Melting Point Determination:
Purity and Identity of Crystalline
Before performing this lab, please review the safety rules that you agreed to follow in the safety training
portion of the course.
Your instructor will indicate any additional safety features of this lab activity and indicate disposal issues.
Introduction:
Most crystalline organic compounds have characteristic melting points that are sufficiently low (50300°C) to be conveniently determined with simple equipment. Organic chemists routinely use melting
points (a) to get an indication of the purity of crystalline compounds and (b) to help identify such
compounds.
Pure crystalline compounds usually have a sharp melting point. That is, the melting point range-the
difference between the temperature at which the sample begins to melt and the temperature at which the
sample is completely melted-is small (narrow). Impurities, even when present in small amounts, usually
lower the melting point and broaden the melting point range. A wide melting point range (more than 5°C)
usually indicates that the substance is impure; a narrow melting point range (O.5-2°C) usually indicates
that the substance is fairly pure. However, there are some exceptions to both of these generalizations.
Small differences in melting point (on the order of 2-3°C) may also result from variations in technique,
thermometer accuracy, and the experience of the person doing the melting point determination.
Melting points can be used in the following way to help identify a compound. Say a sharp-melting,
unknown substance X is suspected of being identical to some known substance A. If the two are identical,
they should have the same melting point. Thus if A is reported in the chemical literature to have a melting
point significantly different from that observed for X, we can be quite certain that X does not have the
same structure as A. On the other hand, if A is reported to have a melting point within a few degrees of
that observed for X, the two substances may be identical (the small difference being due to variations in
technique or purity).
To make certain that X and A are identical , one can determine the mixture melting point-that is, the
melting point of a mixture of X and A (when a sample of A is available). If X and A are identical, the
mixture should have the same melting point as X or A alone. On the other hand, if X and A are not the
same substance (even though they separately have the same melting point), then a mixture of the two
usually has a lower melting point and a broader melting point range than either substance alone. This is
because each substance acts as an impurity in the other.
©2009 Dr. Robert R. Klepper
To summarize, if a crystalline substance is pure, its melting point range is likely to be narrow. If two
samples have identical structures, their mixture melting point is not depressed and the melting point range
is not broadened.
General Technique for Melting Point Determination
To determine the melting point of a crystalline substance, we introduce a small amount of the finely
powdered material into a thin-walled capillary tube that is sealed at one end . The capillary tube is
inserted into a melting point apparatus and heated. Two temperatures are recorded: the temperature at
which the substance begins to liquefy and the temperature at which it becomes completely liquefied. The
observed melting point range is the interval between these two temperatures. The melting point is a
characteristic property of a pure chemical substance.
The observed melting point range can be influenced not only by the purity of the material but also by the
size of the crystals, the amount of material , the density of its packing in the tube, and the rate of heating.
A finite time is required to transfer heat from a hot liquid bath or metal block through the walls of the
capillary tube and throughout the mass of the sample. When the bath or block is heated too quickly, its
temperature rises several degrees during the time required for melting to occur. This can result in an
observed range that is higher than the true one.
When the temperature of the bath or block approaches the melting point of the sample, it is essential for
good results to raise the temperature slowly and at a uniform rate, usually about 2°C/min. The sample
should be small, finely powdered, and packed tightly in a thin walled capillary tube of small diameter.
The column of solid in the capillary tube should be just high enough to be seen clearly during melting
(about 1-2 mm) .
The behavior of a material upon melting should be observed and recorded carefully. Write, for example,
"Melts sharply at 89.0-89.5°C" or "mp 131-133°C, with decomposition" or "Discolors at 65°C; melts
slowly at 67-69°C."
Objectives:
1. Determine the melting points of two different pure solids that have approximately the same melting
point range.
2. Prepare a mixture of the two substances and determine its melting point range.
3. You will obtain a sample of an unknown from your instructor and determine its melting point.
4. Identify the substance by finding a mixture melting point.
©2009 Dr. Robert R. Klepper
The Melting Point Apparatus and Tubes
Figure 1: Fisher Johns melting point apparatus.
Figure 2: Graph for determining setting on the Fisher Johns Melt Temp. Set the Melt Temp at 100% until
the temperature is within 15 OC of the literature value and then use the graph above to determine where to
set it at that time.
©2009 Dr. Robert R. Klepper
Your instructor will supply directions for the proper use of the melting point apparatus available in your
laboratory.*
Procedure:
Determination of Melting Points
Pulverize 50-100 mg of urea by crushing it with a mortar and pestle. Fill a melting point tube with a
sample of urea by thrusting the open end into the powder several times. To work the plug of solid material
down to the sealed end of the tube, vigorously tap the sealed end on the table, or lightly draw a file across
the tube held loosely in the hand, or drop the melting point tube, closed-end down, through a 1.5-ft length
of 8-mm glass tubing onto the bench top. Repeat the procedure until the tube contains a 1-2mm column of
densely packed powder at the bottom. (See Figure 3). Alternatively your instructor may choose to have
you use the circular glass slide method. He will demonstrate this at the beginning of the class period.
Figure 3: a properly packed capillary tube. Take note of the small amount of chemical in the bottom of the
capillary. The white toward the top is reflected light and not chemical.
Insert the tube into the melting point apparatus and start heating. The temperature may be allowed to rise
fairly rapidly to within I5-20°C below the compound's expected melting point. However, during
determination of the actual melting point range, the temperature should not rise more rapidly than 12°C/min. Therefore, decrease the rate of heating when the temperature is about 15°C below the expected
melting point. The melting point of urea is approximately 130°C. Record the melting point range of urea
on the report sheet.
©2009 Dr. Robert R. Klepper
In a similar way, determine and record the melting point range of a sample of trans-cinnamic acid. This
compound also melts at approximately 130°C.
To demonstrate the effect of impurities on the melting point of a pure substance, determine the melting
point range of a 50-50 mixture by weight (use about 50 mg of each compound) of urea and cinnamic acid.
If time permits, repeat the procedure with 75-25 and 25- 75 mixtures. (Alternatively, your instructor may
assign different ratios to different students in the class, and each student can use both his or her results
and those of other.) Using midpoints of the melting point ranges, plot the data on the report sheet.
Identification of an Unknown
When the melting point of an unknown substance is to be determined, you can save time if you first
obtain its approximate melting point using a rapid heating rate-say; 15-20°C/min. Then allow the bath to
cool to 15-20°C below the approximate melting point. Use a second sample of the substance to determine
the melting point accurately, with a slow heating rate of about 1-2°C/min.
Obtain an unknown sample (of one of the substances listed in Table 1) from your instructor. Fill two
melting point tubes with samples of the unknown . Use one tube to determine the approximate melting
point, and use the second tube to determine the melting point more precisely, as described above . Use the
data in Table 1 to make a preliminary identification of your unknown. Then confirm its identity by the
mixture melting point technique: Mix about 50 mg of the unknown with an equal weight of the substance
you suspect it to be from the side shelf and determine the mixture melting point. Repeat the procedure, if
necessary, to determine with certainty the identity of the unknown. Record your results and conclusion on
the report sheet.
Table 1: Melting point of some organic compounds
Compound
Biphenyl
e-Caprolactam
Vanillin
Glutaric acid
Dibenzothiophene
Acetanilide
(±)-Mandelic acid
Benzoic acid
mp, OC
70-71
69-71
81-82
97-99
99-100
113-114*
119-120
121-122
Compound
2-Naphthol
Benzoin
trans-Cinnamic acid
Urea
Cholesterol
Salicylic acid
Benzanilide
Sulfanilamide
mp, OC
121-122
132-133
132-133
132-133
148-150
156-158
164-166
165-166
*If the acetanilide was recrystallized from water and not thoroughly dried, it mar melt at 83-84°C.
Waste Disposal: Discard your used melting point tubes in the waste glass container provided by your
instructor. Place extra solids from the mixture melting point in a solid organic waste container provided
by your instructor.
©2009 Dr. Robert R. Klepper
Report:
Melting Points
Unknown
Mixed melting point
Unknown ID
©2009 Dr. Robert R. Klepper
Questions:
1) You isolated the following compounds in lab and observed the melting points in the table below. For
each compound, look up the melting point in the Tables in the Handbook (or see the online Table of
Physical Constants and Hazards of Some Organic and Inorganic Compounds) and judge the purity of the
compound.
Compound
Naphthalene
Benzophenone
p-anisic acid
salicylic acid acetate
3-chlorobenzoic acid
sulfanilamide
ferrocene
observed melting point
79-80°
45-47°
178-182°
135°
157-158°
165-166°
157.5-161.5°
2) You isolated lidocaine and novocaine in the laboratory. What are the melting points of these
compounds?
3) You think that you have isolated ibuprofen in the lab. Since you don't totally trust your own laboratory
techniques, you want to prove to yourself that you have ibuprofen before you ingest it. Using only
melting point techniques, explain how you can prove that you actually have ibuprofen.
4) The melting point of a pure compound is known to be 110-111°. Describe the melting behavior
expected if this compound is contaminated with 5% of an impurity.
5) You melt and observe decolorization, but miss the melting point. Should you cool the sample and remelt it, or start over?
6) You melt a compound and it disappears: what should you do?
7) What setting should you use on the Fisher-Johns melting point apparatus to determine the melting
point of:
a) acetylferrocene
b) p-anisic acid
8) What setting should you use on the Mel-Temp melting point apparatus to determine the melting point
of:
a) acetylferrocene
b) p-anisic acid
©2009 Dr. Robert R. Klepper
9) You and your lab partner take melting points of the same sample. You observe a melting point of 101107°C, while your partner observes a value of 110-112°C. Explain how you can get two different values
with exactly the same sample.
©2009 Dr. Robert R. Klepper