nimgoldman
Hazard to Others
Posts: 303
Registered: 11-6-2018
Member Is Offline
|
|
Neutralizing HBr reaction mixture
I have attempted synthesis of hydrobromic acid by distilling aqueous mixture of sodium bromide and sodium bisulfate (procedure taken from NurdRage's
YT video).
Basically the mixture is chilled (to precipitate sodium sulfate) and filtered, then the dilute hydrobromic acid is distilled off from the solids. It
is possible to cool stop distillation and re-filter new precipitate - since there is a lot of sodium sulfate produced.
Unfortunately, it is hard for me to distill it to dryness even with heating mantle (NurdRage used hotplate and flat bottom flask so I will try it
too). I am left with residual sodium sulfate mixed with somewhat concentrated HBr and what's worse, Bromine (it is generated by heat decomposition by
the end of the distillation).
Could this residue be neutralized with sodium thiosulfate solution? AFAIK, the sodium thiosulfate should react with free bromine forming sodium
bromide and maybe also with the hydrobromic acid (much like it reacts with hydrochloric acid, only replacing Cl ion with Br ion):
Na2S2O3 + 2HCl (now 2HBr) -> 2NaCl (now NaBr) + S + SO2 + H2O
The reaction with sodium thiosulfate produces gas which I believe is the SO2 and SO4 (from the reaction with bromine).
Could these gases be further neutralized using wash bottle with NaOH solution?
The hydrobromic acid preparation and cleanup is done with good ventilation (later I will use a fume hood), gas mask attached and I am also thinking of
attaching wash bottle or inverted funnel to the distillation adapter to catch any HBr and bromine vapours released (there are fumes only by the very
end of the distillation).
So the proposed safety procedure would be:
- attach wash bottle with NaOH or sodium thiosulfate solution to the distillation apparatus
- attach addition funnel with sodium thiosulfate solution to the boiling flask
- after the end of distillation, slowly open the addition funnel, neutralizing the mixture and cathing toxic/caustic gases (sulfur oxides) in the wash
bottle
Any suggestions are welcome. I am often working with strong and fuming acids but I need HBr this time and it is much cheaper to make it myself so I
just need a safe procedure.
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
Why not neutralize with hydroxide first and then add sodium thiosulfate?
|
|
|
nimgoldman
Hazard to Others
Posts: 303
Registered: 11-6-2018
Member Is Offline
|
|
Yes this makes sense. I would therefore add dilute hydroxide (NaOH), maybe even water to dilute/cool the mixture and dissolve the salt (sodium
sulfate), then neutralize with base (until there is no more bubbling), then remove the bromine with sodium thiosulfate (untril there is no more
discoloration).
I found a nice pressure equalizing addition funnel with two stopcocks so this allows addition without exposing any vapours to the air.
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
Nice. I want one of those....
|
|
|
happyfooddance
National Hazard
Posts: 530
Registered: 9-11-2017
Location: Los Angeles, Ca.
Member Is Offline
Mood: No Mood
|
|
Quote: Originally posted by nimgoldman  | | I would therefore add dilute hydroxide (NaOH), maybe even water to dilute/cool the mixture and dissolve the salt (sodium sulfate), then neutralize
with base (until there is no more bubbling) |
Neutralizing with NaOH won't produce bubbles, unless it is significantly contaminated with carbonate.
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
Yeah, I wouldn't expect a lot of bubbling (that is why you would use NaOH instead of sodium bicarbonate or something). If your fancy addition funnel
has glass stopcocks, don't let NaOH stand in it, or your stopcocks might seize.
|
|
|
UC235
National Hazard
Posts: 565
Registered: 28-12-2014
Member Is Offline
Mood: No Mood
|
|
This prep is from a paper linked on this forum called "preparation of constant-boiling hydrobromic acid" https://pubs.acs.org/doi/abs/10.1021/ed014p187?journalCode=j...
I'm sure we have a PDF somewhere. I did not observe any Br2 formation.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
