Suppose one distills rubbing alcohol (70% ethanol, a few parts acetone, a bit of MIK, and the rest water), for the purpose of testing a new Snyder
column. (Naturally, one distills it so that it drips directly into methanol, to maintain its denatured nature).
One positions the thermometer - a good quality calibrated USA-made spirit thermometer with 76 mm immersion line - so that the immersion line is just
above the top of the distillation head output tube.
Initially, as the distillate drips over, the temperature reads about 75... 76...77.... 78.2. Either the thermometer is warming up, or a varying mix
of acetone and ethanol is coming over.
Next, distillation steadies at 79.5 to 80.0 (mostly 80) for a long time. The azeotrope of ethanol and water is at 78.1, however. One then raises
the thermometer so that the immersion line is just below the thermometer adapter in the distillation head. The temperature falls to 79. It stays
there until it spikes up at the end.
The thermometer was previously tested so that in a steady distillation of water it read about 100.1 when it was a bit low in the tube, immersion line
below the stopper and just at the output of the distillation head. The barometric correction should be about 0.3 deg downward.
What I think is happening is that it is difficult or impossible to measure the temperature correctly because the thermometer and its stopper
are warming up. The 80 degree phase was actually the 78.1 azeotrope, but the thermometer was positioned too low after it had time to warm up.
Comments?
What is the correct way to position an immersion thermometer in a distillation, and is it possible to get genuinely accurate temperatures with a
spirit thermometer?
Endimion17 - 22-2-2013 at 04:44
Many people use too large hot plates and then don't shield the apparatus from the infrared rays. I use a tiny plate (around 10 cm in diameter) and two
layers of aluminium foil to make a "tent" over the flask, so that it appears as the flask is a hen gathering its chicks underneath its wings. That way
infrared radiation is reduced to a tolerable level, and the plate can be operated at a lower setting because the heated air accumulates under the
foil.
Also, use a Claisen adapter or a Claisen flask if you can, and make the setup like this.
It's far more superior because there's more fractioning surface, and it's farther away from the heat source.
Regular distillation heads (I don't even own one, that's how important they are to me) aren't supposed to be used with liquids where you have to be
careful with the boiling points and all. They're ok for boiling over water or some high boiling point liquids that are pretty pure already, but not
for precise fiddling with low boiling point stuff like organic solvents.
I also use the foil to shield other important parts. Every setup is a little bit different and you have to adapt to it.
Take a photo - maybe the problem is right there and we could see it better.GammaFunction - 22-2-2013 at 11:06
Many people use too large hot plates and then don't shield the apparatus from the infrared rays. ....
This might be the problem. I have a huge hot plate, and I use a metal pie pan with a hole as an air bath, but there still a lot of room for heat flow
from the plate to the apparatus.
As far as using the right head, this was a 6 ball Snyder column extending high up over the plate. The height alone should mitigate the overheating a
bit. But I'll watch the temperature up there the next time I try. Thanks.
