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PROFESSOR: Throughout your laboratory experiences,
you will frequently need to assess the purity or identity
of a crystalline solid.
Both of these goals can be accomplished by determining
a compound's melting point.
When a compound is in a crystalline solid state,
the intermolecular forces that are holding the molecules
together greatly outweigh the kinetic energy that
is trying to pull them apart.
Therefore the molecules appear to vibrate in a fixed location.
If energy is introduced into the system in the form of heat,
the kinetic energy of the molecules
and thus the temperature of the system is increased.
At a certain point, the molecular kinetic energy
is high enough to overcome the attractive forces that are
holding the molecules together.
As a result, the crystal lattice breaks down
and the molecules enter a liquid state.
The temperature at which the solid and liquid phases
of a compound are in equilibrium at a certain pressure
is called the melting point.
In reality, a solid melts over a temperature range
instead of at a specific temperature.
Because this range is a physical characteristic,
it is reproducible for a pure compound.
For example, N-phenylacetamide melts between 114.2 and 114.9
degrees Celsius.
Therefore the term melting point really implies a melting range.
Although a variety of melting point apparatuses
are commercially available, this video
will demonstrate the determination of melting point
by using the Mel-Temp.
The Mel-Temp apparatus consists of an on/off switch, a sample
holder, a light source that illuminates the sample,
an eye piece that magnifies the sample, a digital thermometer,
which indicates the temperature of the system,
and a voltage control.
The voltage control dictates the rate
of heating with a higher setting corresponding to a faster rate.
The voltage control does not control
the temperature of the system.
Use caution when touching the Mel-Temp
and always assume that the aperture is hot,
especially the part that is labeled "caution.
Unit hot."
The first step in determining a melting point
is the preparation of the crystalline solid.
To prepare the sample, you will need a spatula, a watch glass,
a mortar and pestle, capillary tubes that
are sealed at one end, and a long piece of glass tubing,
such as a long stem funnel.
Before proceeding, ensure that the crystalline solid
is clean and dry.
If the solid is wet, you can try to pat
it dry with filter paper.
Do not place the sample in the oven
where it may melt or decompose.
If your sample consists of large crystals,
you must first crush them into smaller crystals with a mortar
and pestle.
Large crystals cannot pack together well,
which creates air pockets.
If air pockets are present in your sample,
then a time lag for heat transfer
will exist, causing the crystals to heat unevenly.
If you do not have a mortar and pestle,
you can use the flat end of a spatula
to grind the sample instead.
After crushing the crystals, place them on a watch glass.
Turned the capillary tube upside down
and thrust the end over the crystals.
A small amount of crystals should be
trapped inside the capillary.
Invert the tube to its correct orientation
and pack the crystals down to the bottom
by gently stroking the tube or by tapping it on your benchtop.
If the crystals fail to fall to the bottom of the capillary,
then drop the capillary through a length of glass tubing.
A long stem funnel works well for this application.
You may have to repeat this action several times
to ensure that the crystals are effectively packed down.
Loose material contains air pockets
which cause the sample to heat unevenly.
Ideally, the smallest manageable amount of material
should be used, which correlates to a depth of approximately 1
millimeter of packed material.
If the depth exceeds 2 millimeters,
inaccuracies may result due to uneven heating.
Prepare four capillary tubes of your sample.
One filled capillary tube will be used to determine
a crude melting point.
And the other three will be used to determine
your actual melting point in triplicate.
If you do not know the expected melting point of your compound,
you must first determine a crude melting point.
Finding the approximate melting point of a sample
will save a lot of time when performing
the actual measurement.
To determine the crude melting point,
set the voltage on the melt temp to zero
and wait until the aperture has cooled to room temperature.
Insert a sample.
Increase the temperature at a rate of 10 to 20 degrees
Celsius per minute and record a rough temperature
when the sample melts.
To determine the melting point of a crystalline solid,
set the voltage on the Mel-Temp to zero,
turn on the power and the digital thermometer,
and then insert a filled capillary tube
into the sample holder.
If the temperature is lower than 20 degrees Celsius
below the expected melting point,
then dial the voltage to a setting that
enables the temperature to increase
at a rate of 10 to 20 degrees Celsius per minute.
After the temperature is within 20 degrees
Celsius of the expected melting point,
dial down the voltage so that the temperature increases
at a rate of 1 to 2 degrees Celsius per minute.
Watch the sample through the eyepiece
and record the temperature when the first crystal starts
to melt, which means when the first droplet of liquid
appears.
Finally, record the temperature when
the last crystal disappears.
Turn down the voltage, remove the melted sample,
and throw it into the proper waste receptacle.
Allow the Mel-Temp apparatus to cool to 20 degrees
below the melting point of the compound
and repeat the process two more times with the remaining
samples to ensure reproducibility.
Never remelt a compound because it
may have undergone a chemical change while
heating such as oxidation, decomposition,
or rearrangement.
The point where the first crystal starts to melt
can seem ambiguous to someone taking a melting point
for the first time.
As stated earlier, this is the point
where the first droplet of liquid forms.
It is not when the material begins
to settle, soften, or shrink.
Only the appearance of a liquid droplet
indicates the beginning of melting.
One common use for melting point is
to assess the purity of a known compound.
If an impurity is present in a sample,
it will interrupt the crystal lattice
and weaken the forces of attraction that are
holding the molecules together.
Therefore the sample will melt at a temperature
that is lower than expected.
The sample will also melt over a wide temperature range.
As a general guideline, if a compound
melts over a range greater than 2 to 3 degrees Celsius,
it is most likely impure.
Another common application of melting point
is the identification of an unknown crystalline solid.
To identify a solid, simply compare the melting point
of the unknown with the melting points of known compounds
that are listed in a reference book.
Typically, just the upper limit is reported in these reference
books.
When combined with other spectroscopic data,
you may be able to identify your compound.
To confirm that your compound is indeed what you think, prepare
three samples containing mixtures
of your unknown compound and the proposed compound.
20 to 80, 50/50, and 80 to 20 are good ratios to start with.
Determine the melting point of each mixture.
If all the mixtures melt at the same temperature
and within the same range, you may have correctly identified
your compound.
If they do not, then you definitely
have not identified your compound.
There are several ways to obtain inaccurate melting point
readings.
The most common mistake is increasing the temperature
too rapidly.
In this situation, the capillary will not
have enough time to equilibrate with the heated block.
As a result, the melting range will appear narrower
than it actually is and the melting temperature will
appear higher.
A sample that disappears upon heating
indicates that it is subliming instead of melting.
To remedy this problem, seal the open end of the field
capillary with a Bunsen burner before placing it
in the sample holder.
A sample that changes color upon heating
is probably undergoing decomposition.
Decomposition must be reported along
with the melting point of the compound.
If the melting point is still incorrect after troubleshooting
or if you need to confirm the accuracy of your result,
you must calibrate the Mel-Temp apparatus.
To do this, find pure compounds with known but dissimilar
melting points.
This video will use naphthalene, N-phenylacetamide,
salicylic acid, and succinic acid.
Determine the melting point of each pure compound
and compare the value you received with what is expected.
Plot the actual melting point of each compound
against its observed melting point.
Use the resulting calibration curve
to determine the expected melting point of your compound.
Determining the correct melting point of a crystalline solid
involves good technique in both sample preparation
and in taking the melting point.
To ensure accurate results, use a minimal amount of solid,
pack the crystals tightly together,
and increase the temperature slowly.
Determining the melting point of a crystalline solid
is the best way to assess its purity
or establish its identity.
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