NaK
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Can you make bromine in the separatory funnel?
Storing bromine is not a very good idea, so i wonder if it is possible to very quickly synthesize small amounts of bromine just in a separatory
funnel.
Basically
- make HBr by adding sulfuric acid to potassium bromide solution
- add the ice-cold solution to a separatory funnel
- add h2o2
- separate bromine layer
- dry bromine with sulfuric acid
Why is it not usually done this way? Is there a fundamental problem I haven't thought of?
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Boffis
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Yes, this is my standard method if I don't need dry bromine, e.g. if I am working in an aqueous environment. I use sodium bromide, sodium bromate and
diluted sulphuric acid. The bromine sinks to the bottom and can then but run off directly into the reaction for which it is intended. If you want to
you can then distill the sodium bisulphate solution to recover the last bit of bromine but I personally don't think its worth the effort.
If I need dry bromine then I usually distill it from excess sulphuric acid from a flask directly into a chilled receiver rather then use a dropping
funnel but you could just run off the bromine into another funnel and add some sulphuric acid to dry it.
I am sure you could use other cocktails to generate the bromine. I find the benefit of this method is the minimum of exposure required of the bromine
to the air (and hence me). The disadvantage is the volume of bromine per unit of solution is about 1:25 so that in my equipment I can only make about
20ml max (about 60g). The strength of the acid is important, too strong and sodium salts ppt about 30% seem optimal. The reaction is fairly exothermic
but you can't chill the sodium salt solutions as this greatly reduces solubility so you need to make up the dilute acid and then chill it in the
freezer beforehand.
Your mixture may be better than mine if it generates less sodium hydrogen sulphate. Hydrobromic acid free of sodium salts would be better still.
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pip
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Most people don’t like to make bromine as they need it, they prefer to practice sealing ampules either commercial ones or homemade ones from test
tubes. I prefer to make a lot at once and ampule it. Only because hobbiests are already limited on reagents and it’s one less procedure to go
through when you have to make a bunch of chemicals you haven’t made before.
I’m a big fan of making a lifetime supply of something if possible (and safe) or at least as much as I think will last me a year or two. I bought
2ml and 5ml ampules from eBay cheap, makes it convenient to have 3-10g at my disposal without spending a few hours to do it. Also wastes less
chemicals due to glassware losses.
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NaK
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Quote: Originally posted by pip  | | I’m a big fan of making a lifetime supply of something if possible (and safe) or at least as much as I think will last me a year or two. I bought
2ml and 5ml ampules from eBay cheap, makes it convenient to have 3-10g at my disposal without spending a few hours to do it. Also wastes less
chemicals due to glassware losses. |
Yeah that's the common practice but I'm just very uncomfortable with having bromine around. It may be slightly irrational but a broken vial of bromine
is one of the worst things I can think of. So for the few times I need bromine I will make it as needed
[Edited on 5-2-2021 by NaK]
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clearly_not_atara
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I kind of imagine the real trick here is to use freezing-point depression so that you can solidify the bromine and just pour off the solution. For
this one excellent candidate salt is potassium acetate, soluble roughly 200% w/v in water (that's not a typo). So roughly:
6 HOAc + 5 KBr + KBrO3 >> 6 KOAc + 3 H2O + 3 Br2
But you should use a 1.1-1.5x molar excess of acetic acid to ensure that the solution does not become too alkaline. Bromine freezes at about -7 C.
Potassium acetate should be dissolved at roughly 50% w/v so the heat of reaction will be a significant factor (some additional KOAc might be added so
the reaction can be run more dilute). To my knowledge, bromine does not attack acetic acid under these conditions.
But I haven't got a setup capable of working with bromine right now anyway  All
work and no play makes Jack a dull boy...
[Edited on 5-2-2021 by clearly_not_atara]
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draculic acid69
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| Quote: Originally posted by NaK | | Quote: Originally posted by pip | | I’m a big fan of making a lifetime supply of something if possible (and safe) or at least as much as I think will last me a year or two. I bought
2ml and 5ml ampules from eBay cheap, makes it convenient to have 3-10g at my disposal without spending a few hours to do it. Also wastes less
chemicals due to glassware losses. |
Yeah that's the common practice but I'm just very uncomfortable with having bromine around. It may be slightly irrational but a broken vial of bromine
is one of the worst things I can think of. So for the few times I need bromine I will make it as needed
[Edited on 5-2-2021 by NaK] |
It's not irrational at all, bromine is an untame-able monster
The horror potential is right up there.one little mistake
could be catastrophic to equipment, health, property or all 3
[Edited on 5-2-2021 by draculic acid69]
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njl
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Similar idea, would an electrobromine cell in an addition funnel be advantageous? Anode at the bottom/cathode at the top, bromine forms and pools at
the bottom, drain and repeat. What electrodes would work here?
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Fyndium
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Seal bromine in glass ampoules, place the ampoules in dual PE ziploc bag and freeze them. Now you have three(four) layers of protection against
catastrophic failure.
Other than that, the funnel idea sounds quite convenient, as you will need to do preparations for a synthesis run anyway, adding an extra funnel
shouldn't be an issue.
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Metallophile
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What is the best way to dry bromine? I tried CaCl2, but it seemed to break apart and contaminate the Br with little bits of salt. Would 3A molecular
sieves work? Or would they absorb the bromine too? Maybe drain cleaner grade H2SO4?
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njl
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Shaking with H2SO4 is common
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draculic acid69
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| Quote: Originally posted by Metallophile | What is the best way to dry bromine? I tried CaCl2, but it seemed to break apart and contaminate the Br with little bits of salt. Would 3A molecular
sieves work? Or would they absorb the bromine too? Maybe drain cleaner grade H2SO4?
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Sulfuric is the only way
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NaK
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Tried it with hydrogen peroxide. I cannot recommend it at all! It can't be run cold because the reaction speed is too low. But when it goes up to room
temperature thermal runaway immediately starts and all the bromine escapes. I wi just go for distillation next time
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