Protium
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dry HBr?
48% HBr is fairly easy to make from H2SO4 and NaBr , but dry HBr seems to bee
rather difficult to acquire.
Does anyone think it be possible to take 48% HBr and drip it onto anhydrous MgSO4 or CaCl2 pellets to release dry HBr gas?
It just depends on how you look at it...
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BromicAcid
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Indeed it is somewhat more difficult to dry HBr due to the reactivity with H2SO4. 48% HBr still contains such a large quantity of water, it would
take a lot of drying agent to get out a majority of the HBr in the solution, a saturated solution is 69% HBr so it would have to be dehydrated to near
that point to even produce HBr significantly. Concentrated phosphoric acid, although syrupy, does a good job of taking the water out of many things
and is even less prone to oxidizing things then sulfuric acid and is just as widely avalible for removing rust so could probably be substitued.
Something like P2O5 would work wonders, but that would be a waste, easy to come by dessicants like MgSO4 and CaCl2 like you mentioned would work, no
clue on how much MgSO4 or CaCl2 would be needed though to free all the HBr, probably lots.
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Protium
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Bromic,
What you're saying is that at low temperatures one could use dessicant to produce 69% HBr from 48% HBr, then merely filter/decant off dessicant??
It just depends on how you look at it...
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BromicAcid
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I'm saying that if something were added like a dessicant to remove the water HBr would not come over significantly till the solution would be
equivelent to 69% HBr so there is a lot of water to tie up before any HBr gas would significantly come off, if concentrated HBr were your goal by
removing water from the solution, there are not too many things that would be insoluble in HBr soltuions that would also tie up the water, molecular
sieves and silica gel come to mind though for that particular application.
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verode
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the best way to get dry HBr is with Br2+tetraline
(also Br2+red P)
Br2 + Fe+benzene etc
It's almost impossible to dry HBr (aq)
Because the H2SO4 reacts to give Br2
and the P2O5 need a lot (52 % water)
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Natures Natrium
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If you've got NaBr and high concentration H2SO4 (>70%), then (IMHO) the best way is to generate your HBr is in situ similiar to some of the
setups seen for HCl generating apparatus. Essentially just drip your conc. sulfuric acid on a large excess of NaBr, lead the gas up through a
condenser or other glass path which will give some of the excess water time to cool and condense out, and then lead it through a good quantity of dry
CaCl2 or MgSO4. I wouldn't actually run water through the condenser or anything, as it's just not necessary. Obviously after the dessicant
you can lead it into whatever reaction you are using it for.
Although I say this mainly to remind myself as I often forget, if you are leading the HBr into a liquid mixture don't forget a suckback bottle in
between the reaction vessel and the dessicant. It only takes an extra minute to setup, and can be the difference between success and disaster.
\"The man who does not read good books has no advantage over the man who cannot read them.\" - Mark Twain (1835-1910)
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BromicAcid
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Verode, making HBr through the reaction of Br2 with organics is a good way indeed to make HBr if you have bromine to spare (considering in the best
cases half of it is taken up by the bromination). But if you had read the 4 posts previous to yours you would have seen that I had indeed mentioned
that H2SO4 can oxidize HBr (also take note of this Natures Natrium) and also note that I too mentioned that it would take lots of P2O5 and it would be
a waste in my opinion.
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unionised
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IIRC the 48% is the azeotrope with water. Azeotrope compositions vary with pressure so vacuum distillation would work one way or the other. (This
might be tough on the pump unless you had a good trap.)
Adding something else, perhaps NaBr, would also perturb the azeotrope and let you get a higher concentration of HBr.
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alchemie
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If you mix MgSO4 or CaCl2 to dry 48% aqueous HBr the HBr will react and form H2SO4 or HCl.
Even if it works in the first case it should give a mixture of HBr and Br2, and in the second a mixture of HBr and HCl.
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BromicAcid
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Alchemie, it takes very concentrated H2SO4 to oxidize bromine. If you add magnesium sulfate to hydrobromic acid and it is concentrated enough, the
magnesium sulfate will absorb water and make the relative amount of water avalible to the system less, and though there will be an equilibrium,
magnesium bromide is also a powerful dessicant and will also work to remove water from the system. If the solution was 68% HBr to begin with I have no
doubts that adding even a weakly drying salt like MgSO4 or CaCl2 is going to help to remove water from the system and increase the saturation of HBr
to the point where the system may loose some (though in the case of CaCl2 some HCl would probably come off too).
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BromicAcid
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I've always known that adding bromine to a solution of formic acid results in the oxidation of the formic acid and the production of a solution
of HBr.
Br2(l) + HCOOH(aq) ---> 2HBr(aq) + CO2(g)
Possibly this could be applied to anhydrous formic acid to get total recovery of HBr?
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trilobite
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A way of preparing anhydrous HBr is described here:
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0338
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