Melting Points

 

http://www.chemistry.mcmaster.ca/courses/2oa3/labmanual/2oa3_exp3.pdf

Melting points are an easily measured physical property of solid-state organic compounds useful in product characterization and purity determination.  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. 

Since melting points are relatively easy and inexpensive to determine, they are handy identification tools to the organic chemist.  If you want to use the melting point to identify a solid compound that you have isolated in the lab, you will need to compare its melting point with that of the true compound.  Melting points are listed in various sources of scientific data, as referenced on the Chem Info page or in the physical data tables on this orgchem site: 

Chem Info

Table of Physical Constants and Hazards of Some Organic and Inorganic Compounds

 

Melting points of pure compounds are also 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://chemfinder.cambridgesoft.com/

 

Measurements of the melting point of a solid will provide you with information about the purity of the substance.  Pure, crystalline solids melt over a very narrow range (melting range) of temperatures, whereas mixtures melt over 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. 

 

 

 

 

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. 

 

 

Procedure

See http://orgchem.colorado.edu/hndbksupport/meltingpt/mtfill.html for techniques of how to fill a capillary. 

Melting points are best determined using a finely divided powder.  Grind the sample using a mortar and pistil to ensure homogeneity.  Fill a capillary tube to a height of about 10 mm with the packed urea.  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 45 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 70).  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.

 

Prepare a melting point diagram for a mixture of two compounds. 

We will use urea and cinnamic acid.  Work in groups of two for this part.  Record the melting point ranges of pure urea, pure cinnamic acid, a 4:1 urea:cinnamic acid mixture, a 1:1 urea:cinnamic acid mixture, a 1:3 urea:cinnamic acid mixture, and a 1:5 urea:cinnamic acid mixture.    

 

Plot your data in a phase diagram. 

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

Record the literature melting points of all substances you have used and compare the values you have determined.  Comment on any discrepancies. 

Prepare a graph of the melting points of your mixtures as a function of composition of the mixture.  Plot both the high and the low temperatures of the temperature range that you obtained.  Connect the data points for the high end and the data points for the low end.  Try to identify the lowest point of the mixture. 

 

 

Questions

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? 

5.  Why does the sample in the melting point capillary tube have to be packed tightly?