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4.4 DETERMINATION OF FREEZING TEMPERATURE

      The temperature at which a substance passes from the liquid to the solid state upon cooling is a useful index to purity if heat is liberated when the solidification takes place, provided that any impurities present dissolve in the liquid only, and not in the solid. Pure substances have a well-defined freezing point, but mixtures generally freeze over a range of temperatures. For many mixtures, the freezing temperature, as determined by strict adherence to the following empirical methods, is a useful index of purity. The method for determining freezing temperature set forth here is applicable to substances that melt between –20º and 150º, the range of the thermometer used in the bath. The freezing temperature is the maximum point (or lacking a maximum, the point of inflection) in the temperature-time curve.

Apparatus

      Assemble an apparatus similar to that illustrated, in which the container for the substance is a 25-mm × 100-mm test-tube. This is provided with a suitable, short-range thermometer suspended in the center, and a wire stirrer, about 300 mm long, bent at its lower end into a horizontal loop around the thermometer. (Use a thermometer having a range not exceeding 30º, graduated in 0.1º divisions, and calibrated for, but not used at, 76-mm immersion. Other temperature-measuring devices may be used if they are validated for this procedure. Dimensions should be within ±20 per cent of those given in the illustration.)

      The sample container is supported, by means of a cork, in a suitable water-tight cylinder about 50 mm in internal diameter and 110 mm in length. The cylinder, in turn, is supported in a suitable bath sufficient to provide not less than a 37-mm layer surrounding the sides and bottom of the cylinder. The outside bath is provided with a suitable thermometer.

Procedure

      Use a thermometer having a range not exceeding 30º, graduated in 0.1º divisions, and calibrated for, but not used at, 76-mm immersion. Melt the substance, if a solid, at a temperature not exceeding 20º above its expected freezing temperature, and pour it into the testtube to a height of 50 to 57 mm. Assemble the apparatus with the bulb of the test-tube thermometer immersed halfway between the top and bottom of the sample in the test-tube. Fill the bath to about 12 mm from the top of the tube with suitable fluid at a temperature of 4º to 5º below the expected freezing temperature.

      In case the substance is a liquid at room temperature, carry out the determination using a bath temperature about 15º below the expected freezing temperature.

      When the test sample has cooled to about 5º above its expected freezing temperature, adjust the bath to a temperature 7º to 8º below the expected freezing temperature. Stir the sample continuously during the remainder of the test by moving the loop up and down between the top and bottom of the sample, at a regular rate of 20 complete cycles per minute.

      Solidification frequently may be induced by rubbing the inner walls of the test-tube with the thermometer, or by introducing a crystal of the substance being examined. Pronounced supercooling may cause deviation from the normal pattern of temperature changes. If the latter occurs, repeat the test, introducing small particles of the material under test in solid form at 1º intervals as the temperature approaches the expected freezing temperature.

      Record the reading of the test-tube thermometer every 30 seconds. Continue stirring only so long as the temperature is gradually falling, stopping when the temperature becomes constant or starts to rise slightly. Continue recording the temperature in the test-tube every 30 seconds for at least 3 minutes after the temperature again begins to fall after remaining constant.

      The average of not less than four consecutive readings that lie within a range of 0.2º constitutes the freezing temperature. These readings lie about a point of inflection or a maximum, in the temperature-time curve, that occurs after the temperature becomes constant or starts to rise and before it again begins to fall. The average to the nearest 0.1º is the freezing temperature.