Melting Points
Background
The melting point of a solid is a reproducible, easily measured property of organic compounds. Melting points are one of
the primary ways to characterize and identify products in organic laboratory class. The melting point can also tell you
about the compound’s purity since impurities both lower and broaden the melting range of a compound. As you
learned in general chemistry, impurities elevate the boiling point of substances and depress the freezing point (a.k.a.
melting point). Melting “point” is a bit of a misnomer because all compounds melt over a range. Pure samples generally
melt over a narrow (2 degree) range. Impure compounds generally melt over larger ranges. Melting points can be used
to confirm the identity of an unknown compound and to assess the purity of a known compound.
Preparing a Melting Point Sample
When taking a melting point, your sample should be a fine, dry powder. If your crystals are not finely powdered, crush
a small amount with a mortar and pestle before filling the melting point tube. If your sample is wet from solvent, wait
until the next class period for it to dry. If your sample looks oily it may be due to high boiling impurities that can only be
removed through recrystallization.
To fill a melting point tube, tap the open end of the tube into your powdered sample to pick up a few grains. Flip the
tube and gently tap the closed end on the bench to knock the material to the bottom of the tube. Long narrow tubes
are also available to pack the crystals down. The tubes allow you to drop the melting point tube from a height without
breaking it. No more than 2 millimeters of densely packed crystals are needed to take a melting point. Loosely packed
samples and large samples lead to uneven heating.
Calibrating Your Thermometer
The thermometer you are assigned for this class may read low or high by varying degrees at various temperatures so it
must be calibrated. Known samples of high purity are used to calibrate the thermometers. To calibrate, compare the
high temperature in your experimental range to the high temperature in the literature melting point range (often only
the high temperature reported for literature values). The difference between the two high numbers should be added to
(for thermometers that read low) or subtracted from (for thermometers that read high) each value in melting ranges
that you take in the future. Your thermometer may be off by different amounts in different temperature ranges. Use
whichever calibration is closest to the temperature your solid melts at.
Using the Mel-Temp
The melting point machines can be set to heat at different rates (see graphs on page 2). It is important to heat at a slow
rate (2 degrees per minute) near the melting point to get an accurate reading. If you have no idea where your sample
will melt, make up two sample tubes. Heat the first tube at a fast rate to get a rough idea of the melting point. Allow
the machine to cool to 10 degrees below your estimate, insert the second sample and heat at a slow rate until melted.
Making two samples and using this procedure much faster than making one sample and heating at 2 degrees per minute
until the sample melts, particularly if it is a high melting sample. Note that the machine does not need to be completely
cooled between measurements; it only needs to be cooled lower than the first melting point. Additionally, each machine
holds up to three samples – please utilize all slots if you are doing a lab that requires more than one melting point.
Finally, Mel-Temp barrels get very hot – be careful not to burn yourself.
The melting point (range) begins when the first crystal grain just begins to liquefy and ends when the sample is
completely liquid. Even very pure samples melt over at least a 2 degree range. The melting points quoted in the
literature usually only print the highest temperature. You will always quote both temperatures – always record
melting points as ranges.
How to Take a Mixed Melting Point
One way to confirm the identity of an unknown is to mix it with the compound you believe it to be and take a melting
point. If your unknown and the known compound are identical, the melting point of the mixture should not change. If
your unknown is not identical to the known compound, it will act as an impurity and the melting point range will be
lower and broader.
To take a mixed melting point mix a small amount of each compound (~1:1 ratio is recommended) and thoroughly crush
them together with a mortar and pestle, then proceed with the melting point as usual.
A note on thermometers: The thin glass of the thermometer bulb will break if cooled or heated too rapidly. Do not put a
cold thermometer in a hot mel-temp apparatus. Do not put a hot thermometer in cold water.
Procedure
Part I: Calibration
Your thermometer will be calibrated with pure samples of benzophenone, naphthalene and benzoic acid. Since you
know the literature melting points of the three calibration solids, a quick melting point determination is not necessary.
Measure the melting point range of each compound at least twice. Group members should take turns at the
measurements. Heat at a rate of 2 degrees per minute near the three melting points. Between the melting points,
please heat faster! Repeat until you get consistent ranges. Record all melting ranges in your notebook and compare the
highest number in each recorded range to the high literature value.
Part 2: Identification of an Unknown
You will be given a sample of an unknown to identify through mixed melting points. The sample will have one of three
identities: urea, benzoin or cinnamic acid. All three compounds melt at a similar temperature. Make the following
mixtures: 1) your unknown mixed with urea; 2) your unknown mixed with benzoin; 3) your unknown mixed with
cinnamic acid. Record the melting point range of the three samples (utilize the three slots in the mel-temp). Use this
information to determine the identity of your unknown.
Chemicals: benzophenone, naphthalene, benzoic acid, urea, benzoin, cinnamic acid
Waste: melting point tubes and excess solids should be put in the melting point waste jar. Contaminated weigh-boats
go in the plexiglass bin. No glass should ever go in the regular trash.