Reminder: These notes are meant to supplement, not replace, the lab manual.
Determination of Melting Points
History and Application:
Since antiquity one of the simplest ways to identify an unknown substance was to
determine some physical property of that substance and compare it with physical
properties of known materials. Physical properties are observable properties that
are determined without changing the composition of the material. Common
physical properties include color, melting point, boiling point, hardness,
conductivity and density. In this lab the melting point will be used to help identify
a material and a mixed melting point will be used to assess the purity of
materials.
1.
Here is some terminology related to melting point.
Melting point: The temperature at which a solid transforms into a liquid. (Don’t
say “temperature at which a substance melts” to define melting point;
melts is too close to melting to make a good definition.)
Melting point range: The interval between the temperature at which a solid
sample just begins to turn to liquid and the temperature at which the entire
sample becomes liquid.
Phase transition: When a material changes from one physical state of matter to
another physical state. Common phase transitions are solid to liquid, liquid
to gas, and liquid to gaseous.
2.
The reasons that chemists determine melting point ranges of compounds
include:
To identify unknown compounds by comparing experimental melting points with
melting points of known substances found in the chemical literature.
To determine the purity of samples. Pure samples will have narrow melting point
ranges, close to the literature value for that substance; impure samples
will have melting point ranges broader and lower than literature values.
(Note: The literature is usually, but not always correct.)
To characterize new compounds (so that others will have a published melting
point range to compare with).
3.
Here are the structures of the three known compounds for this experiment, and
the eight other possible unknown compounds along with their literature melting
pointsi. (Draw all 11 structures in your notebook.)
naphthalene
(80.5oC)
acetanilide
(114.3 oC)
urea
(135 oC)
The structures of the other possible unknowns are:
p-nitrotoluene
(54.5 oC)
glutaric acid
2-methyl benzoic acid
(pentanedioic acid)
(99 oC)
(107 oC)
salicylic acid
sulfanilamide
(158 oC)
(165 oC)
succinic acid
(butanedioic acid)
(188 oC)
benzoic acid
(122.1 oC)
3, 5-dinitrobenzoic acid
(205 oC)
4. Soluble impurities in a solid cause the solid to melt at a lower temperature. This is
true even if the impurity itself melts at a higher temperature. If a pure material
melts at 111-112oC and another pure material melts at 115-117 oC, then a 50:50
mixture of these materials will melt at a lower and broader range. A reasonable
expected range may be 95-109 oC. The exact range observed will depend on
how well the sample is mixed and the heating rate. This range cannot be easily
calculated, but can easily be experimentally determined. The fact that soluble
impurities cause melting points to lower and broaden, allow experimentally
determined melting points to be used to assess the purity of samples. Melting
points of isolated or synthesized compounds will be taken throughout the rest of
2230L and 2240L to assess the purity of the derived compounds.
5. Phase Diagrams explain the temperature of phase transitions (solid to liquid, liquid to
solid) of mixtures of materials. Below is a phase diagramii of tin (Sn) and lead
(Pb). Pure lead melts at 327 oC. Pure tin melts at 232 oC. A 50:50 mixture of tin
and lead will start to melt at 183 oC and continue melting until approximately 275
o
C. Its melting point range will be 183-273 oC. The point marked with an asterisk
(*) is called the eutectic point. This is the lowest temperature at which liquid may
exist in the mixture. For this mixture the eutectic point is at 183 oC with a
composition of 62% tin and 38% lead. At the eutectic the mixture will melt over a
narrow range.
6. Insoluble materials or materials with a very high melting point, such as sand or glass,
will have no impact on the melting point of a substance.
7. All melting point measurements using the Mel-temps apparatus need to be reported
as a melting point range not a single melting point. The range starts at the
temperature the first bit of liquid is observed. The range ends at the temperature
that the entire solid has disappeared. In order to get an accurate melting point
range it is necessary to have a slow rate of temperature increase.
8. A practical balance must be struck between allotting only a small amount of time for
obtaining a melting point range and a slow temperature increase of the Mel-temp
to obtain good data. A very slow heating rate (2o C/min) will give good data but
will take more than an hour to go from room temperature to 150 oC. This is not
reasonable for a 2 hour and 50 minute lab period. If a 20oC/minute is used, it will
only take about 7 minutes to acquire, but the data will be very poor to useless.
Often a balance can be struck by ramping the Mel-Temp up quickly to a
temperature 20 or 30 degrees under the literature value, then slowing the
temperature increase down to achieve good data. If the literature value is not
known, a very quick melting point can be taken to identify an approximate melting
point range, and then with another sample, a slow melting point range is taken
starting about 10 degrees below the anticipated value.
9. There are many ways in conduct a poor melting point analysis and hence obtain poor
quality data. Some of the most common mistakes include packing too much
material into a capillary tube, ramping the temperature of the Mel-Temp too
quickly, not properly identifying the onset or completion of melting, or simply
looking away and missing the phase transition. It is much more common to
obtain an artificially lower melting point range than to obtain a melting point range
higher than the literature value. When conducting mixed melting point analysis, if
the two solids are not thoroughly and homogeneously mixed, poor quality data
will result.
10. Detailed instructions on how to fill a capillary tube and take a melting point reading
can be found on this web siteiii:
https://web.coas.missouri.edu/~chemweb/chem2130/doc/fs13/how-to-fill-themelting-point-capillaries-for-the-mel.pdf
If the capillary is overfilled a poor measurement will result. The depth of sample
in the tube should be about 2 mm or about the thickness of a quarter.
11. Here is a procedure for identifying an unknown compound from melting point data.
First determine the melting point range of the unknown.
Compare the melting point range you determined with literature melting points of
known compounds. As a general guideline, consider any compound as a
possibility whose melting point comes within plus or minus 10° C of the
range of the unknown as a possible ID of the unknown (plus or minus 15°
C if the melting point range is above 200° C or if the scientist has only
limited experience taking melting points). The 10° C range allows for
thermometer inaccuracies, sample packing irregularities as well as
impurities in your unknown. As expertise is gained, these values may be
decreased by 5° C.
Mix a small sample of your unknown with a similar size small sample of three of
the possible ID compounds, and determine the melting point ranges of
these mixtures.
The mixture whose melting point range differs least from the melting point range
of the pure unknown is most likely the same composition as the unknown.
12. If a set of melting point range data were provided, you should be able to predict the
identity of the material. You are not expected to memorize melting points
of materials.
13.
The behavior of samples when they melt:
Pure samples melt over a narrow range, close to the literature value of the
melting point.
Samples containing soluble impurities have a melting point range that is broader
and lower than the melting point range of a pure sample. Example: Pure
substance, 121-122°; same substance when impure, 109-117°.
Samples containing insoluble impurities have a melting range similar to that of a
pure sample (except that you might see particles of the impurity floating
around after all of your compound has melted).
References
i CRC Handbook of Chemistry and Physics, 65th ed, CRC press, 1984, p c-65-c-575
ii Phase diagram http://www.chemguide.co.uk/physical/phaseeqia/snpb.html
(December 10, 2010)
iii Capillary tube filling and Mel-Temp use, University of Missouri, College of Arts and
Sciences,
https://web.coas.missouri.edu/~chemweb/chem2130/doc/fs13/how-to-fillthe-melting-point-capillaries-for-the-mel.pdf.. (September 7, 2015)
Hack’s Chemical Dictionary, 4th Editor, J. Grant, McGraw-Hill, 1966 p 414, 506,
Revised September 6, 2015 S. L. Weaver