April 13, 2021, 04:22:56 AM
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Topic: It suddenly turned red (Is it Br2?) (preparation of HBr)  (Read 177 times)

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Offline Monoamine

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It suddenly turned red (Is it Br2?) (preparation of HBr)
« on: March 15, 2021, 12:53:05 AM »
Hi, so I tried to make some HBr the other day, using NaBr and H2SO4 as precursors. I measured out 2 molar equivalent of NaBr, and 1 molar equivalent of H2SO4. I then dissolved the NaBr in distilled H22O (maybe a bit too much), and then slowly dripped the sulphuric acid into the solution (to keep the temperature below about 60°C.  :-\

So far so good...

Next I tried to distill off the excess water, which worked fine at first, but after a while the solution being distilled turned cherry red! Its pH was below 0 and the pH of the clear solution in the collection flask had pH about 1.

At this point I stopped the experiment. I tried to pour the red solution into a bottle for storage, but as I was pouring it immediately crashed out of solution (looking a bit like an orange slushy). Adding more water helped it dissolve the slushy again and made its colour more yellow.

Could anybody help me understand what had happened? (As you can tell I still have much to learn...  ???)


My plan for now is this: Distill everything off at about 125°C and collect it (this should be aqueous HBr). The red slushy that will be left over will likely be Na2SO4.
Since Br2 has its uses I wouldn't mind collecting it... its soluble in organic solvents so maybe I can rinse it with ether or something, since ether has a lower boiling point than Br2. That way I could keep some.

Alternatively if people think this isn't a good idea I would also be curious how one can properly get rid of Na2SO4 contaminated with Br2?

Any avice would be greatly appreciated, since I certainly don't just want to flush it down the drain  ;D

Offline chenbeier

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Re: It suddenly turned red (Is it Br2?) (preparation of HBr)
« Reply #1 on: March 15, 2021, 02:50:09 AM »
Using NaBr and sulfuric is not a good idea, because Bromide will oxidised by the sulfuric.This works only with flouride or chloride.
Better is to use PBr3 and water.

Offline Monoamine

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Re: It suddenly turned red (Is it Br2?) (preparation of HBr)
« Reply #2 on: March 15, 2021, 07:51:56 PM »
Thank you for the advice!

Offline Monoamine

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Re: It suddenly turned red (Is it Br2?) (preparation of HBr)
« Reply #3 on: March 15, 2021, 08:13:49 PM »
Quick update: I ended up continuing the distillation at about 120°C. When the liquid was almost distilled off, it reverted back to a yellow and then almost clear colour. At the bottom of the flask remained a solid, which I would assume to be Na2SO4.

So I think I'll just collect all the distillate and redistill it at about 100°C to concentrate the HBr acid.

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