15B Analysis of Alum
Purpose
Confirm the identity of a sample of alum using melting point, water of hydration, and a
test for sulfate.
Background
Once a synthesis has been completed, the identity of the compound produced should
be confirmed. A number of tests can be used to confirm the identity of potassium
alum, KAl(SO4)2•12H2O.
Melting point, water of hydration, and presence of sulfate in the compound will be
used in this lab to verify that the substance you have is actually potassium alum.
Materials
Equipment
Xplorer GLX
Glass stirring rod
Stainless steel temperature probe
Hot plate or combination hotplate/ stirrer
Ring stand with ring
Watch glass or mortar and pestle
Buret clamp
Bunsen burner
Crucible
Porcelain triangle
Crucible tongs
Beaker, 250 mL
Consumables
Potassium alum (KAl(SO4)212H2O)
Distilled water
0.2 M Barium chloride solution (BaCl2)
Capillary tubes
Safety Precautions
•
Wear safety glasses and follow all standard laboratory safety procedures
Procedure
Melting point determination
1)
Setup the ring stand, and hot plate
as shown in the illustration. Fill a
250 mL beaker three quarters full
with water to use as a water bath.
Place the beaker on the hot plate
or a combination hot plate/stirrer.
Do not turn on the hot plate yet.
2)
Connect the temperature probe to
one of the temperature ports ()
on the left side of the Xplorer.
3)
Crush ~0.5 g of alum crystals to a
fine powder in a watch glass using
a glass stirring rod. Scrape the
powdered alum into a pile in the
middle of the watch glass.
4)
Push the open end of a capillary
into the alum, then invert the
capillary tube and tap the closed end on the bench top to pack the alum into the
end. Repeat this procedure until the capillary tube contains about 1 cm of firmly
packed alum.
5)
Attach the capillary tube to the end of the temperature probe with a small rubber
band so that the alum is even with the end of the temperature probe. Be sure that
the closed end of the capillary tube is down.
6)
Clamp the temperature probe to the support stand using a utility clamp so that the
tip of the temperature probe and the portion of the capillary tube containing the
alum is immersed in the water bath. Be sure that the open end of the capillary
tube is above the water level and no water is able to get into the capillary.
7)
Turn on the hot plate to begin heating the water bath at a gradual rate. Stir the
water continuously while the water heats.
8)
Turn on the Xplorer and press  to open a digits display.
9)
Observe the alum in the capillary tube until it has completely melted. You may
need to peridoicaly tap the temperature probe gently to loosen bubbles that
attach and may interfere with the view of the alum crystals. Record the
temperature in your lab notebook.
Water of hydration determination.
1)
Setup the ring stand, ring, porcelain triangle, crucible with lid and bunsen burner as shown in the
illustration.
2)
Heat the crucible with the bunsen burner until it is
red hot. Allow the crucible to cool.
3)
Weigh the crucible and lid and record the mass in
your lab notebook. Use crucible tongs and do not
touch the crucible or lid with your fingers.
4)
Put about 2 g of alum into the crucible and weigh
the crucible, alum and lid. Record the weight in
your lab notebook.
5)
Place the crucible on the ring stand using the
tongs and place the lid slightly ajar so that water vapor can escape.
6)
Begin heating the crucible slowly with the bunsen burner.
7)
When vapor can no longer be seen escaping from the crucible, heat the crucible
strongly for 5 minutes. Allow the crucible to cool.
8)
Weigh and record the mass of the crucible, alum and lid.
9)
Reheat the crucible with alum for 5 more minutes then allow it to cool. Weigh the
crucible, alum and lid and record the mass as before.
10) If the two masses are very nearly the same, the procedure is complete, otherwise
repeat step 9 until a constant mass is obtained.
11) Calculate the change in mass due to drying of the alum.
Sulfate test
1)
Place a few crystals of alum into a test tube. Add distilled water dropwise with stirring until the alum is dissolved.
2)
Add one drop of 0.2 M barium chloride solution. Record your results in your
notebook.
Analyze
Record calculations in your lab notebook as you complete your analysis.
Data Table
Run 1
Me
ltin
g
poi
nt
(°C
)
Ma
ss
of
cr
uci
ble
an
d
co
ver
(g)
Ma
ss
of
cr
uci
ble
,
co
ver
an
d
alu
m
Run 2
bef
or
e
he
ati
ng
(g)
Ma
ss
of
cr
uci
ble
,
co
ver
an
d
alu
m
aft
er
1st
he
ati
ng
(g)
Ma
ss
of
cr
uci
ble
,
co
ver
an
d
alu
m
aft
er
fin
al
he
ati
ng
(g)
Ma
ss
of
an
hy
dr
ou
s
alu
m
W
ate
r
of
hy
dr
ati
on
Pr
eci
pit
ate
wit
h
Ba
Cl2
(
ye
s/n
o)
Analysis Questions
1)
Did your tests confirm the presence of alum? Explain using your results to
support your answer.
2)
What other tests could be used to confirm the identity of your sample?
3)
If the melting point was the only data you had to confirm the identity of your
sample, how confident would you be of your identification? Explain.