Winter 2007
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PCC Sylvania
Jenkins
Exp #1 Melting Point, Boiling Point, and Index of Refraction
Pre-Lab
Complete items numbered 1-8 (omit number 3) as described in your lab syllabus. In addition to
the known compounds, make sure your data table includes the appropriate physical properties for
all the possible “unknowns” for each experiment (6 compounds for the melting point, 5
compounds for the boiling point and refractive index). Remember to write in pen in your lab
notebook, and to turn in your carbon copies before you start the lab.
Organic chemists determine the purity and characterization of intermediate and final products by
simply measuring a few physical properties, including the melting point, boiling point, and index of
refraction.
Purpose of the Experiment is to find answers to these questions (Question of the Day):
How do the physical properties, melting point, boiling point, and index of refraction allow one to
determine the purity of the product? What are the limitations of each technique?
A. Melting Points
Pure, crystalline solids have a characteristic melting point, which is expressed as the
temperature range over which the solid melts to become a liquid. The transition between the
solid and the liquid is so sharp for small samples of a pure substance that melting points can be
measured to ±0.1oC. Typically it is no more than ±1oC. Melting points of pure compounds are
recorded in handbooks, such as the Handbook of Chemistry and Physics (CRC) or the Merck
Index. Alternatively, you can find this information on the Internet, for example at
http://www.sigmaaldrich.com/Area_of_Interest/The_Americas/United_States.html
Measurements of the melting point of a solid can also provide information about the purity of the
substance. Pure crystalline solids have a sharp melting point. They melt in a very narrow range
(melting range) of temperatures, whereas mixtures melt with a broad temperature range.
Mixtures also tend to melt at temperatures below the melting points of the pure solids.
Many solid substances prepared in the organic laboratory are initially impure. These impurities
affect the melting point of a substance. In a sample that contains a mixture of two compounds,
each component usually depresses the melting point of the other, giving an observed melting
point range that is lower and broader than the melting point of either component. A melting point
composition diagram for two hypothetical solids, A and B, is shown below, as a graph of
temperature versus composition.
FIGURE 1
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PCC Sylvania
Jenkins
The eutectic point is the lowest temperature of the mixture and is determined by the equilibrium
composition at which A and B melt in constant ratio. A sample whose composition is exactly that
of the eutectic point will exhibit a sharp melting point at the eutectic temperature. This means a
eutectic mixture can be mistaken for a pure compound since both have a sharp melting point.
Because it is difficult to heat solids to temperatures above their melting points, and because pure
solids tend to melt over a very small temperature range, melting points are often used to help
identify compounds.
We will use the Mel-Temp apparatus for measuring the melting point in our lab. The Mel-Temp
apparatus uses closed-end capillary tubes. The sample is placed into a pre-designed slot and its
melting behavior observed through a magnifying glass. Keep in mind that we have 5 Mel-Temps
for the labs. Schedule your lab experiment to minimize waiting time.
Procedure
I. Melting points are best determined using a finely divided powder. Grind the sample using a
mortar and pestle to ensure homogeneity. Fill a capillary tube to a height of no more than 2-3
mm with the packed urea. The sample can be packed tightly by dropping the capillary tube
through glass tubing on a table top or the floor. Put the tube into the Mel-temp apparatus closed
end down. Make sure that you can see the sample through the magnifying glass. Set the voltage
to zero and turn on the Mel-temp. Turn the voltage to 4.5 and observe both the sample and
temperature reading as you heat.
http://orgchem.colorado.edu/hndbksupport/meltingpt/mtset.html. (Never set the voltage at more
than 7). Note (a) the temperature at which the column of urea first collapses or shows some
liquid and (b) the temperature at which the sample is completely liquid. This is the melting range,
which we call a melting point. Always report a melting range.
The melting point is not accurate if the thermometer and the sample are not at the same
temperature. For accuracy the sample should be heated through the melting range at a rate of 1
oC or less per minute. Turn off the apparatus and let it cool. If you did not get a good result for
the melting point of urea, prepare a sample in a new capillary, and repeat the measurement.
Capillaries cannot be reused. Put used capillaries in the glass waste container.
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II. Work in groups of two for this part. Prepare a melting point diagram for a mixture of two
compounds. We will use various prepared mixtures of urea and cinnamic acid. Record the
melting point ranges of urea (from part I), pure cinnamic acid, a 1:1 urea:cinnamic acid mixture, a
4:1 urea:cinnamic acid mixture, and a 1:4 urea:cinnamic acid mixture. If you prefer to have more
data points, feel free to make your own mixtures with various compositions. Plot your data in a
melting point composition diagram similar to Figure 1. Make an accurate diagram using graph
paper or a computer program and record your melting point ranges.
http://ull.chemistry.uakron.edu/organic_lab/melting_point/
III. You will be given a solid unknown. Your unknown is one of the following compounds:
salicylic acid, benzoic acid, succinic acid, acetanilide, benzophenone, or napthalene. Samples of
all of these compounds are available in the lab. In the procedure part of your Prelab explain how
you plan to identify your unknown.
Data and Observations
Record the melting ranges obtained directly into your lab notebook. List any important
observations you make while performing the experiment. For example, describe the appearance
of a compound when it melts and any other visible changes occurring prior to, or during, the
melting process, i.e. water vapor, gas bubbles, color changes, clarity of the liquid melt.
Analysis
Compare the literature melting points of all substances you have used and compare to the values
you have determined experimentally. Comment on any discrepancies.
Compare the melting point ranges of your pure urea and cinnamic acid with the mixture. Is it
possible to estimate the eutectic point from your graph?
Questions for Postlab
1. How fast do you heat the sample in the Mel-temp when determining a melting point?
2. If you heat too fast, will your observed melting point be higher or lower than the true value?
Explain.
3. What is meant by the term melting range? What happens at this range?
4. Why should you always use a new capillary tube with a sample of your compound when doing
a second melting point determination?
B. Refractive Indices
The refractive index is a physical constant that, like the boiling point, can be used to
characterize liquids. It is the ratio of the velocity of light in air to the velocity of light in the liquid.
The angle of refraction is a function of temperature and the wavelength of light. Because the
velocity of light in air is always greater than that through a liquid, the refractive index is a number
greater than 1; for example, hexane n20D 1.3751. The superscript 20 indicates that the
measurement was made at 20 oC, and the subscript D refers to the yellow D-line of a sodium
vapor lamp, light with a wavelength of 589 nm.
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The measurement is made with a refractometer using a few drops of liquid.
Compensation is made within the instrument for the fact that white light and not sodium vapor
light is used, but a temperature correction must be applied to the observed reading by adding
0.00045 for each degree above 20 oC.
n20D = ntD + 0.00045(t – 20oC)
equation 1
The refractive index can be determined to 1 part in 10,000, but because the value is quite
sensitive to impurities, there is not always very good agreement with the literature with regard to
the last figure. To master the technique of using the refractometer, you will measure the
refractive indices of several known, pure liquids of your choice before measuring an unknown.
For more information on the Abbe-3L refractometer (the instrument we have in the lab)
and some video clips demonstrating its use, please go to
http://web.uccs.edu/bgaddis/chem337/expts/nD/nD.htm
You may also find this link helpful.
Procedure
Measure the refractive index for a known liquid compound. Place two or three drops of
the sample on the open prism using a polyethylene pipette (to avoid scratching the prism face).
Close the prism and turn on the light. Position the light for maximum brightness as seen through
the eyepiece. If the refractometer is set to a nearly correct value, then a partially gray image will
be seen. Turn the knob so that the line separating the dark and light areas is at the crosshairs.
Sometimes the line separating the dark and light areas is fuzzy and colored. Turn the chromatic
adjustment until the demarcation line is sharp and colorless. Then read the refractive index by
pressing the button down to light up the scale in the field of vision. In your notebook record the
temperature on the thermometer attached to the refractometer and make the appropriate
temperature correction to the observed index of refraction using equation 1. Repeat this
procedure with a second known compound. If the index of refraction is within experimental error
for at least one compound, then proceed to the identification of an unknown sample as described
in the boiling point procedure.
C. Boiling Points
The boiling points of pure organic liquids are, like the melting points, characteristic physical
properties. The process of determining the boiling point is more complex than that for the
melting point. It requires more material, and because it is less affected by impurities, it is not a
good indication of purity. Like the melting point, the boiling point of a liquid is affected by the
intermolecular forces that attract one molecule to another-ionic attraction, dipole-dipole
interaction, hydrogen bonding, and van der Waals forces. A very clean liquid in a very clean
vessel will superheat and not boil when subjected to a temperature above its boiling point. If
boiling does occur under these conditions, it occurs with explosive violence. To avoid this
problem boiling stones or a boiling stick is always added to liquids before heating them to
boiling.
Procedure
1. Practice your boiling point determination techniques, by placing about 0.3 mL of one of the
liquids provided and a boiling stone in a reaction tube. Fit a distillation head (connecting adapter)
on top of the reaction tube to ensure that the system is open to the atmosphere. REMEMBER
TO NEVER HEAT A CLOSED SYSTEM! Using a thermometer adapter clamp a thermometer so
that the bulb is just above the liquid, and then heat the liquid with a sand bath or a water bath
(depending on the boiling point of your liquid, above 100 oC a sand bath, below 100 oC a water
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bath). Heating is regulated so that the boiling liquid refluxes (condenses the drips down) about 3
cm up the thermometer bulb in order to heat the mercury thoroughly. The boiling point is the
highest temperature recorded by the thermometer and maintained over about a 1-minute time
interval. True boiling is indicated by drops dripping from the thermometer and a constant
temperature recorded on the thermometer. If the temperature is not constant, then you are
probably not observing true boiling.
2. You will be given a liquid unknown. Your unknown is one of the following liquids: toluene,
ethanol, cyclohexane, 1-butanol, or distilled water. All of the liquids will be available in the lab. In
the procedure part of your lab, explain how you plan to identify your unknown using the boiling
point and refractive index.
Observation
Record all your data directly in your lab notebook and compare experimental results with
literature values. Comment on any discrepancies.
Analysis
Explain how you identified your liquid unknown using the boiling point and index of refraction.
Comment on the technique of boiling point determination and refluxing and the use of the
refractometer.
Questions for the Postlab
5. What are the consequences of heating a closed system?
6. Which would you expect to have a higher boiling point, ethanol or dimethyl ether? Explain.
7. Answer the question of the day in your notebook.
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