Melting point determination

The general procedure is to heat the oil bath with the micro burner using the technique described on, or to use the heating control to give a temperature rise of about 10QC per minute. When the temperature is about 20°C below the melting point, the rate of heating must be reduced to about 2°C per minute and continued at this rate until the compound has melted. The melting point is the temperature range from where the first drop of liquid appears to where the last crystal dissolves into the liquid.The following points of technique should be considered:

• The most common error is heating the sample too quickly. There is often a lag between slowing the heating rate and the reduction in temperature increase, resulting in a reading of the melting point which is too high.
• If you do not know what the compound is, you will not know its expected melting point. Therefore, you should determine an approximate value and then repeat the procedure to give an accurate measurement, by reducing the heating rate, particularly for the final stage near the melting point. The boost heater on the electrical apparatus can be used for finding the approximate melting point but not for accurate determination.
• If you are carrying out several melting point measurements, it is common sense to measure the melting point of the lowest melting compound first - less time for the apparatus to cool to room temperature.
• If you miss the melting point, do not allow the sample to solidify and then retake the melting point with the same sample: some decomposition may have occurred on melting.