±0.1o C. Typically it is no more than ±1o C.
Experiment #3
Melting Points
Since melting points are relatively easy and
inexpensive
PURPOSE
to
determine,
they
are
handy
identification tools to the organic chemist. If you
To gain skill in lab procedure and
measuring techniques and to learn about
the properties of organic compounds.
want to use the melting point to identify a solid
compound that you have isolated in the lab, you
will need to compare its melting point with that of
OBJECTIVES
the true compound. Melting points are listed in
1. Determine several melting points
(mp's).
various sources of scientific data, as referenced on
2. To see how mixing different
compounds changes their melting
points.
on this organic chem site from the University of
the Chem Info page or in the physical data tables
Colorado:
Chem Info
3. To determine the effectiveness of
using melting points in determining
compound purity
(http://orgchem.colorado.edu/cheminfo/cheminfo.
MATERIALS
Table of Physical Constants and Hazards of Some
html)
Organic and Inorganic Compounds
• Mel-Temp, chemicals Time:~2 hour
(http://orgchem.colorado.edu/hndbksupport/phys
BACKGROUND
data.html)
How to set up the Mel-Temp Apparatus.
Melting Points
http://orgchem.colorado.edu/hndbksupport/meltin
gpt/mtset.html.
Melting
points
easily
Melting points of pure compounds are also
measured physical property of solid-state
recorded in handbooks, such as the Handbook of
organic
Chemistry and Physics (CRC) or the Merck Index.
compounds
characterization
are
useful
an
in
product
and
purity
Measurements of the melting point of a
crystalline
solids
solid will provide you with information about the
point,
purity of the substance. Pure, crystalline solids
which is expressed as the temperature
melt over a very narrow range (melting range) of
range over which the solid melts to
temperatures,
become a liquid. The transition between
broad temperature range. Mixtures also tend to
the solid and the liquid is so sharp for
melt at temperatures below the melting points of
small samples of a pure substance that
the pure solids.
determination.
Pure,
have a characteristic melting
melting
points
can
be
measured
to
whereas
mixtures
melt
over
a
Most solid substances prepared in the
organic laboratory are initially impure. These
1
impurities affect the melting point of a
Procedure
substance. In a sample that contains a
mixture
of
two
compounds,
each
I. Melting points are best determined using a
component usually lowers the melting
finely divided powder. Grind the sample using a
point of the other, giving an observed
mortar and pestle to ensure homogeneity. Fill a
melting point range that is lower and
capillary tube to a height of about 2-3 mm with
broader than the melting point of either
the packed urea. Put the tube into the Mel-Temp
component. A melting point composition
apparatus closed end down. Make sure that you
diagram for two hypothetical solids, A
can see the sample through the magnifying glass.
and B, is shown below, as a graph of
temperature versus composition.
Turn the voltage to 3.5 and observe both the
The eutectic point is the lowest
melting temperature of the mixture and
is
determined
by
the
equilibrium
composition at which A and B melt in
constant
ratio.
A
sample
Set the voltage to zero and turn on the Mel-temp.
whose
composition is exactly that of the eutectic
point will exhibit a sharp melting point at
the eutectic temperature. This means a
sample and temperature reading as you heat.
(Never set the voltage at more than 7.0). Note (a)
the temperature at which the column of urea first
collapses or shows some liquid and (b) the
temperature at which the sample is completely
liquid. This is the melting range, which we call a
melting point. Always report a melting range.
The melting point is not accurate if the
eutectic mixture can be mistaken for a
pure compound since both have a sharp
thermometer and the sample are not at the same
melting point.
temperature. For accuracy the sample should be
Mel-Temp
heated through the melting range at a rate of 1 o C
apparatus for measuring the melting
or less per minute. Turn off the apparatus and let
point using closed-end capillary tubes.
it cool. If you did not get a good result for the
The sample is placed into a slot and its
melting point of urea, prepare a sample in a new
melting observed through a magnifying
capillary, and repeat the measurement. Capillaries
glass.
cannot be reused.
We
will
use
the
2
Note:
Put used capillaries in the glass
waste container.
be. Identify your unknown based on the best
match.
List any important observations you make
II. Prepare a melting point diagram for a
while performing the experiment. For example,
mixture of two compounds. We will use
describe the appearance of a compound when it
urea
the
melts and any other visible changes occurring
melting point ranges of urea (from part
prior to, or during, the melting process, i.e. water
I),
vapor, gas bubbles, color changes, clarity of the
and
pure
cinnamic
acid.
cinnamic
Record
acid,
a
1:1
urea:cinnamic acid mixture, and a 1:5
liquid melt.
urea:cinnamic acid mixture.
Analysis
III. Determine the melting point of a solid
Record the literature melting points of all
unknown. Your unknown is one of the
substances you have used and compare the
following
values you have determined. Comment on any
compounds:
salicylic
acid,
benzoic acid, succinic acid, acetanilide,
discrepancies.
benzophenone, or napthalene. Samples
Prepare a graph of the melting points of
of all of these compounds are available in
your mixtures as a function of composition of the
the lab. In the procedure part of your
mixture.
Prelab explain how you plan to identify
temperatures of the temperature range that you
your unknown.
From the found melting
obtained. Connect the data points for the high end
point of your unknown, decide which of
and the data points for the low end. Try to identify
the listed compounds it might possibly
the lowest point of the mixture.
Plot
both
the
high
Questions
1. How fast do you heat the sample in the Mel-temp when determining a
melting point?
2. If you heat too fast, will your observed melting point be higher or lower
than the true value? Explain.
3. What is meant by the term melting range? What happens at this range?
4. Why should you always use a new capillary tube with a sample of your
compound
when
doing
a
second
3
melting
point
determination?
and
the
low