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len1
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As a corollary to this
Venting BrCl to the atmosphere in an efficient 0-5C reflux will convert max 56% to Br2, and lose 44% to air.
Thus if one uses BCDMH as the bromine source with HCl, essentially all bromine will come out as BrCl and in a simple distillation setup aimed at
liquifying the bromine little yield will be obtained. Setting up one efficient reflux on the distillation products will yield about 50-55% of the
bromine. Using a two stage reflux setup, the second stage must be colder than the first so as to trap most BrCl while the first stage is active and
pass the Cl2, followed by vent to air, will net about 75% of the bromine.
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len1
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Further on this topic, I got interested in the maximum yield of bromine available from BCDMH. The question is not without merit, as due to the large
market for this chemical it has the cheapest cost per bromine atom compared to say bromide or bromate salts. Surprisingly cost a factor not just for
the amateur, professionals are expected to economise too these days.
Coproportionation of BCDMH and say NaBr using HCl has been discussed up this thread, but as the bromine comes 3:1 from the bromide, its essentially
comes at the same cost as the bromide. Using BCDMH as per my chlorine generator and with reflux collumn to decompose the BrCl gave a 50% yield with
one collumn, still cheaper than bromide, but can one do better?
Yes, Na2S2O5 also discussed up this thread is a large market reducing agent and therefore cheap, it can outperform the HCl method, but only with some
fine tuning.
A balanced equation between the Na2S2O5 and BCDMH was written up this thread, but real life is more complicated.
The following reductions of the (BCDMH==RBrCl) are all possible
a) 2RBrCl -> 2RClH + Br2
b) 2RBrCl -> 2RBrH + Cl2
c) 2RBrCl -> 2RH2 + Br2 + Cl2
d) Br2 -> 2Br-
e) Cl2 -> 2Cl-
Which are important is all a matter of the equilibrium constant AND the kinetics, the latter is hard to come by for BCDMH, which lies outside the
class of classical chemicals. Could be measured - and this I did just enough to get a workable method. The formula up the thread corresponds to c)
and e) occuring together, I find a) accounts for ~70 of the reduction processes, with d coming second. This is somewhat surprising as chlorine is the
more easily reduced species, and must be a matter of kineics rather than thermodynamics. It is also clear that d) can not occur unless either b) and
c) do not occur, or e) has completed.
So the main reaction is
4RBrCl + Na2S2O5 + 3H2O -> 2NaHSO4 + 2Br2 + 4RClH
1/4 of the moles of metabisulphite are required, which is very efficient.
Take 120 gms (0.5 mole) BCDMH place in a 1 liter three neck flask with thermometer, distilation head, droping funnel. The distilation head leads to a
west condenser at 30-45 degrees fed with 0-10C water from a recirculating pump. Add 40-50mls H2O and mix by shaking to a thick slurry. Add 10ml conc
HCl. Place 80ml metabisulphite solution made by dissolving 160gms Na2S2O5 in water to 500ml in funnel. Adjust drip rate so the drops are just too
fast to count (about 3 per second). The entire flask will soon turn a deep red and the contents will boil, with temperature rising to 83-87C. The
addition rate is such that bromine essentially boils off as soon as its formed. After all reducer has been added wait for boiling to subside and turn
on heater raising temperature to about 97-99C until gas in flask loses red colour (becomes orange due to BrCl). Yield 30gms bromine phase, + about 1
gm in water phase. Note that insufficient reducer added for anything but a) to occur substantially.
Interestingly small amount of HCl raises yield somewhat.
What happened to the 25% Br that didnt come out? Most is still bound to the hydantoin although more than stoichiometric amount of metabisulphite has
been added needed to reduce it. If one keeps adding bisulphite and heating brown-yellow fumes of BrCl appear, a tell tale sign that the remaining a)
now occurs together with b) (that is c), and to recover the remaining bromine takes a bit more work.
[Edited on 8-11-2009 by len1]
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len1
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And Ive got more to say on this.
After ALL evolution of bromine had ceased, as indicated by lightening of vapour in flask I added 100ml conc HCl with the expectation of a BrCl flux
due to 25% bromine remaining according to previous experiment. I got just a Cl2 flux! No Br2 condensed in my second stage BrCl reflux reactor. This
eliminates possibility of Br- in solution AND Br remaining in the hydentoin complex. So I revise my earlier claim of a 75% yield, its closer to
100%, difference being due to BCDMH containing DCDMH, which happens to be cheaper than the former.
So conclusion is that metabisulphite releases Cl from above compound with difficulty, it mostly reduces bromine.
And so we have a method. Reduce BCDMH with bisulphite as per above to release the bromine, then reduce with HCl to release the chlorine. And so you
have used it to its outmost.
[Edited on 8-11-2009 by len1]
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Polverone
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I take it from the course of events above that your BCDMH source did not disclose its substantial content of DCDMH. Was this just an over the counter
spa/pool disinfection product? I have seen products based on this compound that claim various percentages of the active ingredient on the label, but
have not seen a disclosure of the balance of composition.
PGP Key and corresponding e-mail address
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len1
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The BCDMH I used is intended as a dissinfectant for the consumer market, and on a per bromine basis appeared to be a factor of 2 cheaper than bromide
from the cheapest chemical supplier, which got me interested. The label states BCDMH as the active ingredient, but more than that, it claims
650gm/kg available bromine and 280gm/kg chlorine. Thats pretty much in the ratio of the atomic masses of these elements, which means no DCDMH is
claimed in the product. The available bit is a funny trick which doubles the apparent mass of active ingredient, for chlorine you will get this
amount by reacting with HCl since it supplies the missing half, but for Br this is nonsensical, since the consumer is not supplied with sodium bromide
or hydrobromic acid. BUT If you were to react the BCDMH with HBr you would get about 650gms/kg bromine. Hence they are claiming almost pure BCDMH,
certainly not at the 75% level. Further on their web site they claim 89-97% active ingredient. So the product is outside spec. Its a bit like the
sultanas in the cornflakes.
[Edited on 9-11-2009 by len1]
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1281371269
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I've been away for a while due to having a lot of schoolwork but it's easing up a bit so I've been back in my lab and have been doing some basic
organic experiments.
Ozonelabs kindly sold me some cyclohexanol and 2-ethyl-hexanol, quantities of which I will be dehydrating to form the corresponding alkenes. To test
if the dehydrations have been successful, some Br2 water will be useful.
As I only need about 1.7g max. of actual Br2 for 50ml bromine water solution I think the easiest way to go about it would be:
Saturate 50ml water with KBr
Make a chlorine generator using HCl and KMnO4
Bubble chlorine through KBr sol. until no more will react (tested by holding damn litmus above reaction vessel)
Distill the Br / KCl sol. until all the water and bromine has gone over
Can anyone see any problems with this method or suggest an easier way around?
Thanks for the help!
[Edited on 6-3-2010 by Mossydie]
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woelen
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I have tried that reaction and I must say it is a failure. I bubbled the chlorine through a fairly concentrated solution of NaBr, but the chlorine
does not react with the bromide sufficiently fast. So, at least 90% of the chlorine bubbles through the liquid and the rest reacts with the bromide.
This was not a success at all.
At a certain point, things became even worse. The air-mix above the liquid became bright yellow/orange, indicating that the chlorine, which bubbles
through the bromide takes with it quite some already formed bromine, which gives its color to the air above the liquid.
I am considering making a web-page about this failed experiment (I made nice pictures of cool glasswork to show a nice synthesis, but the outcome was
otherwise, but I still have the pics).
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1281371269
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The reason I wanted to go by this route is that as toxic gases go, chlorine bothers me fairly little, and the reaction would of course be done outside
on a windyish day. Then I could take the Br / KCl sol into the lab for distillation. This in my mind makes it one of the safest routes as there's no
potential for large amounts of Br2 fumes. Poor efficiency isn't really a concern, I've got 500g of the KBr
I have tried the reaction and it seemed to go well, however I could smell Cl2 from the start and the solution anyway had under a gram of KBr in it. It
could be improved though, by warming the KBr sol, by diffusing the chlorine via a funnel, or even by adding the chlorine to a sealed flask which was
warmed / shaken, until it reacted, then adding more and repeating. With those improvements would it be satisfactory, or is there a better way around?
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entropy51
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There is a prep somewhere on the internet for bromine water. It's basically dissolving KBr in diluted bleach and acidifying with a little HCl.
| Quote: | | I have tried that reaction and I must say it is a failure. I bubbled the chlorine through a fairly concentrated solution of NaBr, but the chlorine
does not react with the bromide sufficiently fast. So, at least 90% of the chlorine bubbles through the liquid and the rest reacts with the bromide.
This was not a success at all. |
Woelen, I've made lots of Br2 that way. I can't imagine what went wrong, but
it does work. Maybe it just doesn't work in the Netherlands.
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1281371269
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Entropy - thanks, I've found the synthesis. There were two on the page, the first one went something: Get liquid bromine and add it to distilled
water. Gee thanks.
But the second one was as you said NaBr + NaClO + HCl.
What is the actual reaction there?
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entropy51
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Without balancing the equation, acidifying the hypochlorite makes Cl2, which just displaces the Br2 from NaBr. Same as bubbling Cl2 through NaBr as
far as I know. I can't imagine why it didn't work for Woelen. He's forgotten more chemistry than I'll ever know.
I've used HCl + KMnO4 to make Cl2 many times, without incident. But some have said there is danger of explosion, for what it's worth.
[Edited on 6-3-2010 by entropy51]
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woelen
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Quote: Originally posted by entropy51  | Woelen, I've made lots of Br2 that way. I can't imagine what went wrong, but it does work. Maybe it just doesn't work in the Netherlands.
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How fast did you bubble the chlorine through the solution and what concentration of bromide did you have in your experiment?
The experiment I did was just for educational purposes, I wanted to demonstrate the classical schoolbook reaction for making bromine from a bromide
and chlorine gas. I made the chlorine gas from Ca(OCl)2 and 15% HCl.
Of course I did get bromine, but the reaction simply was not as good as I expected. Too much of the chlorine simply bubbled through the solution of
NaBr while this solution was quite a concentrated one. I simply did not want to spoil so much chlorine in this reaction and after a few minutes I
stopped. I used 10 grams of NaBr and all of this I have converted to Br2 by adding KBrO3 (which in the past I have made from electrolysis of KBr).
Just for the fun, I post the setup I used for this experiment.
The total setup is shown here:
At the right is the chlorine generator, 15% HCl is dripped slowly and regularly on solid Ca(OCl)2. The gas is lead through some glass tubing, through
atest tube. The test tube contains a rubber stopper with two holes and a second glass tube leads the chlorine gas into a concentrated solution of
NaBr in a second test tube. The final glass tube is a pasteur pipette with a very fine tip, allowing the bubbles of chlorine to be small.
A safety tube is inserted as well. If for whatever reason the system has a tendency to suck back, then I could turn open the little handle on the
table, allowing air to be sucked in.
Some detail pictures of the chlorine generator, the safety setup and bubbler:
Finally I want to show how the bubbles of chlorine went through the NaBr. There was a nice constant stream of chlorine gas, with a few bubbles per
second. But you can see in the picture that the bubbles are not absorbed by the liquid very much.
After a few minutes I had to quit the experiment, even with the window above the setup fully open, the stench of the chlorine became unbearable 
[Edited on 7-3-10 by woelen]
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entropy51
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One difference in my method was the use of KBr instead of NaBr. According to my notes I used 25 gm KBr dissolved in 100 mL H2O. The Cl2 was
generated using KMnO4 + HCl, and the Cl2 was bubbled into the KBr using just a simple glass tube. My recollection is also that the Cl2 delivery tube
was near the top of the KBr solution instead of the bottom as your pictures show. The flow of gas was relatively slow, but after a few minutes the
KBr has turned dark red and soon Br2 begins to separate on the bottom of the flask. Some Br2 vapor definitely did escape the flask, but in the hood
it was impossible to say how much. The KBr flask was an ugly red fuming mess contrasted to your picture.
I separated the Br2 in a sep funnel and then distilled quite a bit more from the aqueous solution. I did not keep an accurate record of the yields,
but rather just repeated the prep a few times until I judged I had enough Br2 to last me for a while.
I wonder if you used enough Ca(OCl)2 to generate enough Cl2, but I'm sure you would have calculated it out to react with all the NaBr. There is a
rather large volume of air to displace in your setup however, and that would require some additional Cl2. Because of the air being displaced I don't
think the bubbles would have been absorbed initially. I don't think that the Cl2 is sufficiently absorbed in the NaBr solution to conduct this
experiment without a hood or discharging the escaping Cl2 into caustic absorber. Perhaps if you had been able to add more Cl2 to the NaBr you would
have seen more Br2?
[Edited on 7-3-2010 by entropy51]
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woelen
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I used a large excess of Ca(OCl)2 (maybe two times as much as needed). I have taken into account losses of Cl2 (I assumed loss of at least half a
liter of gas) and the bubbles you see in the picture are at a time when the flask is really full of chlorine (you see the green color of the gas in
the flask). Of course, the first bubbles of gas are not absorbed, but that is what I expected. But when the apparatus is full of Cl2 then still the
absorption of gas is slow. I did not get any blob of Br2 with the Cl2. I had the chlorine in the bottom, because I thought it would be better because
the bubbles of gas are in the liquid for a longer time. I really find it strange that the solution absorbs the chlorine so slowly.
But because you report success, I will retry this experiment soon. I have plenty of Ca(OCl)2 and HCl and I just ordered 1 kg of NaBr. So, I can repeat
this experiment soon. I will make the production of Cl2 much slower than I did in the last experiment, I start thinking more and more that the high
speed of chlorine production is the problem. Next time, I'll also use a delivery tube with a wide mouth instead of the thin needle-like tube I used in
this experiment. In that way, the Cl2 might be absorbed by the liquid without many bubbles going through the liquid. This is the only thing I can
think of what might have gone wrong. I had pure Cl2 gas, I had excess of Cl2 gas and the apparatus itself looks quite good for doing this kind of
experiments. I have done this before in making KIO4 and Na3H2IO6 with Cl2-gas bubbled through alkaline iodide solution or iodate solution.
http://woelen.homescience.net/science/chem/exps/KIO4_synth/i...
http://woelen.homescience.net/science/chem/exps/Na2H3IO6/ind...
Both experiments are very similar, with chlorine bubbled through a liquid. In these experiments, this was very succesful and hardly any chlorine could
be smelled.
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len1
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A saturated solution of Br- will disolve about 4gms Br2/100ml before any is precipitated out. Also Cl2 bubbling through Br2 will volatalize it since
the equilibrium favours BrCl - so the tube must not be at the bottom.
And finally this is an experiment for the fume hood - the chlorine bubbles do not dissolve fully that would mean you would see no gas reaching the
liquid surface. Well over 50% of chlorine is lost as Cl2 and BrCl (this is the brown colour, Br3- is dark red). So while this method is
demonstative it is inefficient.
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woelen
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Yes, I can confirm that it is not as efficient as I hoped it would be. But I did not expect it to perform so bad as I have experienced in my
experiment. I'll retry it with the Cl2-delivery near the surface of the liquid and much slower generation of Cl2-gas.
My goal with this experiment indeed is not to have a perfect bromine generator. I have much better methods for that, based on making KBrO3 and
acidifying a mix of KBrO3/NaBr. I just wanted to make a cool demonstration of a common schoolbook reaction which can be carried out by many people who
do not have the equipment and the knowledge to make their own KBrO3.
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len1
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You can have your reaction ttube, same as your safety - with a gas outlet. This way you can lead the chlorine outside (to your neighbours  ) and the experiment can run longer (why not use silicone tubing instead of glass
tubes). Reducing concentration of Br- will decrease time before Br2 appears as it has drastic effect on its solubility - i suspect it will not much
affect reaction rate as that is determined be Cl2 mixing. I dont believe slower gas generation is what you want - halve the Cl2 rate, double the time
you have to wait before Br2 appears. The amount of Cl2 released is about same in both cases
[Edited on 8-3-2010 by len1]
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woelen
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I do think that Cl2 flow rate must be reduced, because that allows the gas to be absorbed by the solution. Actually no bubbles should be formed at
all, the gas simply must be absorbed. If the gas is produced too fast, then the gas forms bubbles and these may make it to the surface and get lost in
the air (cough cough !!).
The reason why I use glass tubing simply is that I have a lot of it and I have bent them in all kinds of shapes. I like glass better than other
materials, it is clean, is not corroded. The only disadvantage is that it easily breaks, but I work carefully and till now I never broke one of these
tubes.
I expect my new stock of NaBr to arrive this week (I ordered 1 kg) and then I'll try again with a somewhat larger quantity.
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entropy51
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In an attempt to see why the preparation of Br2 didn't seem to work for Woelen, I repeated this on a scale, and with conditions, intended to resemble
those he reported.
This time I used NaBr instead of KBr. I dissolved 2.5 g of NaBr in 10 mL of water and poured it into a large test tube.
This time I made the Cl2 by dripping dilute HCl onto Ca(OCl)2 instead of the KMnO4 I used previously. I did not use a trap like Woelen did, since
there is no suck back. The Cl2 delivery tube was immersed about half way into the NaBr solution, about 2 cm. The experiment was conducted in the
hood so that I could let the Cl2 bubble thru the NaBr solution fairly rapidly. The Cl2 was generated from 5 g of Ca(OCl)2 and about 15 mL of 32% HCl
diluted with an equal amount of water.
In less than a minute after starting the flow of Cl2, which bubbles up through the NaBr solution and doesn't appear to dissolve, the solution turned
dark orange and orange fumes of Br2 filled the tube. After 3 or 4 minutes longer a large drop of liquid Br2 collected in the bottom of the test tube
and the reaction appeared to be over.
I feel certain that Woelen will be able to make Br2 using this method when his NaBr arrives and he tries it again.
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len1
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| Quote: Originally posted by woelen | I do think that Cl2 flow rate must be reduced, because that allows the gas to be absorbed by the solution. Actually no bubbles should be formed at
all, the gas simply must be absorbed. If the gas is produced too fast, then the gas forms bubbles and these may make it to the surface and get lost in
the air (cough cough !!).
|
Thats what I wrote - "the chlorine bubbles do not dissolve fully as that would mean you would see no gas reaching the liquid surface".
But the rate of bubbling has little to do with the efficiency of their dissolution provided they do not subsequently coalesce - thats because once a
bubble is formed its rate of passage through the liquid is independent of the time interval with which the bubble behind it is formed. The latter is
governed by the size of the initial bubble, its path length in solution, and reaction kinetics. The size of the bubbles is governed by the
apparature, and to first order (provided the bubbling is not too fast) is independent of the rate of gas flow.
If you want to go without using bubbling - just a slow two-phase reaction, it will be more efficient but will take much longer. The rate is
proprtional to the area of contact of the two phases, so with a 5mm tube its 100 times slower than if you used an inverted glass funnel 5cm diameter,
whose tip is connected to a chlorine gasometer - if you wanted to do it in a beaker. Magnetic stirring underneath the funnel will also speed the rate
up. But I prefer bubbling
[Edited on 12-3-2010 by len1]
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woelen
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Yes, I agree, once a bubble is moving upwards, its rate of absorption does not depend on how fast it was formed, hence my idea to prevent formation of
bubbles at all. I also realize that I need to have a glass tube with a wide open end, a small inverted funnel might do the job, but maybe it also can
be done (slowly) with a glass tube of 1 cm diameter (I also have quite a few of those). It's just a matter of experimenting and tweaking.
[Edited on 12-3-10 by woelen]
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1281371269
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I did the experiment today and got fairly decent results, certainly made some bromine. I have about 200ml of more than saturated bromine solution
which I will distill at a later date.
This was the best setup I could pull together with the kit that I have. It worked well, though of course most of the chlorine simply bubbled through
the KBr. Thus the yeild was low, but on the other hand I only used about 25mL conc HCl and no more than 50g of KMnO4 so it was hardly wasteful.
A close up of the bromine formed.
As a side note: Could anyone tell me how to clean my glassware? The bromine stains I'll attack with a stong base, but what about the mess left in the
RBF? How about pirahna acid?
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entropy51
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Bromine does not stain glassware, at least not clean glassware. It might color traces of grease already on the glass.
Once you get of the mess left in the Cl2 generator flask, HCl should remove the Mn stains left on the glass. Just use a few mL and swirl it around.
A little mild scouring powder and a brush should do the rest.
Do you have liquid Br2 in the bottom of the beaker? It looks like it. You should be able to distill a lot of Br2 out of the solution. Use magnetic
stirring or boiling chips - it bumps badly. You can probably again gas the KBr solution with more Cl2 after you distill the Br2 out of it.
I wonder why the bromine water is so cloudy? I've seen it turn cloudy briefly but it clears up with continued gassing with Cl2.
For Cl2 generation I use about 6 mL of HCl per gram of KMnO4, which I believe is the ratio in Vogel. 1 gm of KMnO4 should produce about 1.1 gm of
Cl2, so you can calculate how much you should use. It is much easier to clean up the generator if you use Ca(OCl)2 instead of KMnO4.
[Edited on 13-3-2010 by entropy51]
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1281371269
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It looks a lot clearer in the contain I have it in at the moment. There is probably a fair amount of impurity, for example, I suspect that some KMnO4
managed to find it's way in as I poured it into the rbf.
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woelen
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Mossydie, this is exactly what I intend to do with my setup. My new order of NaBr arrived last weekend so I can do the experiment soon. You have a
fairly large contact area between the Cl2 and the NaBr-solution. Did it stull bubble or is the gas simply absorbed by the liquid?
I won't use KMnO4 for making Cl2, but Ca(OCl)2 or TCCA because these are OTC and much cheaper. I'll use the apparatus I have shown further up in this
thread, but I'll use a wide glass tube for delivery of the Cl2 in the NaBr-solution.
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