ethanol/Water (50/50)
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Beaker, 250 mL
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25 mL
Grad. Cylinder
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Funnel
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250 mL
Flask
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Filter Paper, fluted
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Wash bottle (distilled H2O)
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Stirring rod
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Hot plate/stirrer
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Balance
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Parafilm®
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Magnetic stirring bar
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Safety
Goggles must be worn at all times
during this experiment. Sulfuric acid
is is extremely corrosive and potassium hydroxide
is extremely caustic. Use caution when
using these chemicals. Should anything
get on your hands or clothing wash off immediately.
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Procedure
1. Obtain about 1 g of aluminum foil and
weigh to the nearest hundredth of a gram. Tear the foil into small pieces and
place in a clean 250 mL beaker.
2. Add 25mL of 3M KOH solution to the
aluminum foil in the 250 mL beaker using a 25 mL pipette. Allow
reaction to proceed until all the aluminum is dissolved.
3. Flute a piece of filter paper and
place in a funnel. Place the funnel
into 500 mL flask.
Filter the dissolved aluminum while hot to remove the carbon. Wash the beaker and the filter paper with a
small amount of distilled water.
Discard the filter paper and rinse the funnel. If necessary, this is a place to stop and
store solution over night.
4. Obtain 35 mL
of 3M H2SO4 in a graduated cylinder. Acidify the dissolved aluminum/KOH by
adding slowly while stirring the 35 mL of 3M H2SO4. Use a glass rod to stir the solution. You will notice that a white precipitate
forms and then dissolves in the acid.
You will also notice that the solution becomes very hot as a result of
mixing a strong acid with a strong base
5. If precipitate remains, filter the
solution again as described above.
6. Boil the solution to a volume of
about 40 to 50 mL.
Allow the solution to cool slowly. This is a good stopping place.
7. Once the solution has cooled to room
temperature, cover with Parafilm® or plastic
wrap. You may wish to experiment with
placing some of the flasks in the refrigerator. Crystals
should form overnight.
8. Collect the crystals on a piece of
pre-weighed filter paper and wash briefly with the 50% methanol.
9. Determine the percent yield relative
to the expected yield using a mole ratio of 1 mol aluminum to 1 mol potassium
aluminum sulfate duodecal (12) hydrate.
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Questions
1.
If a 1:1 ratio of aluminum to alum exists, what mass of
aluminum would be required to produce one pound of alum.?
2. In the
procedure to produce alum, name two places where the overall yield of alum
would be reduced?
3. Why is the
final filtration a wash with 50% methanol instead of just water?
4. After the
digestion of the aluminum foil with KOH there was a dark suspended material
in the solution which was removed by filtration. What is this material? Whay is it
present? What is the basis or source
of your answer?.
5. What is
the purpose of fluting the filter paper during the filtration steps?
6. What was
the purpose of boiling the alum reaction after the addition of the sulfuric
acid?
7. Alum can
be used as a flocculent. What is a
flocculent and where are they used?
8. What is an
astringent?
9. What is
the gas that is evolved during the initial reaction of the aluminum with the KOH.
10. What are
the different chemical reactions that are involved in alum synthesis. Describe each one and the evidence for the
type of reaction.
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