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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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Mood: walking on sunshine
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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.
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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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