Title: Determination of Melting Range or Temperature
- Objective: To determine the Melting Point of a solid to provide information on the purity of the substance.
- Principle: Melting point of a solid indicates the strength of the force of attraction between the particles of the solid. On heating a solid, its molecules absorb energy in the form of heat and their kinetic energy increases.
As the kinetic energy increases, the temperature of the solid increases. As a result, the force of attraction between the molecules decreases and the molecules become more and more separated. This increases the potential energy of the molecule and the particles leave their fixed positions and start moving more freely.
At a particular temperature, the separation of the molecules increases by a large amount and the solid melts and converts into liquid. This particular temperature is the melting point of that solid.
3. Procedure:
- Precautions:
- For precise result maintain a heating rate of 1 to 2 degrees per minute.
- For unknown sample, run the first sample at a high rate of heating to get an approximate melting point range. Then repeat the procedure but slow down the rate of heating as you approach the expected melting point.
- Do not fill sample in the capillary too little or too large which can lead to errors.
- Generally fill 4 to 6 mm in the bottom of a capillary tube. The sample should be firmly packed in the bottom of the tube to ensure efficient heat transfer.
- Method I: Use Method I unless otherwise specified in the individual monograph.
- Apparatus:
- A glass heating vessel of suitable construction and capacity containing one of the following or any other suitable bath liquid to a height of not less than 14 cm.
- Water for temperature up to 60°C.
- Glycerin for temperature up to 150°C.
- Liquid paraffin of sufficiently high boiling range for temperature up to 250°C.
- Sesame oil or a suitable grade of silicone oil for temperature up to 300°C.
- A suitable stirring device capable of rapidly mixing the liquids.
- An accurately standardised thermometer suitable for the substance being examined. The thermometer must be positioned in the bath liquid to its specified immersion depth and yet leave the bulb about 2 cm above the bottom of the bath.
- Thin-walled capillary glass tubes of hard glass, closed at one end, about 12 cm long, with a thickness of 0.2 to 0.3 mm and an internal diameter of 0.8 to 1.1 mm. The tubes should preferably be kept sealed at both ends and cut as required. A suitable magnifying glass may be used for observation of melting in the capillary tube.
- A source of heat (open flame or electric heater).
- Method:
Reduce the substance to a very fine powder and, unless otherwise directed, dry it at a temperature considerably below its melting temperature of at a pressure of 1.5 to 2.5 kPa over self-indicating silica gel for 24 hours. Introduce into a capillary glass tube, a sufficient quantity of the dry powder to form a compact column 4 to 6 mm height. Heat the bath until the temperature is about 10°C below the expected melting temperature. Remove the thermometer and quickly attach the capillary tube to the thermometer by wetting both with a drop of the liquid of the bath or otherwise and adjust its height so that the closed end of the capillary is near the middle of the thermometer bulb. Place the thermometer and continue the heating, with constant stirring, sufficiently to cause the temperature to rise at a rate of about 1°C per minute. Continue the heating and note the temperature at which the column of the sample collapse definitely against the side of the tube at any point, (melting point of the sample) when melting may be considered to have begun and note also the temperature at which the sample becomes liquid throughout as seen by the formation of a definite meniscus. (sample collapse temperature to definite meniscus formation temperature is called melting range ). The two temperatures should fall within the limits of the melting range.