2013: Advent Chemistry: Piping hot

by on December 12, 2013
Still don't do this.

Still don’t do this.

I’ve mentioned concepts like making things hot before, but I haven’t gone into detail about how. The rather charming image of a chemist in a dark laboratory, tending to a bunsen flame, while the varnish on the wooden bench gleams in the light, and green acids bubble on the shelf… that rather charming image has certain inaccuracies, chief amongst them that Bunsen burners are not usually the right tool for the job.

Using a flame for heat gives you very directed, high temperatures. It can be ideal for melting crystals, but it’s not great for refluxing a mixture, or gently heating something to recrystallise it. For that, you want steady, even heat across the whole base of the flask.

The primary means of heating I used in my degree, then, were the water bath and the stirrer-hotplate.

A water bath is an electrically heated container of water, with a partially closed top, like a lidded saucepan with a hole in the middle of the lid. You get the water bath up to temperature, so that it steams, and you rest your flask over the hole in the top. The steam condenses on the base of the flask and heats it up in the process.

This works quite well, but you can’t heat anything above the boiling point of water – the flask will go as hot as the steam is, and no hotter. To reach higher temperatures, even quite modestly higher ones like 120C, you need to use an oil bath.

The oil baths I used were simple straight-sided bowls of oil, with a paperclip in it, sat on a heated metal plate. The paperclip is there to stir the oil and prevent convection from forming hot spots on the flask, which is suspended partly submerged in the oil, held up by a clamp stand so it doesn’t rest on the bottom.

No, this isn’t a little-known magical property of paperclips. The paperclip does not do the stirring all by itself. It is constantly spun around by a magnet, which is attached to a metal arm underneath the heating element, and revolves at a steady speed. You can probably get fancy digital ones with precise rate options, but in the undergrad lab we had two knobs on the front of the stirrer-hotplate – one for temperature and one for speed.

Paperclips are fine for oil baths, but for stirring your reaction mixture, you use a little plastic-coated iron rod, which goes by the charming name of a magnetic stirring flea, and fish it out at the end with a magnetic wand – meaning a magnet on a stick, also plastic-coated. The plastic stops the fleas reacting with your mixture and makes them easier to clean between uses.

You can see how refluxing can get very dull when the stirrer-hotplate is doing all the work of keeping it going.

Leave a Reply