The congealing temperature is that point at which there exists a mixture of the liquid (fused) phase of a substance and a small but increasing proportion of the solid phase. It is distinct from the freezing point which is the temperature at which the liquid and solid phases of a substance are in equilibrium. In certain cases, this may happen over a range of temperatures.
The temperature at which a substance solidifies upon cooling is a useful index of its purity if heat is liberated when solidification takes place. The following method is applicable to substances that melt between -20°C and 150°C
The inner tube with its jacket is supported centrally in a 1-litre beaker containing a suitable cooling liquid to within 20 mm of the top. The thermometer is supported in the cooling bath (see Fig.).
Melt the substance, if a solid, at a temperature not more than 20° above its expected congealing point, and pour it into the inner test-tube to a height of 50 to 57 mm. Assemble the apparatus with the bulb of the thermometer immersed halfway between the top and bottom of the sample in the test tube.
Fill the bath to almost 20 mm from the top of the tube with a suitable fluid at a temperature 4°C to 5°C below the expected congealing point. If the substance is a liquid at room temperature, carry out the determination using bath temperature about 15°C below the expected congealing point.
Record the reading of the thermometer every 30 seconds and continue stirring only so long as the temperature is falling. Stop the stirring when the temperature is constant or starts to rise slightly. Continue recording the temperature for at least 3 minutes after the temperature again begins to fall after remaining constant. The congealing point will be the mean of not less than four consecutive readings that lie within a range of 0.2°C.
The temperature at which a substance solidifies upon cooling is a useful index of its purity if heat is liberated when solidification takes place. The following method is applicable to substances that melt between -20°C and 150°C
Apparatus
A test-tube (about 150 mm x 25 mm) placed inside another test -tube (about 160 mm x 40 mm) the inner tube is closed by a stopper that carries a stirrer and a thermometer (about 175 mm long and with 0.20 graduations) fixed so that the bulb is about 15 mm above the bottom of the tube. The stirrer is made from a glass rod or other suitable material formed at one end into loop of about 18mm overall diameter at right angles to the rod.
The inner tube with its jacket is supported centrally in a 1-litre beaker containing a suitable cooling liquid to within 20 mm of the top. The thermometer is supported in the cooling bath (see Fig.).Melt the substance, if a solid, at a temperature not more than 20° above its expected congealing point, and pour it into the inner test-tube to a height of 50 to 57 mm. Assemble the apparatus with the bulb of the thermometer immersed halfway between the top and bottom of the sample in the test tube.
Fill the bath to almost 20 mm from the top of the tube with a suitable fluid at a temperature 4°C to 5°C below the expected congealing point. If the substance is a liquid at room temperature, carry out the determination using bath temperature about 15°C below the expected congealing point.
Record the reading of the thermometer every 30 seconds and continue stirring only so long as the temperature is falling. Stop the stirring when the temperature is constant or starts to rise slightly. Continue recording the temperature for at least 3 minutes after the temperature again begins to fall after remaining constant. The congealing point will be the mean of not less than four consecutive readings that lie within a range of 0.2°C.
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