á651ñCONGEALING 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 congealing temperature,as determined by strict adherence to the following empirical methods,is a useful index of purity.The method for determining congealing temperatures set forth here is applicable to substances that melt between -20
and 150,the range of the thermometer used in the bath.The congealing temperature is the maximum point (or lacking a maximum,the point of inflection)in the temperature-time curve.
and 150,the range of the thermometer used in the bath.The congealing 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,
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Congealing Temperature Apparatus
,graduated in 0.1divisions,and calibrated for,but not used at,76-mm immersion.Asuitable series of thermometers,covering a range from -20to +150,is available as the ASTM E1series 89Cthrough 96C.Other temperature-measuring devices may be used if they are validated for this procedure (see Thermometers á21ñ).Dimensions should be within ±20%of those given in the illustration.
The specimen container is supported,by means of a cork,in a suitable water-tight cylinder about 50mm in internal diameter and 11cm 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— Melt the substance,if a solid,at a temperature not exceeding 20
above its expected congealing point,and pour it into the test tube to a height of 50to 57mm.Assemble the apparatus with the bulb of the test tube thermometer immersed halfway between the top and bottom of the specimen in the test tube.Fill the bath to about 12mm from the top of the tube with suitable fluid at a temperature 4to 5below the expected congealing point.
In case the substance is a liquid at room temperature,carry out the determination using a bath temperature about 15below the expected congealing point.
below the expected congealing point.When the test specimen has cooled to about 5above its expected congealing point,adjust the bath to a temperature 7to 8below the expected congealing point.Stir the specimen continuously during the remainder of the test by moving the loop up and down between the top and bottom of the specimen,at a regular rate of 20complete cycles per minute.
above its expected congealing point,adjust the bath to a temperature 7to 8below the expected congealing point.Stir the specimen continuously during the remainder of the test by moving the loop up and down between the top and bottom of the specimen,at a regular rate of 20complete cycles per minute.Congelation frequently may be induced by rubbing the inner walls of the test tube with the thermometer,or by introducing a small fragment of the previously congealed substance.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 1intervals as the temperature approaches the expected congealing point.
intervals as the temperature approaches the expected congealing point.Record the reading of the test tube thermometer every 30seconds.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 30seconds for at least 3minutes 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 0.2constitutes the congealing 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 0.1is the congealing temperature.
constitutes the congealing 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 0.1is the congealing temperature.