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Author: Subject: dry HBr?
Protium
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[*] posted on 4-1-2005 at 20:29
dry HBr?


48% HBr is fairly easy to make from H2SO4 and NaBr:D, 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?




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BromicAcid
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[*] posted on 4-1-2005 at 20:52


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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[*] posted on 4-1-2005 at 20:58


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??




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[*] posted on 4-1-2005 at 21:03


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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[*] posted on 22-3-2005 at 06:37


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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[*] posted on 22-3-2005 at 07:39


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.;)




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[*] posted on 22-3-2005 at 10:02


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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[*] posted on 22-3-2005 at 13:55


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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[*] posted on 24-3-2005 at 15:15


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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[*] posted on 24-3-2005 at 20:33


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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[*] posted on 26-3-2005 at 11:25


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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[*] posted on 28-3-2005 at 18:50


A way of preparing anhydrous HBr is described here:

http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0338
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