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Author: Subject: HBr from NaBr & H2SO4, and disappearing acetic acid!
bjuice
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shocked.gif posted on 9-11-2012 at 22:42
HBr from NaBr & H2SO4, and disappearing acetic acid!


I tried to make a 40% solution of HBr in acetic acid today and ran into some issues. Here was the setup I used:



Basically just used my pressure equalized addition funnel to drip concentrated H2SO4 onto NaBr, and had the gas bubble through the acetic acid, then through Na2CO3/H2O. I'd weighed the acetic acid beforehand so I could keep track of the HBr concentration.

Over the period of about an hour I reacted just about all the NaBr and the AA solution turned from yellow to orange to red. Looked like it had gone well!

But no. I weighed the final solution and it was about 25% LIGHTER than when I started. This means I somehow lost acetic acid.

Where did it go??? I have my doubts that I lost that much due to evaporation. Could there be some reaction going on that would cause me to lose acetic acid as a gas?

[Edited on 10-11-2012 by bjuice]

[Edited on 10-11-2012 by bjuice]

[Edited on 10-11-2012 by bjuice]
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tetrahedron
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[*] posted on 10-11-2012 at 01:49


you must have been producing some Br2 as well, which would explain the color. did you not notice any brown fumes? and yeah the weight loss is probably due to evaporation.
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bjuice
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[*] posted on 10-11-2012 at 09:56


I'm fairly certain the coloration is normal and actually a good sign for a successful gassing. Here is one site that describes the color: http://www.wsgchem.com/cgi/search-en.cgi?f=product_en_1_+new...

Any tips on preventing evaporation? I'd just dilute it once the process is finished, but I need an accurate start/finish weight in order to determine the concentration.

I guess a simple ice bath on the acetic flask would help?
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[*] posted on 10-11-2012 at 10:06


myst32 has developed a procedure here;

http://www.youtube.com/watch?v=JmS4roFTr08

that is fairly easy and seems to work well.




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12AX7
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[*] posted on 10-11-2012 at 18:20


Would the assembly shown not be susceptible to suck-back? With only H2SO4 in the preceeding flask, suckback would be hazardous to say the least!



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bjuice
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[*] posted on 10-11-2012 at 20:08


Quote: Originally posted by 12AX7  
Would the assembly shown not be susceptible to suck-back? With only H2SO4 in the preceeding flask, suckback would be hazardous to say the least!


I didn't have any issues with suckback at all. What exactly would the mechanism behind the suckback be?

Edit: I guess maybe the sulfuric acid flask cooling down and creating a bit of a vacuum?

Quote:
myst32 has developed a procedure here;

http://www.youtube.com/watch?v=JmS4roFTr08

that is fairly easy and seems to work well.


That actually looks pretty nice. Only problem is I need the HBr in acetic acid. Would it be feasible to separate the 48% HBr from the water?

[Edited on 11-11-2012 by bjuice]

[Edited on 11-11-2012 by bjuice]
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mr.crow
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[*] posted on 10-11-2012 at 21:12


Well HBr will boil as a water azeotrope, so anything with water will not work. Try heating NaBr with phosphoric acid. It will not oxidize it to Br2. H2PO4 can etch glass when hot though.

For the bicarbonate wash use an inverted funnel to prevent suckback. Also cool the acetic acid with an ice bath to prevent evaporation and increase solubility.




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bjuice
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[*] posted on 10-11-2012 at 22:12


Hey that's pretty nifty. I saw lots of people talking about H2SO4 oxidizing the HBr into Br2, but everyone seems to have mixed opinions on what extent this happens. I wonder why I never came across the H3PO4 method. Thanks for the tip.

I just broke my small glass funnel :( so I cannot use that for suckback. An empty flask inline would work though, right?
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12AX7
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[*] posted on 10-11-2012 at 22:12


Suckback can occur when the absorbing liquid absorbs the gas rapidly, sucking itself up into the tube carrying said gas. Ammonia absorbing into water is an excellent example: it's easy to generate, and readily soluble, enough that it can create a fair vacuum if pressure is not released.

A partly submerged funnel helps by increasing the absorption area and giving a greater volume for the liquid to suck up into; if placed just barely into the liquid surface, that volume will drop the liquid level until the funnel ends up below the liquid level, causing a bubble to rise and releasing the difference.

I'm assuming HBr into HOAc (diluted or glacial, by the way?) has a similar affinity to HBr in H2O, which is in turn similar to HCl and NH3 into H2O.

Tim

[Edited on 11-11-2012 by 12AX7]




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bjuice
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[*] posted on 10-11-2012 at 22:18


Very interesting thanks for the helpful information.

The acetic acid is glacial.
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[*] posted on 14-11-2012 at 14:33


Quote: Originally posted by bjuice  


I didn't have any issues with suckback at all. What exactly would the mechanism behind the suckback be?

Edit: I guess maybe the sulfuric acid flask cooling down and creating a bit of a vacuum?


I think hes talking about the suckback that occurs when bubbling an extremely soluble gas into a solvent, the gas dissolves so rapidly, it gets displaced from the flask quicker than it can be regenerated resulting in a vacuum in the reaction flask. If no suckback occured, I'm guessing HBr isn't violently soluble in acetic acid.

Quote: Originally posted by bjuice  
I saw lots of people talking about H2SO4 oxidizing the HBr into Br2, but everyone seems to have mixed opinions on what extent this happens.

I've seen it happen at room temperature. To prevent Br2 from being formed, I had to keep the solution cool.

[Edited on 14-11-2012 by mycotheologist]
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