CHEM 1102
Preparation of Alum
When waste materials are recycled or converted to useful
chemicals our natural resources are also conserved. Re-use of aluminum cans in particular,
not only conserves our supply of aluminum, but it also facilitates removal of an otherwise
long-lasting source of litter. While the best re-use of aluminum cans is to recycle them
as cans, they may also be used to produce other useful products.
For example, alum [KAl(SO4)2×12H2O], a chemical
used in dyeing cloth and in making pickles, can easily be made from aluminum cans, even if
they are mixed with iron contaminants. Aluminum is easily separated from iron since
aluminum (but not iron) dissolves readily in hot aqueous potassium hydroxide according to
the equation:
2Al(s) + 2KOH(aq) + 6H2O(l)
® 2KAl(OH)4(aq)
+ 3H2 (g)
The addition of sulfuric acid first forms the precipitate
Al(OH)3, which dissolves when the solution is warmed.
2KAl(OH)4(aq) + H2SO4(aq)
® 2Al(OH)3(s)
+ K2SO4(aq) + 2H2O(l)
2Al(OH)3(s) + K2SO4(aq)
+ 3H2SO4 ® 2KAl(SO4)2(aq) + 6H2O(l)
On cooling, alum forms according to the following reaction:
KAl(SO4)2 +
12H2O(l) ® KAl(SO4)2×12H2O(s)
Thus, the overall equation for the formation of alum from
aluminum is:
2Al(s) + 2KOH(aq) + 4H2SO4
+ 22H2O(l) ® 2KAl(SO4)2×12H2O(s) + 3H2(g)
Purpose
To prepare alum from aluminum and to determine the yield
and melting point.
Procedure
Materials
aluminum scrap (you may bring aluminum cans from home)
scissors or tin snips
250 ml beaker
KOH (80 gm/liter)
50/50 mixture of alcohol and water
suction flask -- to be checked out from Stockroom
Buchner funnel-- to be checked out from Stockroom
There are no toxic wastes in this experiment. Excess KOH or
H2SO4 can be poured into a large beaker of water in the sink and
then poured down the drain. Solids can go in the trash.
1. Put on plastic gloves to protect your hands. Wear
safety goggles.
2. Accurately weigh out 1 gm of aluminum scrap, cut into
small pieces (about 1-2 mm square) and put the pieces into a 250 ml beaker.
3. In the HOOD, add 50 ml KOH solution. BE CAREFUL! DO NOT
SPATTER THE SOLUTION. THE KOH IS VERY CAUSTIC.
4. Heat the beaker gently over an open flame or on a hot
plate. Large quantities of H2 will be evolved. (Remember H2 is
explosive, so do this in a HOOD!!!). Continue heating (approx. 10 minutes) until the
aluminum is completely dissolved and no "fizzing" around each piece occurs.
There may be some impurities (perhaps black specks) which do not dissolve.
5. If the volume drops below 1/4 the original volume, add
distilled water from your wash bottle to bring back to about 1/2 the original volume. This
will help reduce spattering.
6. Using your tongs, remove the beaker from the heating
unit. Use a glass rod to remove small pieces of solids.
7. If necessary to remove suspended solids, filter hot
solution through glass wool or through filter paper.
8. Cool the solution.
9. While continuously stirring, slowly add 9 M
H2SO4 until pH paper shows that the pH is about 1--2. 2 to 5 ml may
be required. Large lumps of Al(OH)3 may appear.
10. Heat gently for at least 10 minutes at 60°C - 80°C--DO NOT BOIL!--or until all the
Al(OH)3 dissolves. (Periodically check the pH and add more 9 M H2SO4
if the pH rises above 1-2). But do not heat more than 20 minutes. A clear solution should
result (but small black specks may be observed in the clear solution).
11. Remove from heat.
12. Cool in an ice bath for 20 minutes to form alum,
(KAl(SO4)2×12H2O), crystals.
13. Collect the crystals by vacuum filtration using a
Buchner funnel and a filtering flask.
14. Wash the crystals with 20 ml of a 50/50 mixture of
alcohol and water. Dry the crystals by suction.
15. Weigh the crystals. Enter weight in your notebook.
16. Obtain the melting point (m.p.) of the crystals.
In your notebook indicate the answers to:
1. The number of grams of Al recovered in the alum.
2. The theoretical yield of alum if aluminum scraps were
pure. (Hint: consider Al to be the "limiting" reagent and that all
other reagents are present in excess quantities.)
3. The percent yield.
4. The melting point.
