Melting Point Verification of Compound Purity
Introduction:
The melting point of a pure compound is a physical property related to the
compound’s composition and bonding character. The melting point is the
temperature at which the compound changes from a solid to a liquid. Most pure
compounds melt over a defined range that is diagnostic for that compound.
In your previous lab, you synthesized acetylsalicylic acid from its precursor salicylic
acid. One method to check the effectiveness of your synthesis is to measure the
melting point of your product.
The synthesis you performed is an equilibrium reaction, meaning that there should be
some salicylic acid remaining in your product – this is the “impurity”. By comparing
the melting point of your product to the melting points of pure salicylic and
acetylsalicylic acids, you can learn something about the success of your synthesis.
Purpose:
The purpose of this experiment is to determine melting points of reagent grade
acetylsalicylic and salicylic acids in order to evaluate the purity of your synthesized
acetylsalicylic acid product.
Equipment/Material
Mel-Temp Apparatus
Thermometer
Mortar and pestle (optional)
Glass tubing (optional)
Capillary tubes
Pure salicylic acid
Pure acetylsalicylic acid
Student-synthesized acetylsalicylic acid
Procedure:
1. Obtain three capillary melting point tubes and samples of salicylic acid,
acetylsalicylic acid and your synthesized product. Be certain you have
completed all massing of your product for percent yield calculations before
you take any product for this procedure.
2. For each compound, place a small amount on a clean surface (a weigh boat
would work well for these compounds). Tap the open end of the tube into the
compound, invert the tube, and tap the closed end of the tube on the tabletop.
This will deliver the compound to the bottom of the capillary tube. If the
compound does not move into the bottom of the tube, take a piece of glass
tubing and holding it perpendicular to the tabletop, drop the capillary tubes
from a few inches, in the glass tubing, onto the table. The capillary tube will
bounce within the glass tubing and deliver the compound to the bottom.
3. Place all three samples in their respective capillary tubes into the Mel-Temp
Apparatus. Raise the temperature of the melting chamber by adjusting the dial
on the apparatus. Slow incremental change in temperature will allow you to
estimate melting points most accurately, so do not rush this step. You may run
the temperature up to about 90 C at a relatively quick pace, then turn the dial
back and proceed slowly from there. The compounds you are examining melt
within the range of 100C - 200C.
4. Record the melting range of each compound, which begins when the
compound begins to melt and ends when melting is completed.
Data Table:
Compound
Compound
Compound
Melting
range
Melting
range
Melting
range
Expected
range
Expected
range
Expected
range