Sciencemadness Discussion Board

Bromine contamination in Hydrobromic Acid

sbreheny - 19-3-2014 at 19:47

Hi all,

I recently obtained a sealed bottle of hydrobromic acid (48%). I was surprised to find, upon opening it, that it was colored brown. If you draw some into a pipette, you can see through it, but if you try to look through the entire bottle of it, it is opaque.

After some Google searching and thinking, I came to the conclusion it the contamination must be elemental Bromine. I read somewhere that HBr can break down in long term storage. Can anyone here confirm this?

Can anyone recommend a way to remove the Bromine contamination from the acid? So far, the only way I can think of doing it without contaminating the acid with something else would be to dehydrate the acid with a drying agent to liberate HBr gas and then pass that gas through something that would react with the Br but not HBr, and the finally bubble the gas through water to get the acid again - but that sounds very dangerous.

Ideas?

Thanks,

Sean

thesmug - 19-3-2014 at 21:21

Yes, HBr does decompose (much like H2O2). From my understanding of it, HBr is always yellowish to brown in color. If you really need to remove all bromine you would need to de-brominate it right before you actually need it as it decomposes somewhat quickly. Purely hypothetical, but you could try to brominate some methylbenzene (toluene) with UV light. This would make benzyl bromide, which is insoluble in water (and therefore in HBr).

AJKOER - 20-3-2014 at 06:45

First before anything else, take a sample and add a little water and a touch of Sulfur. References, per Wiki on Bromine, to quote:

"The acid may be prepared by several other methods, as well, including reaction of bromine either with phosphorus and water, or with sulfur and water:[11]

2 Br2 + S + 2 H2O → 4 HBr + SO2 "

No comment on the practicality of the following method (see http://www.ucc.ie/academic/chem/dolchem/html/elem035.html ) based on Bromine's similarity to Chlorine:

Br2 + H2O = HBr + HBrO

2 HBrO + uv → 2 HBr + O2 (g)

Note, upon heating HBrO to 60 degrees per Mellor, page 294, link: http://books.google.com/books?id=1iQ7AQAAMAAJ&pg=PA294&a... forms Bromine and water.

Also, you may create some HBrO3 as well:

3 HBrO + uv → 2 HBr + HBrO3

[Edited on 20-3-2014 by AJKOER]

Metacelsus - 20-3-2014 at 08:05

You wouldn't need to add water, as there is already plenty. Adding sulfur is actually a pretty good idea.

sbreheny - 20-3-2014 at 09:14

Quote: Originally posted by Cheddite Cheese  
You wouldn't need to add water, as there is already plenty. Adding sulfur is actually a pretty good idea.


Thanks for all the replies. I will try adding sulfur to a small sample and see if the brown color goes away. Just to be clear, though, I don't think that is really a suitable method for removing Bromine from the whole batch because it would leave SO2 dissolved in the acid, right?

sbreheny - 20-3-2014 at 09:16

Quote: Originally posted by thesmug  
Yes, HBr does decompose (much like H2O2). From my understanding of it, HBr is always yellowish to brown in color. If you really need to remove all bromine you would need to de-brominate it right before you actually need it as it decomposes somewhat quickly. Purely hypothetical, but you could try to brominate some methylbenzene (toluene) with UV light. This would make benzyl bromide, which is insoluble in water (and therefore in HBr).


When HBr breaks down like this, does it release H2 gas?

thesmug - 20-3-2014 at 13:25

Quote: Originally posted by sbreheny  


When HBr breaks down like this, does it release H2 gas?


Yes, but it will come out as H+ as well so venting the bottle shouldn't be necessary.

DraconicAcid - 20-3-2014 at 13:29

Quote: Originally posted by sbreheny  
Quote: Originally posted by thesmug  
Yes, HBr does decompose (much like H2O2). From my understanding of it, HBr is always yellowish to brown in color. If you really need to remove all bromine you would need to de-brominate it right before you actually need it as it decomposes somewhat quickly. Purely hypothetical, but you could try to brominate some methylbenzene (toluene) with UV light. This would make benzyl bromide, which is insoluble in water (and therefore in HBr).


When HBr breaks down like this, does it release H2 gas?


I suspect it's more likely to be 4 HBr + O2 -> 2 Br2 + 2 H2O than 2 HBr -> H2 + Br2.

sbreheny - 20-3-2014 at 18:52

Well, adding sulfur to a sample of the contaminated HBr did not seem do to anything, either at room temp or with gentle heating to maybe 70C. In fact, the sulfur did not dissolve or seem to react in any way. I tried to find out more about the Br2 + S + H2O reaction but couldn't other than in the wiki article, so I don't know whether heating or some catalyst are needed, or maybe if the very low pH of the HBr is a hindrance.

Ideas?

Töilet Plünger - 20-3-2014 at 21:37

I think the tribromide ion (Br3-) is forming as the bromide comproportionates with the bromine. I don't know if this is stable though.

sbreheny - 22-3-2014 at 14:55

Does anyone know if I could separate the Br2 from the HBr+H2O by mixing with some kind of solvent which dissolves Br2 but not H2O or HBr? If so, can you recommend a solvent?

gdflp - 22-3-2014 at 15:13

Mixing it with a solvent is unlikely to work. Instead you could heat the mixture to boil off the bromine, bromine has a boiling point of 59C whereas hydrobromic acid's boiling point is about 110C.

sbreheny - 23-3-2014 at 17:10

Thanks. I think I will try distillation but I will try a small amount first and also I am going to wait until I have my fume hood set up.