EXPERIMENT 3
SYNTHESIS OF AN ALUM
KAL(SO4)212H2O
INTRODUCTION
Potassium aluminum sulfate dodecahydrate, KAl(SO4)212H2O, belongs to a class of inorganic
compounds called alums (which is accented on the first syllable). The composition of alum is represented
by the general formula M+M3+(SO4)212H2O. In this formula, M+ is a monoatomic ion such as Na+, K+,
Tl+, or Ag+; or is NH4+. Although the name alum sounds as if the compound must contain aluminum, the
M3+ might be from any of several main-group metals (p block elements) and transition metals (d block
elements) that form triply charged ions: Al, Ti, V, Cr, Mn, Fe, Co, Ga, In, and Ir. Some examples are
NaAl(SO4)212H2O, which is a component of some baking powders; NH4Al(SO4)212H2O, which is used
in dyeing textiles; and KCr(SO4)212H2O, which is used in tanning leather.
The term double salt also describes KAl(SO4)212H2O. A double salt is an ionic compound that
contains two different cations or two different anions. In aqueous solution, double salts give positive tests
for all three of the ions they contain. KAl(SO4)212H2O would give positive tests for K+, Al3+, and SO42.
Hydrate is another term that describes KAl(SO4)212H2O. A hydrate is a substance that contains a
specific number of water molecules per formula unit in its solid form. Hydration is common with ionic
compounds, for example Na2SO410H2O and CoCl26H2O. The water of hydration is included when
calculating the compound’s formula mass. The water molecules are part of the crystal of a compound
along with cations and anions, but they can break out of the crystal when it is heated. This dehydration
might occur at a specific temperature characteristic of the particular compound. If the compound is heated
in a relatively closed container, the escaping water might dissolve the remaining salt forming a liquid
solution. Consequently, this combination of dehydration and dissolving looks like melting; the
temperature range at which it occurs (the solid-to-liquid transition) can be measured in the same way that
a melting point is measured.
Reactions Involved in the Synthesis
Metals dissolve in water or aqueous solutions only by reacting with water or some other component
of the solution. The reaction converts the metal to a positive ion. Aluminum dissolves in acids by reacting
with H3O+ and dissolves in bases by reacting with OH. The reaction you will use to dissolve aluminum
involves OH from KOH:
2 Al(s) + 2 OH- (aq) +6 H2O  2 Al(OH) 4 (aq) + 3 H2(g)
(3-1)
Next, you will add sulfuric acid to the solution containing Al(OH) 4. As H3O+ reacts with Al(OH)4,
insoluble Al(OH)3 forms and precipitates. As more acid is added, the precipitate Al(OH)3 dissolves:
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3-1
Al(OH) 4 (aq) + H3O+(aq)  Al(OH)3(s) + 2 H2O(l)
(3-2)
Al(OH)3(s) + 3 H3O+(aq)  Al3+(aq) + 6 H2O(l)
(3-3)
The product forms and precipitates when the solution containing Al 3+, K+ (from KOH), and SO42
(from H2SO4) is cooled:
K+(aq) + Al3+(aq) + 2 SO 24  (aq) + 12 H2O (l)  KAl(SO4)212 H2O(s)
(3-4)
EQUIPMENT NEEDED
balance
beakers
Büchner funnel
hot plate
clamp
conical funnel
Erlenmeyer flasks
filter flask
filter paper—12.5cm and 7.0cm
graduated cylinder
melting point capillary tubes
ring stand
rubber O-ring
slit stopper
temperature probe
spatula
watch glass
glass stirring rod
CHEMICALS NEEDED
isopropyl alcohol
Al foil
ice
6 M H2SO4; sulfuric acid
3M KOH; potassium hydroxide
thymol blue solution
PROCEDURE
Dissolving Al by Reaction with KOH
1. Weigh a piece of aluminum foil on the laboratory balance. Remove some Al from it or add more Al
to it until you have ~0.5–0.6 grams; record the mass. Tear the Al into pieces about the size of large
postage stamps and put these pieces into an Erlenmeyer flask.
2. Take a beaker to the hood, measure out ~20mL of 3M KOH solution into the graduated cylinder
provided, and pour the measured solution into the beaker. Take the beaker back to you bench area and
pour the 3M KOH solution into the flask containing the aluminum foil. Within a few minutes the Al
and KOH will begin to react producing hydrogen; this reaction will become progressively more
vigorous. Do not handle the flask; the reaction is so exothermic that the entire flask, including
the neck, will become extremely hot. After the obvious evolution of hydrogen ceases, let the flask
stand for a few minutes more to ensure complete reaction of the Al. Then cool the flask slightly by
wiping the neck with a wet sponge. Cool the flask further under cold, running water until you can
handle the entire flask comfortably.
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3-2
3. If the solution in the flask is absolutely clear, proceed to the next step in the synthesis. If the solution
is not absolutely clear, filter it by gravity filtration. Collect the filtrate in another Erlenmeyer flask.
Discard the filter paper in the regular trash container.
Converting Al(OH)4 To Al3+ by Reaction with H2SO4
4. To the clear solution in an Erlenmeyer flask, add 16.0 mL of 6M H2SO4 a few milliliters at a time.
After each addition, swirl the flask to mix the contents. Initially, Al(OH) 3 will precipitate; then some
of it will dissolve as more H2SO4 is added. If solid sticks to the inside wall of the flask, wash it down
with a few drops of H2SO4. After you have added all the H2SO4, gently heat the flask for several
minutes; have the liquid barely bubbling. If the amount of H2SO4 added to the reaction mixture was
sufficient for the amounts of Al and KOH you used, all of the solid in the flask should dissolve.
5. If solid remains, continue the gentle heating for several more minutes; do not increase the intensity of
the heating. If solid still remains, gradually add more H2SO4 dropwise with gentle heat until all of the
solid dissolves and you have a clear solution; this should not require more than one mL (about 20
drops) of H2SO4.
Precipitating KAl(SO4)212H2O by Cooling
6. Place the flask containing the reaction mixture into a beaker of ice water; be sure the water in the
beaker is at least as deep as the solution in the flask, and that there is plenty of ice in the beaker.
Occasionally stir the solution with a glass rod. After crystals of the product begin to form, cool the
mixture for at least another ten minutes; continue to stir it occasionally. While you wait for the
product to precipitate, set up the equipment for filtration.
Collecting, Washing, and Drying KAl(SO4)212H2O.
7. Set up a Büchner funnel apparatus. Clamp the filter flask to a ring stand and connect the sidearm of
the flask to the vacuum line with a long piece of pressure tubing. Place the funnel on top of the flask,
and insert a piece of filter paper. Open the vacuum line so that it is pulling air through the funnel. Wet
the filter paper in the funnel with some water from a wash bottle until it forms a seal with the base of
the funnel. Swirl the flask with your product so that the solid is dispersed in the solution, then pour it
into the funnel. Wash any remaining solid left in the flask into the funnel with a small amount of icecold water. The water must be as cold as possible; since the product is slightly soluble in water, using
cold water ensures that the amount of product that is dissolved away is minimized. When no more
drops of water are seen coming out of the tip of the funnel, turn off the vacuum.
8. The washing of the precipitate is done to purify the product by removing the excess ions present in
the starting materials from the product. This step takes advantage of the differing solubility properties
of the impurities and the product; the ionic impurities are all readily soluble in water, while the alum
is only slightly soluble in water. One of the ionic impurities is hydronium ion; we can monitor the
progress of the washing by using an indicator to detect the presence of hydronium ion. Put 2 or 3
drops of distilled water into a small test tube, add one drop of thymol blue solution, then remove the
funnel and tubing from the filter flask and transfer a few drops of the filtrate (the liquid in the filter
flask) into the thymol blue solution with a disposable pipet. The first filtrate that you collect should
Experiment 3
3-3
give a pink color with the indicator showing that an excess of acid was used in the reaction. After
testing this first filtrate, keep the test tube of indicator and filtrate to use to make color comparisons
with subsequent washings.
9. Pour the filtrate out of the filter flask into a beaker (you will be pouring this into a waste container at
the end of the lab period), rinse the flask with water, reconnect the tubing and replace the funnel with
your product. Open the vacuum line and wash the product with 10 mL of ice-cold water. Test the
filtrate as above with a fresh water-and-thymol blue solution in a clean test tube. If the thymol blue
solution turns pink, repeat this washing step. If the thymol blue turns yellow or perhaps a pale orange,
the product has been adequately washed. When all the excess acid has been washed out of the
product, you can be confident that any other excess starting material has also been washed away.
When you complete the filtering and washing, the filtrate can be discarded by washing it down the
drain. The thymol blue solutions can also be discarded by washing them down the drain.
10. Obtain 20 mL of isopropyl alcohol in a dry container. With the vacuum line open, pour about 5 mL of
isopropyl alcohol over the entire surface of the solid in the funnel. Repeat this procedure three more
times. The isopropyl alcohol washes away the water; isopropyl alcohol is much more volatile than
water and it will evaporate fairly rapidly to give a dry product. Allow the vacuum to pull air through
your product for a few minutes to speed up the drying process.
11. Obtain a large piece of filter paper. Lift the wet filter paper and product from the funnel and use a
spatula to scrape the solid onto the larger piece of dry filter paper. Use a spatula to spread the solid
out as much as possible; cut through it repeatedly with a spatula to expose new solid surface and
hasten drying. If the large piece of filter paper becomes quite wet, transfer the solid to another large
piece of filter paper and continue cutting through and spreading the solid for a minute.
12. Transfer a small amount of the solid to a watch glass. Spread it out completely so that it can dry
thoroughly in a few minutes. While it dries, set up the equipment for measuring the melting point.
Disposing of Acid Waste.
When your TA instructs you to do so, take the beaker containing the acidic filtrate from the initial
vacuum filtration (step 9) to the hood and pour it in the Acid Waste container.
Experiment 3
3-4
Determining the
KAl(SO4)212H2O.
Note:
“Solid
to
Liquid”
Transition
Temperature
of
Wait until you are sure that the small sample of the product is completely dry before
performing this part.
13. Use an ice bath to calibrate the temperature probe, following the procedure from EXPERIMENT 1.
14. Place a small beaker on wire gauze on a ring on a ring stand with a hot plate beneath the beaker.
Obtain a capillary tube.
15. Push the powder into a mound. Press the open
end of the capillary tube into the mound of
powder once. Turn the tube upright and
repeatedly tap the sealed end against the
desktop until all the powder falls to the
bottom of the tube. Again, press the open end
of the tube into the heap of powder once and
tap the bottom of the tube against the desktop
until all the powder falls to the bottom of the
tube. Repeat this process until there is solid in
the bottom of the tube to a height of about 5
mm. Use a rubber o-ring to hold the capillary
tube onto the temperature probe so that the
bottom of the capillary tube is even with the
end of the temperature probe.
Figure 3-1. Determining the melting point.
16. Set up an apparatus like FIGURE 3-1, using a
hot plate instead of the burner. Note: the beaker may be placed directly onto the hot plate, so the iron
ring and wire gauze are not needed. Put the temperature probe into a slit stopper and clamp it onto
the ring stand. Put clear tap water into the beaker to a great enough depth that the tip of the
temperature probe is completely immersed in the water. Be sure that the open end of the capillary
tube is above the water level. Also, have the rubber o-ring (rubber band in the figure) above the water
level.
17. Begin to heat the bath and stir it gently and continuously. Record the temperature at which you first
see solid change to liquid; the solid will seem to collapse as it begins to melt and will be translucent
or transparent, rather than opaque. Record the temperature at which all solid finally changes to liquid.
If the melting range is more than 3C, you were probably heating the water bath too rapidly. Repeat
the measurement with a fresh sample of product in a fresh capillary tube. The narrowness of this
range and the accuracy of your result depend on the purity, including dryness, of your product. While
you are carrying out this step, you should occasionally cut through and redistribute your remaining
product to facilitate drying. Dispose of the used capillary tube in the wastebasket.
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3-5
Determining Yield.
18. Place a piece of weighing paper on a balance. Zero the balance, and then place the alum product on
the weighing paper. Record the mass of the alum.
Disposal of Product.
After the mass of the alum has been determined, it may be thrown into a wastebasket.
Experiment 3
3-6
EXPERIMENT 3
REPORT SHEET
Name: _______________________________________
Date:__________
Mass of Al foil used
Moles of Al used
Volume of 3M KOH used
Moles of KOH used
Mass of KOH used
Volume of 6M H2SO4 used
Moles of H2SO4 used
Mass of H2SO4 used
Limiting reactant
Theoretical yield of KAl(SO4)212H2O, moles
Theoretical yield of KAl(SO4)212H2O, grams
Actual yield of KAl(SO4)212H2O, grams
% yield of KAl(SO4)212H2O
“Solid to Liquid” transition temperature range
Experiment 3
3-7
Laboratory Notes
Exp. 3
Synthesis of an Alum
Chemistry 111
You may work in pairs on this experiment.
Changes in Procedure and Helpful Hints
Step 2:
You may get the reaction started by heating the flask on a hot plate. As
soon as the reaction starts to bubble vigorously, turn off the hot plate!
Step 3:
The filtration works better if you wet the filter paper thoroughly with
water before pouring in the reaction mixture.
Step 6:
This step takes a while; you can use the time to start working on your
theoretical yield calculations.
10/10
Experiment 3
3-8