Melting Point (using the MelTemp)
(from Mohrig, p. 96) The MelTemp apparatus consists of an electrically heated aluminum block that accommodates three capillary tubes. The sample is illuminated through the lower port and observed with a sixpower lens through the upper port. The heating rate can be controlled by a rheostat, and with a special thermometer the apparatus can be used up to 500 oC, far above the useful limit of silicone oil (about 350 oC).
Melting points may be run from 8 AM to 5 PM Monday through Friday only. If you want to run one during another lab, please get the instructors approval.
The most critical factor in determining an accurate melting point is the rate of heating which can be controlled on the MelTemp. At the melting point the temperature rise should not be greater than 2-4 oC per minute. This may seem extraordinarily slow, but it is necessary in order for heat from the bath to be transferred equally to the sample and to the glass and mercury of the thermometer.
How do you know the correct setting on the MelTemp?
Since you may not know the melting point of your solid you will first determine a quick or approximate melting point. Heat the sample such that the temperature rises rapidly. Watch for the temperature when the solid begins to melt. Record this as your approximate melting point. This will not be used to report your final melting point.
Now obtain the accurate melting point by controlling the heating rate more carefully. (Note: If you are continuing to use the same MelTemp, wait until the temperature drops atleast 20oC below the approximate melting point). Use this chart to determine the power or voltage setting on the rheostat. Then set the MelTemp to a temperature rise of 2-4o per minute. The melting range obtained using this slow technique is what you will report.
For example: If the M.pt was determined to be 200oC in the quick trial, wait till your Melt-Temp has cooled to 180oC, then refer to the chart for MelTemp setting that will be used in the slow method. Using this example, it turns out to be a power level of 4.7 or voltage to 58V.
Important Note about Melting Points! If you click on the picture below you will see the melting of solid crystals. The melting point for this compound and any other should really be called a melting range, since the crystals will never melt instantaneously. When recording a melting point, the first temperature noted is the temperature at which you see the first drop of liquid. The second temperature listed in a melting point is the temperature in where you see the last crystal melt into liquid. For example, following the slow technique and the MelTemp setting described above the crystal in the movie on the left melted between 182-184 oC.