Question

1. When measuring the melting point of an unknown, why is it necessary to prepare two samples for determination? (Hint: Included in the answer should be the RATE at which the temperature should be allowed to rise when determining an accurate melting point).

Answer 1

Melting Point of an Unknown Tthe melting point range of a pure unknown will be determined and a preliminary identification will be made. Fill two capillary tubes with ~1-2 mm of unknown. Note, the Melting-Temperature do not allow for a single melting point to be determined. Only ranges of melting points are possible when using a Melting-Temperature.

Do not wait for the Melting-Temperature to cool to room temperature before starting this determination. First, perform a rapid and crude melting point analysis to assess the approximate melting point range of the unknown, then repeat using a slower temperature ramp to obtain a more accurate and precise value. This is the reason it is necessary to prepare two samples for determination. Insert one capillary with the unknown into the Melting-Temperature and adjust the voltage control to a heating rate of ~15 ^oC per minute is obtained. As soon as the sample begins to melt, note the temperature and shut off the Melting-Temperature. Allow the Melting-Temperature to cool to at least 10 ^oC below the temperature at which the sample just began to melt. Remove the first sample from Melting-Temperature and insert the second capillary containing the unknown. Adjust the voltage control to a heating rate of ~1 ^oC per minute is obtained. Record the melting point range. From this melting point tentatively identify your unknown. Remember that any soluble impurities will lower the melting point. Therefore, if you did the procedure correctly, you are more likely to be under the actual melting point than over it.