MELTING POINT DETERMINATION
PURPOSE
Identify an unknown compound by its melting point.
MATERIALS
•Mel-Temp melting point apparatus
•capillary tube
•unknown sample
•Table of Melting Point Data
•laboratory notebook
THEORY
The melting point (mp) of a substance is one of the physical properties that chemists use
to identify a substance. The melting point is the temperature at which a substance
changes from a solid to a liquid state. A pure crystalline organic compound usually has a
sharp and characteristic melting point range of 0.5 to 1 °C. The melting point range is
determined by recording the temp at which melting first begins and the temp at which
melting is complete. You may recall that impurities depress the melting (freezing) point
of a substance. They also increase the range of melting. When a sample melts at a lower
than expected temperature over an extended range, this is a sign that the sample was not
pure. Consequently, the melting point of a compound is also a criterion for purity and
well as for identification.
A technique known as a "mixed melting point" may be used as additional evidence in
identifying a given compound. First, a melting point is taken of the unknown and a
tentative identification is made using literature data. Then the unknown sample is mixed
with some authentic sample of the suspected compound and the melting point is taken of
the mixture. If the mixture shows no depression in the melting point, the two compounds
almost certainly were the same and the identification of the unknown is confirmed. If the
mixture shows a depression of melting point, the two compounds were not identical.
Often solid substances undergo some unusual behavior prior to actual melting.
Compounds may decolorize, decompose, soften, or shrink. It is normal for compounds to
appear to shrink immediately before melting. Actual melting begins when the first drop
of liquid becomes visible. When you research your compounds, be sure to note any
decomposition temperature listed, as some solids decompose into other products before
they melt.
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TECHNIQUE
Pulverize a few DRY crystals of your solid using the rounded end of a test tube. Obtain a
glass capillary tube that is open at one end but sealed at the other end. Jam the open end
of the capillary into your sample so that some solid enters the tube. Invert the capillary
and gently let the tube "bounce" repeatedly on the bench top until most of the solid
vibrates down to the closed end. Your sample should be about 1 cm high (the height of
your fingernail). If you have trouble filling the tube, your sample either needs more
grinding or it is too wet.
Make sure the Mel-Temp apparatus has cooled to near room temperature if it has been
used recently. Insert your capillary tube into a slot that will hold the tube next to the
heating block. Look through the eyepiece to see the solid sample. Make sure the rheostat
is set to "zero" and switch "ON" the on-off switch for the heating block. If you know the
expected melting point of your sample, set the power level to about "5" until the
temperature reaches about 15 °C below the anticipated melting point. Then adjust the
power so as to slowly approach your melting point. The rate of temp increase in the
vicinity of the melting point must be small, about 2 °C per minute. A graph will be made
available that indicates the approximate maximum temp of the apparatus for any given
power setting. This may help you find the proper power setting, but be advised that each
apparatus varies in its performance and these guidelines may not be accurate.
As the temperature is increasing, observe the crystals through the eyepiece, but be careful
not to accidentally touch the hot part of the apparatus. Record both the temp at which
melting begins and at which the last crystal disappears. Report a range such as 88-90 °C.
Turn off the apparatus, reset the power to "zero", careful remove your pipet, and discard
it in the nearby waste container.
If you do not know the melting point of a compound (as in THIS experiment), first take a
crude melting point by heating rapidly. Then cool the apparatus to about 20 °C below the
crude melting point, and then take a more careful measurement on a second sample of
compound.
PROCEDURE
All information should be recorded directly into your lab notebook, but if this is your first
lab meeting, you will be allowed to transfer the data to your notebook at a later time.
Obtain a sample from the instructor and immediately record the unknown code letter.
Prepare the sample as described and take a crude melting point (record this value). Then
determine an accurate melting point. Use the Table of Melting Point Data given to
determine the identity of your unknown and record your conclusion. If you feel you
cannot decide between two different compounds, discuss your choices and explain what
you could do to make a final decision between the two choices.
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TABLE OF MELTING POINT DATA
OF SOME ORGANIC COMPOUNDS
COMPOUND
benzophenone
4-nitrotoluene
palmitic acid
acetamide
naphthalene
1-napthol
o-toluic acid
m-toluic acid
acetanilide
benzoic acid
benzamide
trans-cinnamic acid
urea
cholesterol
salicylic acid
sulfanilamide
succinic acid
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MP ( °C)
48-50
52-54
61-63
79-81
80-82
95-96
105-106
111-113
113-114
121-122
126-128
132-134
134-136
144-146
158-159
165-166
184-185