BromicAcid
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Making constant boiling hydrobromic acid, a write-up.
Today, after getting sunburnt and having to pull a decayed skunk out of a well crock at work I decided to make some hydrobromic acid and document my
procedure here seeing as how I miss lab write-ups. My basic procedure was taken from "Inorganic Preparative Chemistry" Volume 1.
H2SO4(l) + KBr(aq) ----> KHSO4(s) + HBr(aq)
Was the supposed equation but I deviated from it immediately. First step, dissolve 120 g KBr in 200 ml H2O. Too bad I didn't have any KBr so I
figured NaBr would work just fine. I adjusted the figures, the inital step called for 1 mol KBr as you can see so I decided 1 mol of NaBr would do
just fine, however my NaBr has sit around for awhile and may have picked up some water, converting some of it to the dihydrate, I decided a comprimise
was in order so I assumed my NaBr was equivelent to a monohydrate which actually gave a figure of about 120 g/mol if it is the 'monohydrate'
so that figure worked out regardless.
So I added my NaBr to a 1 L flask and dropped in a stir bar. Then I ran into the house and got 200 ml of distilled H2O chilled (I drink distilled
water constantly so it's always chilled  ) It dissolved quickly with the aid
of the stir bar and I measured out my 90 ml of concentrated H2SO4 that it called for. But I only had 70 g of my regular and had to move into the
unknown territory. I bought 1 gal H2SO4 from an estate sale for $2 about a year ago, the date on the bottle says it was made in 1972.... ahhhhh....
vintage..... The jug was really heavy so rather then mess pouring that into the small volumetric flask that I was measuring out the acid in I poured
it in a 300 ml beaker, much wider mouth. And.... the acid was BLACK! Whoah was it black, I've never had dyed acid before but this stuff was
pitch black 
Regardless, I had my 90 ml of H2SO4 and I started adding it slowly to my chilled NaBr mix. I felt the side of the container from time to time and it
started to warm up, seeing as how the procedure said "The temperature should not be allowed to rise above 75C to retard the possible formation of
free bromine." I promptly got a tray of iced cubes in water and set the flask in it to keep it cool during the acid addition. I did it in 5 - 10
ml portions over the course of 10 minutes but the mix was still turning red! Stupid bromine evolution, solution must be getting too hot so I tossed
in an ice cube, forgetting purity and measuring imputs for a moment. So I took the temperature, hmmmmm.... 22C seems a little low.... But sure enough
there looked to be dots of bromine on the surface of the liquid skittering around. Eventually all the acid was added and I was left with what looked
like bloody milk. Next step, cool to room temp, check, filter KHSO4 precipitate.... wait a tick... no precipitate!
Well, you work with what you have so I took the mix and put it in a 1 L two neck flat bottomed flask. Stir bar was in there with it, thermometer in
the other neck. Still head had a thermometer, attached to quite a long Lebig condenser, then to a vacuum adapter and finally to a 500 ml round
bottom. The vacuum tube went off to a suck back trap and from there bubbled though a NaOH solution. All the joints were silicone greased. The
heating was begun with stirring at a moderate level.
It slowly heated up but stupid thermometer in the other neck, USELESS it got occluded by vapors from the start and I couldn't read the mercury
though the glass anyway, ahhhh why do I have a thermometer for that hole!!! Nevertheless, it took quite some time to get the vapors to reach the
bottom of the bulb in the still head but temperatures started to rapidly shoot off and leveled out around 90C. This took awhile since I was heating
cautiously. The solution had turned yellow and the 'Bromine' was all over the surface, but no, it can't be bromine, the freaking
vapors are at 90C, no way is that bromine, it must be the dye from the acid. It all came out of the acid and floated on the top, sticking to the
sides and climbing a little bit.
Temperature continued to rise and some distillate started to come over. Slowly, over the course of almost an hour the temp finally got to 120 C, time
to put a different flask on and discard the forerun (actually just put it in a different flask for messing with later, pH < 1 is all I know about
it). The volume for forerun was exactly 52 ml. The book says to start collecting the azeotrope at 121.5 and continue till the temperature drops.
The solution by this time had turned green and that layer was floating all over the top.
The distillate though came over at exactly 122.5 C, the exact temp in the book, and stayed there for over an hour. EXACTLY 122.5 C, it was
unbelievable, the thermometer did not move but a few tenths of a degree Celsius the whole time. The mixture in the distilling flask started to turn
colors again, this time yellow again then just before I turned it off, yellow-red.
Does turning the stirrer up on high increase the distillation rate?
Three hours after deciding I wanted to do this the temperature of the distillate coming over dropped and I turned off the heat. This was the only
time I had suckback. The total volume of distillate was 159 ml, this added up to more then the total amount of water that I put in the mix....
odd....
Actually working that out as the 9M 48% purity that would be 1.43 mols of HBr, which would put me beyond my theoretical yield and way past the books
statement of an average yield of 85%. Possibly I started collecting distillate too early, in addition my NaBr may have been in excess and there was
the ice cube I originally added.
I poured the hot mix left in the distilling flask out and was rewarded with the painful sting of acid gasses in my lungs. Surprisingly they were not
identical to what I expect from HCl. The red ring left around the inside of the distilling flask came out easily with acetone after it dried.
Overall it was a very smooth procedure and left me with what seems to be good HBr, no red color what so ever, have to see what % it really is
though....
[Edited on 6/16/2004 by BromicAcid]
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unionised
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There are a couple of ways that spring to mind for assaying the %HBr, one of them is to measure the boiling point......
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Magpie
lab constructor
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Mood: Chemistry: the subtle science.
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I needed some 48% HBr so after reading this thread and looking up the procedure in Brauer I proceeded. As did BromicAcid I used 1 mole of OTC NaBr
instead of the 1 mole of KBr called out in the procedure. As for Bromic things went smoothly with just a couple of differences.
After reacting the NaBr solution with sulfuric acid I left the beaker in the ice bath for a while. Then I noticed quite a bit of slush. So I put it
back in the ice bath and had lunch. When I came back there was a lot of crystals in the form of a slush. I set up my 15cm Buchner funnel and removed
them. There were a lot of nice crystals and this decreased the volume of the acid from 350mL to 225mL. This nicely answered my question about using
the 500mL RBF for the distillation pot as called out by procedure. See picture below of NaHSO4 crystals.
The second difference was that my azeotrope came over at 125C, rock solid, as was Bromic's. My Gmehling reference said that the BP would be 124.3C at
101.32 kPa (1atm). My local airport said the barometric pressure was 765mmHg which is just enough above 1 atm to make it 125C.
I think I ended up with about 100mL of the HBr azeotrope. I'll have to titrate it to determine its molarity.
[Edited on 29-4-2006 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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kazaa81
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Hey Bromic, well done
It took me a week to read your post, but it was very interesting
Tell we about any other improvement
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jamit
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I tried making hydrobromic acid last weekend and the result was uneventful. I add 206g of NaBr and 100g of H2SO4 and there was no ppt. I put it into
an ice bath and nothing. I must have put in too much water is my guess as to why there was not any ppt of sodium sulfate. I didn't want to boil it
down either in case it produce bromine gas... so..
I put the 400ml of solution into a 1liter rbf and started to distill. after about 1.5 hrs alot of the water came over and the solution changed color
to transparent red and a ppt of sodium sulfate formed at the bottom of the reaction flask. It was at this time that violent bumping started to
happen. My question is: is the bump the result of the solid ppt of sodium sulfate in solution?
I turned off the heating mantle and allowed the reaction flask to cool down and separated the ppt from the solution and added boiling chip and
redistilled... but it still bump though not as violent. I turned off the heat and notice more ppt of sodium sulfate. My guess is that I have to
completely remove the ppt of sodium sulfate or it will continue to bump in the reaction flask even with boiling chips. Am I correct in thinking this
way? thanks in advance.
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