bjuice - 1-12-2012 at 21:23
Previously made a post regarding making a 40% HBr in Acetic Acid solution by gassing with H2SO4/NaBr. I was having issues with Br2 forming and
decided to try phosphoric acid, which apparently does not oxidize into Br2.
Here is the previous post: https://www.sciencemadness.org/whisper/viewthread.php?tid=22...
I tried to find reference material on H3PO4/NaBr but there doesn't seem to be much information out there. The wikipedia page on H3PO4 states that it
is commonly used in the lab to produce HBr from a bromine salt, and that the reaction requires the addition of heat.
Since that's about all the information I could find I decided to just try it out and see what happens.
Here's the setup: 
I slowly dripped the 85% H3PO4 into NaBr before it was heated because I wasn't sure if gas would evolve right off the bat. Turns out it didn't, and I
probably could have added all the acid at once.
The middle rbf is full of MgSO4 because I assumed there would be water vapor coming through, and I would like the AA to be as dry as possible.
Well, I heated and I heated and I wasn't sure at what temperature the HBr would form, but I assumed it would be lower than H3PO4's boiling point
(153C). Well, I got the solution all the way up to the boiling point and still couldn't discern any formation of HBr. Let it bubble for a bit, even
though it was probably just H3PO4 gas bubbling through, and gave up when the tube coming out of the reaction flask melted.
Right around this time I did notice the solution turning a yellow color, indicating that there probably was some HBr being formed. I'm just confused
about how I'm supposed to produce this gas without having H3PO4 vapor contaminating the end result.
So....any tips? 
elementcollector1 - 1-12-2012 at 22:07
A cooling bath, maybe? Phosphoric acid appears to solidify at ~50 degrees C, while HBr remains a gas.
bjuice - 1-12-2012 at 22:17
That's a great idea.
It makes what I'm trying to do a bit tricky though. The best way I have to know the final HBr concentration is by weighing the acetic acid before and
during/after gassing. This would be impossible with H3PO4 in the mix, and in filtering out solidified H3PO4 I would have mechanical loss of AA
resulting in inaccurate weights.
DJF90 - 2-12-2012 at 16:23
You won't distill the phosphoric acid. It ends up dehydrating and polymerising, yielding meta and polyphosphoric acids. What you'll see boiling is the
water coming off of it.You also give no indication of quantities used in your experiment. When I've made HBr this way (and the other hydrogen halides,
years ago at school) we used crystalline orthophosphoric acid and heated in a test tube connected to a gas delivery tube.
woelen - 2-12-2012 at 23:40
I also made HBr this way, from 85% H3PO4. I first heated the acid to drive off some water and then I added solid NaBr (or KBr, don't remember anymore
which of the two, I don't think it makes a lot of difference). As soon as the solid was added, HBr gas escaped. The reaction mix tends to foam
somewhat. In my case, there was no yellow color, the gas really was free of bromine.