Determination of Melting Point Ranges

PART I

Melting Ranges of Impure and Recrystallized Unknown Solids

NOTE:  The Purified Product MUST be completely dry before its melting point can be determined.

You are to determine the melting range of your recrystallization product using the following procedure:

Capillary Tubes and Sample Preparation

Commercially available capillary tubes have one end already sealed and are open at the other end to permit introduction of the sample.  The sample is put into the tube as follows:

Place a small amount of the solid whose melting point you wish to determine (your recrystallized product) on a clean watch glass and tap the open end of the capillary tube into the solid on the glass so that a small amount is forced about 2mm - 3mm  into the tube.  You may have to grind your crystals first into a fine powder in order to get your material into the capillary tube , then to get the solid to the closed end of the tube, take a piece of glass tubing 1 meter long and place this tube vertically on the benchtop, then drop the capillary tube (sealed end down) through the tube several times.

Melting Point Determination

The determination of the melting point involves taking the capillary tube containing the  purified unknown and placing them in the slots that are seen through the magnifying glass of your melting point apparatus.  The melting point apparatus is then allowed to heat until the contents of both tubes dissolve.  Best results are obtained by heating the sample at a rate of about two degrees per minute.  Many organic compounds undergo a change in crystalline structure just before melting, usually as a consequence of the release of water of crystallization.  The solid takes on a softer, perhaps "wet" appearance, which may also be accompanied by a shrinkage of the sample in the capillary tube.  These changes in the sample should NOT be interpreted as the beginning of the melting process.  Wait for the tiny drop of liquid to appear.  Melting invariably occurs over a temperature range, and the melting point range is defined as the temperature at which the first tiny drop of liquid appears up to and including the temperature at which the solid has melted completely.

PART II

Mixed-Melting Point Determination

Prepare a capillary tube containing only your recrystallized product.

On a piece of glassine weighing paper (DO NOT use filter paper) place a small sample of your purified product and a small amount of known compound you believe to be your product. Make an intimate mixture of the two substances by mixing the solids together thoroughly with your spatula.

Prepare a capillary tube containing the mixture.

On another piece of glassine weighing paper place a small sample of your purified sample and a small amount of another known compound you believe your product could be.

Make an intimate mixture of these two substances by mixing them thoroughly with your spatula.

Prepare a capillary tube containing the mixture from step 6.

Measure the melting ranges of the 3 samples. The apparatus can accommodate three capillary tubes, so melting range determinations can be done simultaneously.

Package the remainder of your recrystallized product in an appropriately labeled sample vial.

NOTES:

a)  The actual melting ranges may differ somewhat from the literature values that are given in the list. Do NOT determine the melting range of any of the standards.

b)  The melting range of your purified benzoic acid may not be exactly the same as the literature value because: a) the solid is not completely pure b) the sample was heated too rapidly in the melting point apparatus c) or thermometer error, etc.

Solid Compounds and their Melting Points