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Harmless
Posts: 4
Registered: 12-9-2016
Location: Cornwall
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Collecting a volatile product
I found a 15 year old fume cupboard on eBay last month which increases my options. It vents back into the room through copious filters and protects
from flying glass - it's the last bit I've always felt more useful in practice. It has room for a heating/stirring mantle, a thermometer head,
horizontal 40cm Liebig condenser and receiving flask. There's not head-height for including a 40cm fractionating column.
I left the fan off, keeping it as a fallback while I checked I could chemically trap something smelly, and it turns out I could.
Procedure: 20g BCDMH (a Bromine tablet for an outdoor spa bath, 1-Bromo-3-chloro-5,5-dimethylhydantoin) simmered in 30% sulphuric acid until all the
halogen is driven off. The Bromine condenses to an extent into a round-bottomed flask immersed in ice. A rubber tube carried any uncondensed Bromine
(and the evolved Chlorine) to the bottom of a beaker of Sodium Thiosulphate crystals which neutralised it all - there was no halogen smell from start
to finish which was partly what I wanted to find out.
Result: The theoretical yield is 2.13cc of Bromine which in practice turned out to be just a few drops. I had thought the ice bath was enough to
condense the vapour but no, it isn't, the equipment retained the brown vapour colouring throughout and that's where most of the yield stayed. The
reaction flask contents were white but the product didn't chill. No Chlorine escaped past the neutralizing trap.
If I'd wanted a bottle of Bromine I'd have started with Potassium Bromide, but I don't want one. If I try again I'll add salt to the ice bath to get
it colder, and finish by flushing the remaining vapour through the system on a stream of air.
Question: If I do actually manage to get the product down to 0°C, will the vapour adequately condense? I have my doubts. How can I improve the
collection?
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Praxichys
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Your yield will be higher if you scale the process up. Bromine vapor at STP has a mass of 7.14g/L, and this is likely where most of your bromine
remains. Unless you can cool that entire volume, you will never get all the bromine to condense.
I recommend scaling up maybe to 10 or 15 grams of bromine yield and using a spiral condenser with ice/salt circulating through at approx 0°C. Keep
the receiving flask submerged in a similar cold bath and you should end up with at least 8g bromine, or around 2.5ml. Even this much won't last long
at room temperature though.
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MrHomeScientist
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To add to Praxichys' comment, the only way to permanently store bromine once you've made it is in sealed glass ampoules. It will escape nearly
everything else given time.
Also, I'm not too fond of recirculating fume hoods that vent back into the room. You have to make sure you have the correct filters for the material
you are working with, and these must be changed out regularly to keep them effective (and there's no way to tell they aren't effective until dangerous
vapors have already escaped into the room). I like your setup to trap the bromine vapors, though - it's good to plan ahead like that and make the hood
a measure of last resort.
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Deathunter88
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Quote: Originally posted by MrHomeScientist  | To add to Praxichys' comment, the only way to permanently store bromine once you've made it is in sealed glass ampoules. It will escape nearly
everything else given time.
Also, I'm not too fond of recirculating fume hoods that vent back into the room. You have to make sure you have the correct filters for the material
you are working with, and these must be changed out regularly to keep them effective (and there's no way to tell they aren't effective until dangerous
vapors have already escaped into the room). I like your setup to trap the bromine vapors, though - it's good to plan ahead like that and make the hood
a measure of last resort. |
I also agree with having a fume hood acting as a last resort. When I just started chemistry one of my first reactions was making bromine. However, I
did not have enough practice with using apparatus and thus made a major mistake. The stopper in the pressure equalising addition funnel with H2O2 fell
out letting 250ml of 30% H2O2 enter directly into the half liter of heated H2SO4 mixed with KBr... The resulting plumes of Br2 blew out the stopper in
the addition funnel and also popped out the collection flask, cause a spill of 25ml of collected bromine. The fume hood really saved me but it
couldn't handle all of the dense fumes and my lab smelled like a swimming pool for the next day.
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spot
Harmless
Posts: 4
Registered: 12-9-2016
Location: Cornwall
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Thank you for the comments. I've now put the fume cupboard on a wheeled bench so if I anticipate fumes I can move it out into the yard before
starting. I also contemplate bolting a water tank and small aquarium pump onto the trolley to circulate ice water through any condenser I may be
using. For indoor use the fume cupboard will keep flying debris - boiling reactants, disintegrating glass vessels - out of my hair while I run from
the room as I presume Deathhunter did. I have a nagging suspicion that making a bit of bromine is high on every beginner's list.
I shall now attempt to grow long thin crystals of iodoform which I undoubtedly did at school because I can remember the smell. Iodoform is not
volatile.
Staying roughly on topic, if I make a little bromoform for comparison is it really as nasty as Wikipedia makes it sound? I'd not have thought it would
be volatile at all and I wasn't planning on touching it. I was thinking I might try crystallizing some without solvent in the fridge to compare with
the iodoform.
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