I finally made a somewhat larger amount of bromine, and I am storing it now under a layer of sulphuric acid. Some of the bromine dissolves in the
sulphuric acid, and some vapor develops above it, but it is MUCH less than without the acid.
Just after bottling, the bromine looks as follows, and there hardly is any vapor above the acid:
Now, after a few days of storage, it looks as follows:
Altogether, I am quite happy with this result. If I open the bottle, and the bromine vapor is gone, then it takes many hours before a fair amount of
vapor is present again. This makes working with the bromine much more pleasant.
But now I have a question about how pure the Br2 will be under the acid. How much of the H2SO4 will dissolve in the bromine? I have not found any info
on the solubility of H2SO4 in Br2.Fleaker - 17-12-2007 at 18:30
I thought it wasn't soluble? I can only think the colour imparted to your sulfuric is from dissolved bromine, I wonder if that necessarily means that
sulfuric is dissolving in your bromine. Just as puzzled as you are. I don't know where to look for solubility of sulfuric acid in bromine. Maybe
Brauer's has a comment?12AX7 - 17-12-2007 at 18:48
Solubility is never zero; even Hg2S is soluble in H2O, if to the tune of 10^-23 or whatever. And most hydrocarbons are soluble in water to the ppm
range. Br2 is nonpolar while H2SO4 is; the situation should be similar here. How much so,.... who knows...
Heh, hell, titrate with Ba!
TimMagicJigPipe - 17-12-2007 at 21:17
You probably have more water in your H2SO4 than you think.not_important - 17-12-2007 at 21:59
Tim's got a good idea - pull out a sample, weigt, dilute with water, titrate with a soluble barium salt, the titrate the Br2.
United States patent 6855845 implies mutual solubility, but at elevated temperatures.woelen - 17-12-2007 at 23:48
I like the titration idea, although it probably will be hard to try with such low quantities, and titrating with formation of a precipitate is hard.
How do I see the end point? Should I centrifuge the solution after each drop of barium salt added, until no cloud appears anymore. That would be a
tremendous amount of work.
@MagicJigPipe: Why this comment? How can you see that it has more water than I think? Initially, before adding the acid, there only was some water
near the glass wall of the bottom, floating on the bromine, but not on its entire surface. The water was spread out like a ring, not on the center
part of the bromine surface, only near the glass wall. With a pasteur pipette I took away most of that water and then I added the H2SO4 (which is 96%
acid, lab grade, not the dirty drain cleaner stuff).12AX7 - 18-12-2007 at 08:17
Precipitation titration isn't very fast, but there's no reason why it can't be done or won't work. You could add a drop (0.05 ml or so?) then swirl
every couple of hours until it stops precipitating. Then calculate the solubility of BaSO4 in the volume to find the remainder. Alternately, you can
add an excess of Ba (reducing solubility by common ion effect) and weigh the precipitate (gravimetry).
Timwoelen - 18-12-2007 at 10:49
I tried the precipitation experiment, just to see if any H2SO4 is dissolved. I took appr. 0.2 ml of bromine and put this in a test tube. The test tube
was heated a little bit in order to drive off bromine vapor. Any H2SO4 certainly would remain behind. With this experiment, not a single indication of
any residue remains. the glass is totally clean when all bromine has evaporated.
I then rinsed with a ml of distilled water, and then added a ml of a solution of Ba(NO3)2. Not even the faintest turbidity could be observed.
I am happy with this result. This means that the bromine is very pure, no solid or liquid residue at all!! If any H2SO4 is dissolved, then the amount
is VERY small.
The only drawback of this method of storage is that it is not so easy to take out some bromine, without also taking out some H2SO4. When a pasteur
pipette is used to take out some bromine, then sulphuric acid sticks to the outside of the glass of the pipette and this slowly goes to the tip. When
the bromine is pressed out of the pipette, then this drop of sulphuric acid also is going with it.
Does anyone have a good idea of how the bromine can be taken from this bottle, without the hassle of sulphuric acid, sticking to the pipette?
[Edited on 18-12-07 by woelen]Magpie - 18-12-2007 at 11:07
How about using a septum and a syringe with a long needle?
Edit: On second thought the H2SO4 wiped onto the septum would probably destroy it. Maybe just forget the septum unless it's acid resistant. A
needle might be easier to wipe clean than a Pasteur pipet.
[Edited on by Magpie]PainKilla - 18-12-2007 at 11:57
This is exactly how I store bromine...
However, for the reasons mentioned it is indeed impractical to use this directly as is for reactions because of contamination.
The best thing to do is to store large amounts this way, taking a small bit out when it is needed, and then separating in a small separatory funnel,
followed by distilling from KBr. This affords very clean bromine. (The procedure was taken from Purification of Laboratory Chemicals 5th ed.... I
unfortunately don't have access to the computer with the files, otherwise I'd post the procedure.)
But while time consuming, it is IMHO, the most effective route towards very clean bromine. I imagine you could alternatively try extract with CHCl2 or
something, but I doubt removing the solvent would be practical.YT2095 - 19-12-2007 at 02:18
Quote:
Originally posted by woelen
Does anyone have a good idea of how the bromine can be taken from this bottle, without the hassle of sulphuric acid, sticking to the pipette?
[Edited on 18-12-07 by woelen]
the way I do it is to fill the syringe with air and put the needle on, then whilst pushing on the plunger to slowly expel air I put the needle down
into the bromine layer constantly blowing the air out, that will stop Anything (your acid) going Into the syringe.
then take the required amount out up into the Syringe, and then leaving the needle in place in the bromine take the syringe off the needle.
this way you will have No contamination in there, only on the needle itself
the important part is to maintain positive pressure until the needle is immersed well into the Br2 level.Sauron - 19-12-2007 at 03:02
How well did you dry your Br2? Normally you dry it by stirring with H2SO4 which then settles to the top just as in your pic. But if your Br2 was
appreciably wet, then the H2SO4 obviously will be somewhat diluted. You might conside taking off the first H2SO4 wash and replace it with a fresh
layer of H2SO4. Stir this and allow to seperate. Repeat this until the Br2 is bone dry.
Once the Br2 is water free, a final insulating layer of dry acid ought to remain nearly colorless.
Mellor may have something to say about the solubility of Br2 in H2SO4. Or Gmelin. The Mellor volume on the halogens is one of the handful that
S.C.Wack scanned and so is available on MadHatter ftp and elsewhere.woelen - 21-12-2007 at 06:41
Sauron, I tried your suggestion, but whatever I try, I always see that bromine slowly creeps up into the H2SO4. Here follows a picture of what I mean:
The sulphuric acid I use, however, is not 100% free of water, it is so-called 66 Baume acid, meaning 95.. 96% H2SO4, the rest being water. Is that
last 4 to 5% of water sufficient to still dissolve the bromine in the acid?
The picture, shown above, is made more than 24 hours after adding the acid to the bromine. You can see that the bromine dissolves in the H2SO4 and
slowly diffuses upwards. Once the bromine reaches the upper surface of the acid-layer, then the color of the vapor above the acid darkens.
[Edited on 21-12-07 by woelen]Sauron - 21-12-2007 at 07:42
The question is what is the concentration after the water from the Br2 is absorbed in the conc acid?YT2095 - 21-12-2007 at 07:45
I`m wondering if it`s Purely a Vapor pressure problem as opposed to actually Dissolving exactly, but rather Diffusing Through it instead.
the same way water will still Boil under a layer of mineral oil.
[Edited on 21-12-2007 by YT2095]evil_lurker - 21-12-2007 at 07:54
Quote:
Originally posted by YT2095
I`m wondering if it`s Purely a Vapor pressure problem as opposed to actually Dissolving exactly, but rather Diffusing Through it instead.
the same way water will still Boil under a layer of mineral oil.
That would be my guess as well.
Keep in mind though, that what your seeing is not a lot of bromine.
IMO your fretting over somthing you shouldn't.Sauron - 21-12-2007 at 09:05
Properly drying bromine is still necessary and quantifying the solubility (however low) of Br2 in H2SO4 of whatever % purity is a valid question,
isn't it?
I still think the answer will be found somewhere in fine print in Mellor's halogens volume.
The usual purpose of storing bromine under sulfuric acid is to reduce attack by Br2 vapor on the cap.
When I have a bottle of Br2 from a commercial supplier and want to dispense some, I just chill the bottle in ice-salt (brine) and the red fumes
condense. I do not store under sulfuric acid, because I don't have to dry reagent grade bromine. Chilling it makes it less obnoxious to handle when
transfering.
