If you’ve ever taken an organic chemistry lab class, you’ve probably done a melting point determination. That’s when you take a small sample of a solid, heat it up, and make note of the temperature at which it melts. This can be used to identify an unknown, but it is often used to assay purity. This is because impurities tend to make solids melt over a range of temperatures rather than at a single point, and because they tend to lower the melting point overall. There are fancy instruments you can buy which will measure melting points, but they’re so simple that I decided to make my own.

One way to do it is to use a Thiele tube, but I didn’t have one of those lying around. So I reached for my volumetric flask, filled it with mineral oil, and set it on a hot plate. Then, I put a tiny bit of the chemical vanillin into a capillary tube; this is my sample to test. I rubber-banded the capillary tube to a thermometer, such that the sample was next to the bulb. I set up a stand and clamped the thermometer in place, suspended in the mineral oil.

This would have worked, except that the samples used are typically so small that they are difficult to see with the naked eye. So I grabbed my USB microscope and clamped it in place, focused on the sample.

do it yerself

With my apparatus assembled, I turned up the heat and sat back to watch. Sure enough, between 80 and 82 degrees C. My copy of the Merck Index actually gives two melting point ranges for this compound, 80-81 and 81-83 degrees, which is a little confusing but seems to confirm that my melting point apparatus works as expected. Sweet!

melting