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jackchem2001
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Registered: 2-6-2024
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I have not done this hazardous procedure so these are just my speculations on some of the posts in this thread:
I do not think that using a quartz boiler and having the rest of the setup be borosillicate is advisable - the difference in thermal shock resistances
of these glasses is due to the different coefficients of thermal expansion. I think it would be dangerous to have a male borosillicate joint inserted
into a female quartz joint at high temperature.
I don't think teflon stir bars/tape is advisable either. The operating temperature of teflon is 260 degrees, and its decomposition occurs around 350
degrees. I think ordinary stir bars would also demagnetize at the high temperature.
As for clips, I have found plastic clips to be a bit useless in general and they only have an operating temperature of around 140 degrees. I would use
metal. They will survive just fine in a well designed setup. Sulfuric acid is a perfectly sufficient joint grease and will probably wick its way into
the joints during the distillation.
Finally, if you plan to use foil to create an air bath around the flask, be absolutely certain to avoid direct foil-glass contact. I had a flask crack
like this during cooling - https://www.sciencemadness.org/whisper/viewthread.php?tid=61...
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macckone
Dispenser of practical lab wisdom
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Location: Over a mile high
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It seems most have not done this procedure.
I use a 1L quartz RBF with a quartz air condenser (one piece L type 75 degree).
I had the condenser custom made so it wasn't cheap.
If you mix borosilicate and quartz you need to be careful of the differing expansion rates.
I use steel keck clips. No sealant as there is only one joint with sulfuric acid vapor.
Teflon tape can become glue at these temperatures but the joint can be cooler than the vapor so mileage may vary.
Instead of a stir bar, use a piece of steel wire sealed in a glass tube.
You can magnetize it if you want. But most hot plate stirrers don't go hot enough.
Too many boiling chips act as an insulator. loose glass thread can work. I use regular glass chips but not a lot. My setup doesn't accommodate it
but an air bubbler would be ideal.
Expect bumping. Plan for it. Mixing a sodium carbonate with the sand and then a top layer of just sand in a steel pan helps prevent disasters. To
understand why sulfuric acid bumps so badly, the temperature that 96% sulfuric acid boils at is higher than 98% due to boiling point elevation and the
surface loses water faster than the bottom. So you have a layer of higher boiling liquid under a layer of lower boiling liquid just like oil and
water only the layers in this case can mix if stirred.
One trick to avoid full scale boiling of sulfuric acid is to use a beaker with a fiberglass sheet over it. This won't remove metal contaminants
(usually not relevant for many reactions) but organics will be removed. Heating it to 200C and bubbling air through it will destroy any organics that
don't boil off. water will come off above 290C but it is slow. Think evaporation rather than boiling.
The primary reason to purify sulfuric acid by distillation is to remove iron and lead contaminants. Those tend to occur due to process equipment.
Removing organics is easier, see above. Removing the water doesn't require distillation. It comes off before the sulfuric acid via evaporation. You
have to get it much hotter to boil off acid water mix, see above about bumping.
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