Molecular Manipulations
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Preventing oxidation of bromide with sulfuric acid
It's quite well know that sulfuric acid will protonate chloride without much oxidation (if any). This is how myself and many people here produce anhydrous HCl gas.
The reaction is:
NaCl + H2SO4 → HCl + NaHSO4
Also twice the molar concentration of NaCl can be used and with strong heating the sodium bisulfate can also protonate the remaining NaCl.
But when it comes to bromide, some of the bromide gets oxidized to bromine. Of course if bromide is getting oxidized, something is getting reduced,
the only possibility I can think of is the sulfur in sulfate. My guess is this reaction is occurring:NaBr +
H2SO4 → .5 Br2 + NaSO3 + H2O.
Is this correct?
Of course this is a equilibrium reaction, which also produces HBr according to an analogous reaction to the one shown with NaCl at the top. Does
anyone where the equilibrium lies and to what extent both reactions take place?
I want to minimize the reaction producing bromine as bromine can easily be produced via oxidation with hydrogen peroxide or similar.
The goal is to find a easy way to make HBr gas from NaBr and sulfuric acid, just like HCl.
Here's another good read, but it still gives no information about the degree of the equilibrium: http://www.chemguide.co.uk/inorganic/group7/halideions.html
[Edited on 16-1-2015 by Molecular Manipulations]
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gdflp
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The equilibrium is very temperature dependent, higher temperatures favor bromine formation. As for the reaction, the sulfur is getting reduced, but
it is getting reduced to SO2, not sulfite. I believe the reaction is as follows 2NaBr + 2H2SO4 --> Br2
+ SO2 + 2H2O + Na2SO4
Phosphoric acid is non-oxidizing and thus it may be a better choice for this reaction.
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Molecular Manipulations
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Thanks for that gdflp,.
Yes, I should have known sulfur dioxide would be produced, as even if sodium sulfite was made first, in the presence of hydronium ions and a
dehydrating agent it would likely decompose into sulfur dioxide and water!
Unfortunately, phosphoric acid is about as available to me as HBr(aq), (I have a few 10's of mLs of both).
So after looking even more I've still found no information about the degree of the equilibrium, based on what you said, I can expect more HBr at lower
temps, which makes sense. If there's not something I can add to the solution to prevent oxidation of bromide, I will have to find a way to either
reduce the remaining aqueous bromine (HOBr/OBr-) with hydrogen or effectively remove it.
I know it seems counter intuitive to add an oxidizing agent, but could this reaction be of any use:?
HOBr (aq) + H2O2 → HBr (aq) + H2O + O2 (g)
There's also the old sunlight/UV that is used for HOCl → HCl, but it seems like it would work even less well for for HOBr...
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gdflp
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Does you HBr need to be anhydrous? If not, diluting the sulfuric acid will also move the equilibrium in favor of HBr formation. If it does need to
be anhydrous, the output gasses can be dried with anhydrous calcium bromide to yield nearly anhydrous HBr.
On a completely different note, if you have access to calcium phosphate, this procedure may be useful for you if you decide to go the phosphoric acid route. If you are in the US, I buy 85% phosphoric acid for $13 a
liter from Duda Diesel, they don't ship chemicals internationally though.
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Molecular Manipulations
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Thanks! I'll be using the HBr to make hydrobromic acid, so no it need not be anhydrous.
Is 70% a good concentration for the sulfuric acid?
I live in the US and bought both my sulfuric acid and phosphoric acid from Dudu (and a host of other chemicals), always a pleasure buying from them.
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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