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Author: Subject: Collection of bromine preps
njl
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[*] posted on 10-9-2020 at 08:02
Collection of bromine preps


Bromine is a useful chemical to have on hand for oxidations, halogenations, etc. While looking for the best way to produce elemental bromine, I've noticed that there are many procedures both on this forum and in the literature available online. I thought it might be useful to compile the methods here.

Most methods start by generating HBr from a bromide source, like Na or K Br. These include the reaction of:

NaBr and Sulfuric acid
NaBr and Phosphoric acid
NaBr and Hydrochloric acid
NaBr and NaHSO4
Analogues reactions with other salts

The HBr can then be distilled or collected as an aqueous solution. From here, the Bromide ion is oxidized to Bromine. Typical reactions include:

HBr and H2O2
HBr and Nitric acid
HBr and Chlorine gas
HBr and Hypochlorite
HBr and Oxone/peroxymonosulfate
HBr and Permanganate

The Bromine product can then be distilled or collected and washed. Another method is to directly oxidize the Bromide ion from the Bromide salt in solution. These reactions include:

NaBr and Hypochlorite
NaBr and Chlorine gas
NaBr and TCCA/trichloroisocyanuric acid
NaBr and Permanganate
NaBr and Oxone

The Bromine product can then be distilled or collected to leave behind solid products.

Once crude bromine is collected, it can be purified by any of the following methods, or a combination of the following:

Distillation (Simple or fractional, or over a desiccant)
Washing with a dehydrating agent like concentrated Sulfuric acid
Washing with a dehydrating solution like saturated brine/bicarbonate solution
Fractional freezing by cooling a solution/multiphase liquid to below ~8 degrees C

This is by no means an exhaustive list, it is instead meant to be a collection of the most common/confirmed preparations accessible for the amateur. Any suggestions for edits or additions is appreciated.

Not included here are electrochemical reactions, because I have little experience with them. However I would love to add more info on them if anyone has suggestions.



[Edited on 9-11-2020 by njl]
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karlos³
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[*] posted on 10-9-2020 at 08:03


A good choice that I used for in-situ chlorine preparation for N-chlorosaccharin that works also well for bromine is, to use a bromide salt and oxone in aqueous solution.
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njl
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[*] posted on 10-9-2020 at 08:11


I am most interested in the reactions of NaBr with either Chlorine gas (prepared externally or in situ) or with Ca(OCl)2. I expect the latter to be lower yielding due to the formation of hypobromite, but it would certainly be easier since no gas generating apparatus is needed.

@karl That seems to be one of the cleaner reactions. I don't have any oxone on hand however. I think KMnO4 might be even cheaper since permanganate is available in large quantities from home depot etc. unless there are problems with this reaction that I don't know about.
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Tsjerk
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[*] posted on 10-9-2020 at 08:22


Bromate, bromide and sulfuric acid is a nice one. Woelen has a nice page on it. You can even partly electrolize a bromide solution to bromate and use that as the bromine/bromate solution.
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[*] posted on 10-9-2020 at 08:53


I would separate methods at least to 2 groups:
1) Bromine seriously contaminated with Chlorine (Br + BrCl mixture)
2) Bromine not (much) contaminated with another halogen

All methods where Cl is present at the start eventually gives only 1).
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njl
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[*] posted on 10-9-2020 at 08:58


@teodor what do you mean by "All methods where Cl is present at the start eventually gives only 1"? Are you saying that the two can't be separated?
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[*] posted on 10-9-2020 at 09:29


Quote: Originally posted by njl  
@teodor what do you mean by "All methods where Cl is present at the start eventually gives only 1"? Are you saying that the two can't be separated?


Yes, I think the separation is possible only by making something like KBr and applying one of these methods again.

[Edited on 10-9-2020 by teodor]
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njl
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[*] posted on 10-9-2020 at 09:39


Speaking specifically to BrCl, it looks like this compound is volatile enough to not contaminate the final product after distillation (although I'm surprised it's even stable enough to be generated as a major contaminant in the first place).
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[*] posted on 10-9-2020 at 11:34


No, if you have BrCl in your Br2 you won't be able to distill this out of the mix in a satisfactory way. Bromine is too volatile to have these separated well.

Getting rid of BrCl can be done by adding a solution of NaBr or KBr to the Br2, contaminated with BrCl. This reacts as follows:

BrCl + Br(-) --> Br2 + Cl(-).

The chlorine goes in solution as chloride ion.

You end up with wet Br2. You can distill off the Br2 (and some water) from the salt mix. Any remaining bromide salt and chloride salt will not distill over. The resulting wet bromine can be mixed with a little H2SO4 and by a second distillation perfectly dry and pure Br2 can be obtained.




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clearly_not_atara
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[*] posted on 10-9-2020 at 14:52


Can solid bromine be crystallized out of water by reducing the freezing point with e.g. NaH2PO4?



[Edited on 04-20-1969 by clearly_not_atara]
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njl
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[*] posted on 10-9-2020 at 15:49


@atara I've never heard of that but it looks like it could be! That's really interesting.
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njl
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[*] posted on 11-9-2020 at 09:18


I would like to add electrochemical procedures, so if anyone has suggestions for the most practical methods (as is the theme of this post) feel free to share. All I'm aware of is anodic oxidation of a bromide salt, but this also suffers from losses due to the aformentioned hypobromite side product unless the cell design is optimized in a way that I am not familiar with.
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[*] posted on 11-9-2020 at 20:34


Quote: Originally posted by njl  

Washing with a dehydrating solution like saturated brine/bicarbonate solution
Fractional freezing by cooling a solution/multiphase liquid to below ~8 degrees C
[Edited on 9-11-2020 by njl]


Perhaps these two can be combined? Washed with saturated brine then freeze out the bromine from the liquid brine.
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[*] posted on 12-9-2020 at 08:05


Elemental bromine wouldn't separate out of water on cooling, you'll end up getting bromine hydrate. This is the same for chlorine (although having a solid form of chlorine is pretty handy).

Of note is that bromine itself has a reasonable freezing point (-7°C) which opens the door to freezing it as a form of purification.




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draculic acid69
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[*] posted on 17-9-2020 at 07:10


I've had a water layer seperate out and sit on top of the br2.
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[*] posted on 17-9-2020 at 07:15


Also , NaBr+h2so4+kmno4+ water = ~95% yeilds of good quality bromine if distilled carefully
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njl
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[*] posted on 17-9-2020 at 08:24


I have also had bromine collect under a layer of water but only after letting a solution sit overnight. Does bromine in solution just reach a critical mass at some point and begin to fall out of solution?
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[*] posted on 17-9-2020 at 12:36


Yes, it doesn't dissolve too well in water.
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[*] posted on 17-9-2020 at 23:04


At room temperature, in 100 ml of water, appr. 3 grams (which is appr. 1 ml) of bromine dissolves.
If bromide ion is present, then the solubility strongly increases. There is an equilibrium:

Br2 + Br(-) <---> Br3(-)

The latter goes in solution as a red anion. So, if you make bromine from a bromide salt, then it is important to have all bromide oxidized to Br2, otherwise a lot of bromine remains in solution. Especially with the bromate/bromide reaction, if you have alittle excess of bromate, then there hardly is any harm, if you have a little excess of bromide, then even a small amount of excess bromide leads to big losses if you simply collect precipitated bromine.

The Br3(-) complex is fairly labile. If you heat such a solution, e.g. with distillation, then it decomposes and the bromine can be driven off. So, this issue only exists when you do not distill, but simply collect the precipitated bromine.




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[*] posted on 20-9-2020 at 02:31


Due to the UK's currently insane rules on sulphuric and nitric acid I tend to use excess phosphoric acid and potassium permanganate and distil off the bromine. It gives a decent product but it does inevitably contain water and would need drying to get pure bromine.



If you're not part of the solution, you're part of the precipitate.
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[*] posted on 28-9-2020 at 01:01


A method I used a few years ago was to oxidize sodium bromide with MnO2. This reaction works for chlorine so I wanted to test if it also worked for bromine. The reaction that actually generates the bromine is;

4 HBr + MnO2 -> Br2 + MnBr2 + 2 H2O

By adding sulfuric acid the yield is improved;

2 HBr + MnO2 + H2SO4 -> MnSO4 + Br2 + 2 H2O

And the hydrobromic acid can be made in situ from sodium bromide;

2 NaBr + MnO2 + 2 H2SO4 -> Br2 + Na2SO4 + MnSO4 + 2 H20

The reaction needs an excess of sulfuric acid. It's an exothermic reaction but not very violent. You need to heat the flask to distill over the bromine and drive the reaction to completion.

This was my procedure:
102.9 g NaBr (1.00 mol) and 86.9 g MnO2 (1.00 mol) were mixed in a 500 mL flask. 220.0 g of 70% sulfuric acid (1.57 mol) was added. All reagents were cooled to below 0 °C and no reaction seemed to happen (In hindsight cooling was not needed).
The flask was attached to a simple distillation setup and heated. Bromine and water distilled over. The bromine was separated in a separatory funnel and washed with concentrated sulfuric acid (98%).
Yield: 58.2 g Br2 (0.73 mol, 73%).

I forgot to divide the mass of the MnO2 by 2 so there was a large excess of it. An excess is probably a good thing though because MnO2 is almost always contaminated.

I stored the bromine in a ground glass bottle with PTFE tape around the joint. This works surprisingly well but glass ampoules are the best way to store it for longer periods.

[Edited on 2020-9-28 by YoctoByte]
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