Synthesis of Alum
Prem Sattsangi
Copyright 2009(Rev.)
#2 Synthesis of Alum
Al    KAl(SO4)2.12H2O
K2SO4 + Al2(SO4)3

2 Simple salts
2KAl(SO4)2.12H2O
A Complex Salt
FW = 474g/mol
Theoretical Yield: (BLB, p. 102)
Al (FW = 27.0)
1mol Al produces 1mol KAl(SO4)2.12H2O
•
Limiting reagent (BLB, p. 100)
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•
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Calculate Theo. yield of Alum (g) from 0.2345 g Al.
[Hint: Al(g)  Al (mol)  Alum (mol)  Alum(g)]
#3 Analytical Balance
Tare, place foil, close door, record wt.
Calculate Theoretical Yield
Of Alum from 0.0566 g Al
#4 Cut Al foil in 16 squares
• Length wise
• Breadth wise
#5 Plastic Reaction vial
with cut Aluminum foil
#6 Mark MP Bulb 1, 2, 3, cm
a.
b. Fill Micropipet with
1.5 M KOH to 2 cm mark
c. Add to reaction vial
#7 Add more KOH (1cm)
a. Fill KOH to 1 cm mark
b. Add to reaction vial
#8 Start reaction, In fume hood,
Label 250 mL beaker with Stn. #
(a) Take hot tap water up to
2cm mark in the coffee cup.
(b) Support the coffee cup
in 250 mL beaker (Stn.#__)
STN# 4
(c) FLOAT reaction vial in
the coffee cup using
forceps.
#9 Oxidation Numbers
HELP KEEP TRACK OF ELECTRONS
•BLB p. 128-129
•Ox.No. of elements in standard states = 0
•Ox.No. of an ION = Charge on it
Ox.No.
•
•
•
•
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Aluminum (foil)
Aluminum ion (Al3+)
Hydrogen (H2(g)
Hydrogen ion (H+)
Hydride ion (H:-)
H in water [H+ OH-]
LEO the lion says GER
Al foil
0
+3 Loss of Electron = Oxidation
0
+1
-1 Gain of electron = Reduction
+1
#10 Allow REDOX reaction to finish
ELECTRONS GAINED = ELECTRONS LOST
2Al(s) + 2KOH(aq) + 6HOH(l)  2KAl(OH)4(aq) + 3H2(g)
0
+1
+3
0
0
6 x +1= +6 2x+3 =
+6
0
•
Oxidation: (Loss of Electrons)
Each electron lost = 1+ charge
2 Al  2 Al3+ + 6eAluminum [0 to +6 ]
Reduction: (Gain of Electrons)
6 [H+ OH-] + 6e-  3 H2 + 6 OHWater [+6 to 0 ]
[Ox.Nos./atom or ion]
Hydrogen gas forming
(FLAMMABLE)
#11 KAl(OH)4(aq) Complex
Finished reaction
AMPHOTERISM: (BLB p. 689-690)
KOH(aq)
+ Al(OH)3(s)  KAl(OH)4(aq)
Two Simple hydroxides ADD, form a SOLUBLE complex.
Black particles are
impurities in “Al”
#12 Remove Black Particles
• Insert a wad of cotton
in a glass pipet.
• clamp to a stand, wet,
remove water, filter.
• May need 2 times
filtration on the same
pipet to get a
clear
filtrate. KAl(OH)4(aq)
#13 To KAl(OH)4(aq) colorless,
Add Sulfuric acid  White ppt. Al(OH)3(s)
Add 1/3 MP H2SO4
Stir with a glass -rod
Colorless Solution  White ppt.
Soluble, More reactive
REMEMBER: 2KOH (aq) + 2Al(OH)3(s)  2KAl(OH)4 (aq) Colorless #10
Base
+ Acid
 Salt
+ water
[2KOH (aq)
+ H2SO4(aq) K2SO4 (aq)
+ 2HOH]
• A. 2KAl(OH)4 (aq) + H2SO4(aq) K2SO4 (aq) + 2Al(OH)3(s) + 2HOH
• B. 2Al(OH)3(s) + 3H2SO4(aq) Al2(SO4)3 (aq) + 6HOH
#14 White precipitate dissolves.
Alum forms.
(a)Stir with a glass rod.
(b)Heat on hot plate
•K2SO4(aq) + Al2(SO4)3 (aq)  2KAl(SO4)2.12H2O
•Simple salts ADD,
form a COMPLEX salt
#15 Crystallization of Alum
• Room temperature
• (15 minutes or
more) SEEDING?
• Ice bath
• (10 minutes)
#16 Alum crystals
Typical result
Single
#17 Suction Filter Alum
• Water aspirator
[Removes
Mother liquor]
Isolation of Alum:
(a)Suction filter.
(b)Rinse 2x with 15 drs
50% ethanol(0oC)
(c) Rinse 2x with 95%
ethanol(0oC)
(d)Continue suction ~5
more minutes to dry
the crystals.
#18 Suction Filter Alum
(a) Water aspirator
[Removes Mother liquor]
(b) 50% ethanol
[Removes Al3+(aq), K+(aq), SO42-(aq)]
(c) 95% ethanol.
[Removes water]
(d) Continue suction ~5 minutes.
[To air dry the crystals]
#19 Dry on a pre-weighed
paper
Weight of paper_____ g
Transfer Alum on PAPER.
Cover with a Funnel.
Allow to dry.
#20 Weigh crystals, Calculate %Yield
Wt of crystals + paper = 0.9802 g
Wt of paper
= – 0.4181 g
Wt of crystals = 0.5621 g
Exp. Yield
= 0.5621 g
•
%Yield
• =
Exp. Yield x 100
Theo. Yield (Slide #2)
Please calculate %Yield now:
=
#21 Summary of Equations
(i) REDOX (Change in oxidation no.)
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2Al(s) + 2KOH(aq) + 6HOH(l)  2KAl(OH)4(aq) + 3H2(g)
(ii) Complex (Add the reagents)
• Al(OH)3(s) + KOH(aq)  KAl(OH)4(aq)
• K2SO4(aq) + Al2(SO4)3 (aq)  2KAl(SO4)2.12H2O
(iii) Acid-Base (Water is formed)
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2KAl(OH)4(aq) + H2SO4(aq) K2SO4(aq) + Al(OH)3(s) + 2HOH
2Al(OH)3(s) + 3H2SO4(aq) Al2(SO4)3 (aq) + 3H2O
#22 Qualitative Analysis of Alum
KAl(SO4)2.12H2O
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1. Test for pH, pink color (Al3+ ion)
2. Test for SO42- ion, [with BaCl2(aq) white ppt)
3. Flame test for K+ ion (LILAC COLOR)
and evolution of H2O(g) and SO3(g) [PUFFS UP].
4. Test for absence of sulfate, [with BaCl2(aq)]
(after THOROUGHLY heating the alum).
• 5. Test for presence of Al3+, 0.15 M KOH. 
gelatinous ppt Al(OH)3(s).
• Add 1.5 M KOH to ppt  dissolves, Al(OH)4(aq),
• Forms SOLUBLE Complex
#23 Prepare saturated Alum
solution.
• Take 2 mm
mound of Alum,
KAl(SO4)2.12H2O,
with 3 drops of
water on the
plastic sheet.
Crush alum with
a glass rod.
#24 pH Test for Al3+
• Alum, KAl(SO4)2.12H2O,
contains Al3+ ion.
• Al3+ ion has:
• a. High charge.
• b. Small radius.
• Such cations, may rip apart
the water molecule to
produce H+.
• Al3+ + HOH  AlOH2+ + H+
#25 Chemical Test for Al3+
0.15 M KOH  Gel (soluble in 1.5 M KOH)
• In the 24 well tray, mix 1 dr. 1.5 M KOH + 9 drs.
Distilled water to make 0.15M KOH
• Alum contains Al3+.
• 1 drop Alum(aq) + 1 drop 0.15 M KOH Gel
• Al3+ + 3KOH(0.15M)  Al(OH)3(gel) + 3K+
Insol(s)
• Add 1 drop 1.5 M KOH to gel  Dissolves.
• Al(OH)3(gel) +KOH(1.5 M)  KAl(OH)4(aq)
• Simple hydroxides ADD  COMPLEX
#26 Test for SO42- ion in Alum
KAl(SO4)2.12H2O
Alum(aq)
1 drop Alum(aq) +
1 drop BaCl2(aq) 
white ppt, BaSO4(s)
Sulfate present.
SO42-(aq) + BaCl2(aq)  BaSO4(s) + 2 Cl-(aq)
#27 Flame test sample prep.
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Place some Alum on
a watch glass.
Heat nail.
•Touch Alum with
Red Hot Nail
#28 Evolution of gases and
Lilac flame, K+ ion.
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Slowly bring close
to the flame.
Watch it swell as
gases H2O(g) and
SO3(g) evolve.
Bring it into the
flame.
Lilac color, “K+”
ion.
#29 Heating Alum (Chemistry)
2SO3 (g)
24H2O (g)
Alum
2KAl(SO4)212H2
O
Al2O3 (s) insoluble
HEAT
K2O (s) soluble
+ H2O
Base
2KOH
#30 Tests in heated Alum sample
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2KAl(SO4)2.12H2O  24H2O(g) + 4SO3(g) + Al2O3(s) + K2O(s)
Remove Alum RESIDUE
Crush it in a ~2 cm pool of
distilled water.
#31 Tests with HEATED Alum
sample
• Check pH, Basic. WHY?
• K2O(s) + H2O(g)  2KOH(aq)
Filter over cotton.
Test with BaCl2(aq)
No SO42-, No ppt.
CAUTION: Any Undecomposed alum
may give positive
test with BaCl2.
Acknowledgements
Experiments:
Ms. Virginia Roll
Power Point Assistance:
Mr. Bill Hager
Christopher Byers