Sciencemadness Discussion Board - Getting all chlorine out of bromine

Getting all chlorine out of bromine

localbroadcast - 11-5-2013 at 10:47

Hello. This is my first post. Hopefully I find this sites users friendly and full of information! So far from what I've read on here, you are a very knowledgeable bunch.

The goal is to produce as pure as possible liquid bromine. The method chosen is to generate chlorine gas using hardware store hydrochloric acid dripped onto potassium permanganate. The chlorine gas is bubbled through a saturated solution of water and potassium bromide. The resulting gas is passed through a condenser and into a collection vessel.

I presume the resulting bromine will be contaminated with chlorine, and maybe some side reaction components im unaware of.

Does anyone have any suggestions on the best method to clean up the bromine? My first thought is fractional distillation, collecting the fraction at the boiling point of bromine.

Any thoughts?

Thanks.

Lambda-Eyde - 11-5-2013 at 11:42

BrCl will form. To get rid of BrCl contamination, distill the bromine from dry NaBr (KBr should also work, I guess). I'm not sure if this step is done before or after drying and distilling the bromine. I think you could get away with drying the crude bromine with conc. sulfuric acid, then distilling from a dry bromide salt.

blogfast25 - 11-5-2013 at 12:49

@localbroadcast:

There are several ways of oxidising bromide to bromine and the Cl2 method is probably not the best (nor the easiest), as indicated by Lambda-Eyde. Use the search facility to uproot what's been said and done here on bromine preparation methods.

Endimion17 - 11-5-2013 at 12:59

There has already been a similar thread.

Distilling is not enough. You need to reflux it with finely grinded bromide salt during one or two hours. Water is required, so if your sample is damp, and it probably is, great.
Then you dry it with conc. sulphuric acid, then you distill it, preferably more than once. You can skip the acid treatment if you distill it from barium oxide or phosphorus pentoxide.
To remove any remaining organic traces, you need to reflux it with powdered chromium(VI) oxide in conc. sulphuric acid before the bromide treatment.
To remove virtually all traces of hydrobromic acid, you need to run several alternate distillations through a "train" supplied with magnesium oxide and phosphorus pentoxide, respectively.
Such purifications are tedious, but yield extremely pure samples.

I've never done this as I've never had enough crude bromine. You need at least 50 ml and a small apparatus in order for this to be efficient enough.

woelen - 11-5-2013 at 13:11

For what purpose do you need the bromine? And what do you mean with 'as pure as possible'?

A practical thing which you can do is shake your bromine with 5% of its volume of a concentrated solution of NaBr (or KBr) in water. This removes all chlorine, but adds water. You can use a pipette to take away the bromine from under the aqueous layer.

Next, shake the bromine with conc. H2SO4 and. You can use a pipette to take away the bromine from under the H2SO4 layer. If you want best results, then distill off the bromine.

In this way, you have bromine, free of chlorine, and practically free of water.

AJKOER - 11-5-2013 at 22:19

Here is an original idea that limits contamination of your sample of Br2/BrCl. Treat with dry Hydrogen bromide. Chemistry:

BrCl + HBr (g) --> HCl (g) + Br2 (g) (see http://www.google.com/url?sa=t&rct=j&q=hbr%20%2Bbrcl... problem #9 where the reaction is reportedly slow)

Possible embodiment: Place a small amount of Hydrobromic acid (sufficient to be an excess for the above reaction) and a dehydrating agent in a beaker. Place the sample of Br2/BrCl in a wide mouth and shallow container alongside the HBr. Cover both with an inverted beaker perhaps with dry freshly dried NaCl spread over the base. Cool the outside of the inverted beaker. In time, the BrCl should be removed and H2O and Br2 may condense on the cooled inverted beaker.

Advantages of this approach: the sample of Br2/BrCl remains untouched and little to no water is expected to be added. Chlorine and BrCl sample contaminates could be removed with time. This method is not completely new, as per the reference below, P2O5 together with dry HBr gas have been used to remove water from Bromine.

Disadvantages: Expected to be slow, but mild heating (or sunlight, which would at least increase convection) may accelerate. Also, apparently some HBr gas will dissolve in liquid Bromine, in the presence of some water, effectively removing the water (see "Physics Letter", Vol 77, No. 3, 1978, page 329 at http://books.google.com/books?id=6zGJJ-9RGnkC&pg=PA714&a... ), but this does imply the consumption of some HBr. Limiting the amount of HBr to a small excess of the estimated concentration of BrCl and water in the Bromine, should help reduce contamination.

[Edited on 12-5-2013 by AJKOER]

papaya - 12-5-2013 at 00:09

What if you oxidize bromide directly with KMnO4, will it give Br2 or bromate ?

woelen - 12-5-2013 at 03:02

Depends on pH. At low pH you'll get bromine, at high pH you'll get bromate. The reaction will be slow at high pH and might require a lot of heating and it becomes more and more difficult due to buildup of a slurry of solid hydrous MnO2.

APO - 12-5-2013 at 14:50

I would suggest abandoning that process and going with this one:

https://www.youtube.com/watch?v=AL9ehxTaYRs

MrHomeScientist - 13-5-2013 at 06:00

You could also try the method I used: electrolysis of sodium bromide solution. It takes quite a while, but no chlorine involved! Here's a link to Part 1 of my video on it: https://www.youtube.com/watch?v=NKjyM2AkZSY

Credit to woelen for the procedure!

If you're sticking with your original method, I second woelen's suggestion above to remove chlorine.

APO - 13-5-2013 at 14:36

Both are very pure, with exceptionally high yields.

localbroadcast - 6-6-2013 at 20:23

I decided to go with this method - https://www.youtube.com/watch?v=AL9ehxTaYRs
sodium bromide mixed with water and potassium permanganate and dripped on with concentrated sulfuric acid.

I like this method because it removes the chlorine component all together, and the glassware setup is less complicated. I had a question about it though.. Would I be able to use potassium bromide instead of the sodium bromide used in the video? The reason I'd rather use potassium bromide is because I already have it!

If I am able to use this method with potassium bromide, how much should I use? Would I use the same amount as the sodium bromide used in the video?

Thanks for all the help everyone

localbroadcast - 6-6-2013 at 21:05

I got this equation on the reaction from another website. I'm confused why there is no "K" for potassium in the equation when it is supposed to be potassium permanganate + potassium bromide + h2o + sulfuric acid. Anyways, from this equation, can anyone tell me how much of each ingredient to use if I'm using, lets say, 100 grams of potassium bromide? Thanks.

(1) MnO4- (aq) + 8H+ (aq) +5e -> 4H2O (l) + Mn2+ (aq)

(2) 2Br- (aq) -> Br2 (aq) + 2e

Equation (1) uses up 5e while Equation (2) uses up 2e. Since the lowest common multiple between 5 and 2 is ten, we ought to apply the mole-multiplier of 2 and 5 respectively on each equation, such that they involve 10e each.

The modfied half equations would be:

2MnO4- (aq) + 16H+ (aq) +10e -> 8H2O (l) + 2Mn2+ (aq)

10Br- (aq) -> 5Br2 (aq) + 10e

Next, combine both modified half-equations to obtain the final chemical equation, ie:

2MnO4- (aq) + 16H+ (aq) + 10Br- (aq) -> 8H2O (l) + 2Mn2+ (aq) + 5Br2 (aq)

UnintentionalChaos - 7-6-2013 at 22:07

Quote: Originally posted by localbroadcast

I decided to go with this method - https://www.youtube.com/watch?v=AL9ehxTaYRs
sodium bromide mixed with water and potassium permanganate and dripped on with concentrated sulfuric acid.

I like this method because it removes the chlorine component all together, and the glassware setup is less complicated. I had a question about it though.. Would I be able to use potassium bromide instead of the sodium bromide used in the video? The reason I'd rather use potassium bromide is because I already have it!

If I am able to use this method with potassium bromide, how much should I use? Would I use the same amount as the sodium bromide used in the video?

Thanks for all the help everyone

Yes, that is my video. Just scale the amount of KBr to match the same number of moles of NaBr. You will also need more water to dissolve it, most likely.

The half reactions omit potassium because it does not participate in any redox or form any precipitate. It's a spectator ion, just like the cation for the bromide salt. Sodium,potassium,lithium. It doesn't matter. Ammonium is susceptible to oxidation so avoid that.

[Edited on 6-8-13 by UnintentionalChaos]

localbroadcast - 8-6-2013 at 03:08

thank you very much! your reply is just in time as im going to perform this reaction this morning.

i did some more research and figured out the molar masses for everything and worked it out as follows. If anyone wants to check my work it would be useful for anyone else who reads this.

10KBr + 2KMnO4 + 8H2SO4 ==> 6K2SO4 + 2MnSO4 + 5Br2 + 8H2O

In order to make 5 moles of Br2 (about 800g at 159g/mol), these would be the required amounts:
KBr - 1190g (molar mass 119g)
KMnO4 - 316g (molar mass 158g)
H2SO4 - 784.8 (molar mass 98.1g)

Scaled down to only use 100g of KBr and create a possible 67.2g of Br2, the amounts I will be actually using are:
KBr - 100g
KMnO4 - 26g
H2SO4 - 64.6g (roughly 36ml although I will load the addition funnel with 50ml)
Distilled H2O - I think 110ml should work. If the KBr doesn't completely dissolve, I'll just use more.

blogfast25 - 8-6-2013 at 05:00

Quote: Originally posted by localbroadcast

Scaled down to only use 100g of KBr and create a possible 67.2g of Br2, the amounts I will be actually using are:
KBr - 100g
KMnO4 - 26g
H2SO4 - 64.6g (roughly 36ml although I will load the addition funnel with 50ml)
Distilled H2O - I think 110ml should work. If the KBr doesn't completely dissolve, I'll just use more.

Have plenty of ice on standby. Add the KMnO4 slowly, to avoid overheating. With a BP of just under 60 C, local overheating will lead to hard-to-control eruptions of bromine. It sounds very little water for a lot of chemicals, providing very little heat sink. Nearly 70 g of bromine is a huge amount. If you've never done this before, scale down even further for a first test, I'd say. This is much more exothermic than the displacement with Cl2.

[Edited on 8-6-2013 by blogfast25]

localbroadcast - 8-6-2013 at 18:54

Ran the reaction as planned, although it did take 200ml of water to get the KBr to dissolve completely, even when heated to near boiling.

All of the KMnO4 was added at the same time to the water and KBr. This part is not exothermic. It only starts to generate heat and evolve Bromine gas when the sulfuric acid is dripped in. This was done at the rate of about 5ml at a time over the period of a couple hours to be safe. I think it probably could have been done quicker, but since this was a first run, I wasn't really too sure. Once all the sulfuric had been added, there was only about 10ml of bromine in the receiving flask. Heat was then added to the reaction vessel until it reached about 160 degrees Fahrenheit. The reaction really took off at that point and a lot more bromine started to come over. Heating was stopped and the reaction vessel was left to cool to room temperature.

The final volume of bromine collected wasn't measured accurately, but it was around 35ml. about 15ml Sulfuric acid was added to the bromine in a 250ml rbf and it was shaken around then left to settle. Only 10ml was required for a different reaction, so that was sucked out of the flask, and everything else was discarded.

Success!

localbroadcast - 8-6-2013 at 21:14

cool

Zwitteron - 17-6-2013 at 12:52

Nice setup.

What kind of tubings are you using?
I often use ordinary PVC, but there is problem with agresive gasses like NOx or so. Tubes just hardens and stops to fit tightly and leaks.

AndersHoveland - 18-6-2013 at 04:49

Pass in hydrogen bromide. That should reduce all the chlorine to hydrogen chloride gas.

localbroadcast - 22-6-2013 at 01:23

Quote: Originally posted by Zwitteron

Nice setup.

What kind of tubings are you using?
I often use ordinary PVC, but there is problem with agresive gasses like NOx or so. Tubes just hardens and stops to fit tightly and leaks.

I believe it is standard latex rubber tubing. Teflon tubing was purchased for this application, but it was too small diameter and not flexible enough to stretch onto the nipples. Perhaps the application of heat combined with needle nose pliers would be able to stretch the Teflon tubing to fit.

The latex rubber did the job fine for this single reaction, although it is yet to be seen if it will hold up to multiple runs of this reaction. It isn't a long distance of tubing that gets exposed to the Br vapors and wouldn't be much of an inconvenience if it needed to be replaced each time. I would however prefer to use something that could be used repeatedly without worry.

A word of caution when dealing with bromine... It is nasty stuff!! When the first drops of sulfuric acid were added, the joint to the 3 way distillation adapter didn't have enough grease and a puff of Br vapors escaped and whisped passed someones bare wrist before the wrist could be yanked out of the way and rinsed with water immediately. This small exposure was enough to cause a chemical burn. Not severe, but still a burn nevertheless. The seriousness of the reaction was realized at that point and caution was instilled instantly in the participants. So be careful!