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How do you tell when a liquid boils, if you only have 10 μL?

This isn’t a subject I think about often, largely because I assumed the analysis would require complex equipment and most of an afternoon. Flipping through “Microscale Techniques for the Organic Laboratory” last week [1], I found an elegant work-around. All that’s needed is a standard melting point apparatus, a melting point tube, and a small bit of glass.  The limit of detection seems to be around 5 μL, though I tested the setup at about twice that.

The trick is to drop a tiny, inverted tube into the melting point tube, before adding the liquid of interest. The trapped pocket of air acts as a micro boiling stone, and as the liquid boils a steady stream of bubbles is evolved. The boiling point is thus the temperature at which sustained bubble evolution is observed [2].

Art is not my forte.  I have other talents.-Art is not my forte.

Most labs probably don’t have the necessary glass inserts handy, but they’re easy to make. Simply hold the end of a glass spotter (made from a Pasteur pipette) at a 15º angle, with the last 2-3 cm resting just above the flame of a Bunsen burner. As the glass heats it will bend, until the tip of the spotter sits in the flame [3].

Melting the glass cools the flame, creating a fiery corona.

Boiling Point - Glass MeltedHeat the spotter for five to ten seconds — until the glass melts into a teardrop — then shut off the flame. Once the glass is cool break off the insert (set the tip of a spatula about 1.5 cm from the end, then bend the open end of the spotter upwards). The final insert should be about 1 cm long, with one end melted into a (relatively) heavy glass blob.  Longer inserts are also viable, and may hold in place better.

Drop the insert into a melting point tube, then tap the tube on a bench until the insert has reached the bottom. Add your compound of interest with a 10 μL Hamilton syringe, and flick the tube to force the liquid to bottom.

From here, the boiling point determination is functionally no different from a standard melting point test. Rapidly heat the sample to ~10-15 °C below the expected boiling point, then climb up at about 2 °C per minute. A stream of bubbles will form at the boiling point (verified with ethyl acetate), and the liquid will condense in the melting point tube about 1.5 cm above the source of heat. If you need another determination flick the tube again to force the liquid down, cool the apparatus, and repeat as necessary. Discolouration or a decreasing boiling point likely indicates decomposition.

Sadly, my camera could not focus on tiny melting point tube.  This was as close as I could get.

Sadly, my camera could not focus on the tiny melting point tube. This was as close to the bubbles as I could get.

 


[1] The book is a good primer on microscale techniques, geared towards an undergraduate curricula. Worth a read if you come across it, but for most probably not useful enough to justify the cost.

[2] Without the air pocket bubbles don’t form. Instead, the level of liquid just smoothly drops.  Here’s the physical chem behind bubble formation.

[3] This setup keeps your hands away from the superheated air.  Fewer burns.

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