SHADYCHASE54
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looking for a route to aluminium tribromide
Hello all,
I was just wondering if anyone was aware of a relatively simple route to AlBr3 without the direct use of bromine. I like to avoid using bromine
whenever possible not to mention I don't even have any at present. I don't require a product of absolute purity, but of course the purer the better
although for my use it doesn't need to be 99.99999% Any ideas are appreciated so thank you in advance.
SHADYCHASE54
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woelen
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It strongly depends on how you want your AlBr3. The hydrated salt (AlBr3.6H2O) is much more easy to prepare without bromine than anhydrous AlBr3.
Making the hydrated salt can be done by adding solid KBr (or NaBr) to 85% phosphoric acid and gently heating the acid. Pure, albeit slightly humid,
HBr will come over and this can be dissolved in water, in which filings of aluminium metal are placed. The metal-acid reaction produces hydrated
AlBr3, dissolved in water.
You can also make aqueous HBr by mixing 40% H2SO4 (not more concentrated, otherwise you'll get contaminination with Br2 and SO2, which will lead to
formation of H2SO4 in your end product acid) with a hot saturated solution of NaBr or KBr. Allow the liquid to stand for a while. Crystals of the
sulfate and bisulfate separate from the liquid. Decant the liquid and distill this liquid. You'll get aqueous HBr of 30 to 40% concentration. Dissolve
the Al in this.
Making anhydrous AlBr3 can be done by leading carefully dried HBr over red hot Al-powder in a quartz tube. This requires a furnace and well sealed
quartz tubing. Not something for the beginner.
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Jor
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Woelen, you can use conc. H2SO4  . I prepared conc. colorless HBr by a procedure
described in Vogel. I dissolved 36g of KBr in 60mL of water and added 27mL of conc. H2SO4 very slowly, such that the temperature does not rise above
60C.
After all is added, cool the solution, filter the KHSO4. Now you have a crude HBr solution. I have made this multiple times. I used it once for making
HBr, so I distilled it twice, and all the other times for making Br2 by adding KBrO3.
Even when you just get a small amount of red color, you should immediately swirl and the Br2 will disappear again, by rxn with the SO2.
When you want really pure HBr, distill the HBr over a little BaBr2.
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Picric-A
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Quote: Originally posted by Jor  | Woelen, you can use conc. H2SO4 . I prepared conc. colorless HBr by a procedure
described in Vogel. I dissolved 36g of KBr in 60mL of water and added 27mL of conc. H2SO4 very slowly, such that the temperature does not rise above
60C.
After all is added, cool the solution, filter the KHSO4. Now you have a crude HBr solution. I have made this multiple times. I used it once for making
HBr, so I distilled it twice, and all the other times for making Br2 by adding KBrO3.
Even when you just get a small amount of red color, you should immediately swirl and the Br2 will disappear again, by rxn with the SO2.
When you want really pure HBr, distill the HBr over a little BaBr2. |
Your not using conc. H2SO4 here, the 60ml of water you use to disslve the KBr is diluting the sulphuric acid, reducing its oxidising capability.
If one were to add conc. H2SO4 to solid KBr then you would get bromine produced, woeelen is right!
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DJF90
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If you bothered to read what Woelen wrote you will see he also adds the acid to a (hot) saturated solution of potassium bromide. I agree with Jor in
that concentrated acid can be used, but the reaction mixture must be kept cool (<50-60*C ideally) or an orange discolouration (due to free bromine
will form). A magnetic stirrer greatly helps by ensuring heat dissipation from the local area where the concentrated sulfuric acid is added. HBr
offgas is a problem when no magnetic stirring is used, so you may want to do this in a "closed system" or outside. The other point I'd like to make is
to allow ample time for the precipitation of the sulfate/bisulfate that forms - failure to do so before distillation means you still have significant
sulfuric acid (either free or as bisulfate) in your reaction mixture and upon distillation the heat causes discolouration due to free bromine being
formed. This discolouration won't pass over, but it will reduce your yields as I suspect the bromine complexes to the bromide forming Br3(-) ion. A
decent suggestion I've seen on Versuchschemie is to add a small amount of red phosphorus to the stillpot - this reduces the bromine using water to
form more HBr.
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Picric-A
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| Quote: Originally posted by DJF90 | | If you bothered to read what Woelen wrote you will see he also adds the acid to a (hot) saturated solution of potassium bromide. I agree with Jor in
that concentrated acid can be used, but the reaction mixture must be kept cool (<50-60*C ideally) or an orange discolouration (due to free bromine
will form). A magnetic stirrer greatly helps by ensuring heat dissipation from the local area where the concentrated sulfuric acid is added. HBr
offgas is a problem when no magnetic stirring is used, so you may want to do this in a "closed system" or outside. The other point I'd like to make is
to allow ample time for the precipitation of the sulfate/bisulfate that forms - failure to do so before distillation means you still have significant
sulfuric acid (either free or as bisulfate) in your reaction mixture and upon distillation the heat causes discolouration due to free bromine being
formed. This discolouration won't pass over, but it will reduce your yields as I suspect the bromine complexes to the bromide forming Br3(-) ion. A
decent suggestion I've seen on Versuchschemie is to add a small amount of red phosphorus to the stillpot - this reduces the bromine using water to
form more HBr. |
Calm down... i did read what Woelen wrote, i just mis-interpreted it.
If you dont have acces to conc. sulphuric acid you could try heating anhydrous KBr with anhydrous NaHSO4. This doesnt produce nearly as much bromine
as c.H2SO4 and it is amost dry enouh to be used in the manufacture on anhydrous AlBr3.
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