MELTING POINT TIPS AND GUIDELINES

A pure organic compound usually melts over a range of two degrees or less.

If the melting point of a pure compound is within a degree of the value found a lab handbook it is presumed to be pure. 

A sample is impure if it has a melting point range that is lower and/or wider than that the literature value.More impurities increase this effect.

In the organic lab, unless something is wrong with the procedure or the equipment, a substance generally cannot be observed to melt at a higher temperature than its melting point!

Experimental melting points should be always reported as a range, for example, 42-44°.

A given sample is only used once for mp determination.Always use fresh samples for additional trials.

Dispose of used capillary tubes in the proper receptacle, not in the wastebasket.

Most errors come from heating the sample too fast. A heating rate of 1 to 2 degrees per minute is will give good results.Going faster than five degrees per minute virtually guarantees poor results in most cases.

TIME-SAVING TIP: If a compound has a high mp, it can take a long time to reach it at 1 or 2 degrees per minute! Unless you have prior knowledge of the expected melting point, it is advisable to have extra samples prepared ahead of time. Run the first sample at a high rate of heating to get an approximate mp range. Then repeat the procedure but slow down the rate of heating as you approach the expected melting point.

Too much or too little sample can lead to errors.Generally 3-4 mm in the bottom of a capillary tube works well.

The sample should be firmly packed in the bottom of the tube to insure efficient heat transfer. 

See the Picture (Right)

The smaller sample is probably too small to properly observe the mp range.The larger sample may be too large, resulting in a mp range that is too wide and possibly high.The middle sample is about right but it is near the maximum size that should be used.Note that it is firmly packed.

·If you have trouble packing a tube, consider discarding it (proper receptacle) and starting over. Difficult cases are often not worth the struggle and mp capillaries are inexpensive.

·When it doubt about a result, check your technique by running a sample of a pure known substance with a similar melting point.

·Samples that melt below 50° require extra care in order to get good results. Slow heating is critical. Also, small amounts of impurities seem to have a greater effect on these low melting substances.

·Mixed Melting Points – The identity of two samples that have the same melting point can be determined by taking a mixed melting point. Grind roughly equal portions of the two samples together to mix well and take a melting point of the mixture. If the two substances are identical the melting point should be the same as that of either sample. If the two substances are not identical, then the melting point will be depressed.

WHAT YOU MAY SEE

THIS IS A SAMPLE OF IBUPROFEN ISOLATED FROM COMMERCIAL TABLETS.

TINY DROPLETS APPEAR AT THE START OF THE MELTING POINT RANGE. OFTEN THE SAMPLE APPEARS TO “SHRINK” AWAY FROM THE SIDES OF THE TUBE. HERE, AS SHOWN IN THE INSET (YELLOW BOX), THE SAMPLE HAS STARTED TO MELT JUST ABOVE 72°.

AS THE TEMPERATURE RISES TO 73° THE LIQUID PHASE BECOMES CLEARLY VISIBLE.

THE LITERATURE VALUE (The Merck Index, 12^th Ed.) FOR IBUPROFEN IS 75-77°.

THE EXPERIMENTAL RESULT SUPPORTS THE PRESENCE OF IBUPROFEN. BUT THE MP RANGE IS WIDER AND LOWER THAN EXPECTED, PROBABLY DUE TO SMALL AMOUNTS OF IMPURITIES.