i was contemplating generating some anhydrous HBR from the crude acid and CaCl2, was suggested. It's inefficent for this purpose. so what about making
a slurry of the mixture then incorporating some dry H3PO4 to further dehydrate the mix? some of the research i've done says this will react with the
calcium chlorides to give off HCl, i don't see how as Chloride is the more negative anion. any input?
wikipedia says H3PO4 could be used to dehydrate this acid, this in concert with a dessicant makes sense to me.
[Edited on 3-11-2008 by jon]
oops this belong in the inorganic forum, sorry.
[Edited on 3-11-2008 by jon]
<sub>Edit by Nicodem: Thread moved to General chemistry and tittle made descriptive.</sub>
[Edited on 3/11/2008 by Nicodem]chemrox - 2-11-2008 at 21:10
There's a procedure for generating HBr gas in Vogel's 3rd. I don't have it here but will look in the am. I seem to recall it was a lot like making
HCl which comes off anhydrous.smuv - 2-11-2008 at 21:21
Your best bet would be to make the anhydrous HBr directly from phosphoric acid and any readily available bromide.
Of course i could be wrong, this is an easy one for you to test on microscale.
Remember when you are drying your HBr to avoid using other halides and avoid things which can oxidize the HBr (sulfuric acid).
EDIT: Took out something I was wrong about
[Edited on 11-3-2008 by smuv]jon - 2-11-2008 at 21:21
i saw the one on orgsyn and oh what a pain! why do people go through the elbows to get to the asshole (a rhetorical question of course).
H3PO4 + NaBr = NaH2PO4 + HBr looks good on paper but real world application another story why do you say avoid halides?
metathesis reactions? can you spell out some of the halides it would swap with?
[Edited on 3-11-2008 by jon]smuv - 2-11-2008 at 21:31
I guess now that I think about it, it should only realistically swap with Iodide, because there is no way bromine is going to oxidize chloride.
A lot of people including myself have used this method to make anhydrous HCl; its not the best method IMO but its not too bad.
as was stated before it takes a tremendous amount of heat to get that to work you'll likely eat the silicates on your pyrex before anything happens
there. it looks dandy but it aint gonna worknot_important - 2-11-2008 at 23:15
Avoid other halides for drying, because
2 HBr + CaCl2 <=> 2 HCl + CaBr2
both halides are in ionic state, the more negative ion doesn't matter that much. As you are passing the HBr over the CaCl2, HCl is constantly being
swept away, driving the equilibrium to the right.
NaBr + H3PO4 => HBr + NaH2PO4 does work for making nearly dry HBr, use an excess of H3PO4 to keep it fluid, keep the temperature below 150 C or
thereabouts to avoid trashing the glassware.smuv - 2-11-2008 at 23:33
Br2 tetralin is another method, it sucks though because half of the bromine is lost; although it is apparently very convenient.
In Brauer CaCl2 is used to dry HBr (see dibromodisulfane prep).
I realize though that my rationalization above makes no sense at all. If it were not in brauer I would be inclined to agree that CaCl2 would be a bad
drying agent because not only are you continuously shifting the equilibrium, HBr is also a stronger acid than HCl.
[Edited on 11-3-2008 by smuv]S.C. Wack - 3-11-2008 at 03:57
That is odd. I would think that they would have used CaBr2, which is commonly used for this. I thought maybe it was a misprint, but it is also in the
German edition, and the author in the book is also a journal reference author.smuv - 3-11-2008 at 09:10
From Brauer's HBr prep.
Quote:
The HBr-H2 mixture finally passes through a drying tube H filled with CaCl2 (CaBr2 is better, of course)
So it looks like the swap of halides occurs, but it is not hugely detrimental to the purity of the effluent gas.
I would guess though that if you had very wet HBr the swap of halides would be more significant.
[Edited on 11-3-2008 by smuv]woelen - 3-11-2008 at 10:59
Another good option for drying is finely powdered MgSO4, which one easily can make from Epsom salt MgSO4.7H2O. With this there will not be any
exchange of gaseous products, and no oxidation of HBr to B2 occurs. That oxidation only occurs with conc. H2SO4, not with dry salts.jon - 3-11-2008 at 13:13
right it's got to be concentrated and above a certain temperature 60 C I think for that to happen.
with H3PO4 good mechanical or magnetic stirring would be also nessecary because the salts don't mix well.
so it's plausible one could use MgSO4 on wet HBr in conjuction with phosphoric acid as a drying agent this would be a lot more convenient then using
H3PO4 because of the whole stirring heating thing.
somewhere i read that MgSO4 swaps out with HBr to make H2SO4 is this also true? it would seem if so it would'nt be to any great extent because the
equilibrium would be (fixed) because of the rising concentration of H2SO4 it might work on wet HBr.
[Edited on 3-11-2008 by jon]
[Edited on 3-11-2008 by jon]woelen - 3-11-2008 at 15:23
There may be formation of some HSO4(-) in combination with the wet HBr, but I do not expect a main loss due to this. If you want a really lossless
drying agent, then you most likely want P4O10, but for many of us, this is not easy to obtain, although it is not a watched or restricted chemical.
P4O10 is not the easiest thing to handle. It tends to form bigger lumps with a somewhat glassy appearance at the outside and having still completely
dry P4O10 inside, effectively blocking the inside P4O10 from exposure to the gas to be dried. This is a really annoying property of P4O10.jon - 3-11-2008 at 17:45
that was exactly what i read on a page from a chinesse supplier ( about the clumping)the easiest way to get this stuff is overseas either customs
seizes or they don't, no muss no fuss.jon - 27-12-2008 at 20:47
well instead of starting a new thread i should have done this.
what about heating NaHSO4 with NaBr???
the sulfate *ion* oxidizes bromine but this is dry so unionized what do you think?grind - 30-12-2008 at 10:55
Quote:
Originally posted by woelen
If you want a really lossless drying agent, then you most likely want P4O10
P2O5 can´t be used for drying HBr, because it reacts to some extent with HX (X = halogen).
Mg(ClO4)2 is nearly as strong as P2O5 for drying purposes, should not react with HX, is more easy to handle and is easy to recycle.
CaBr2 is easy made from CaCO3 and aqueous HBr. It should be dehydrated cautiously to avoid partial decomposition into basic calcium bromide and HBr
(similar with CaCl2).
Wet HBr gas can be partially dried by freezing out the water in a good freezing mixture.
