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Author: Subject: Possible semi-catalytic bromine synthesis
pHzero
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Possible semi-catalytic bromine synthesis

I was playing around with some AgCl when I came up with an idea for a method of bromine synthesis.

AgNO3+KBr --> AgBr+KNO3. Filter off the insoluble AgBr

Place the AgBr into a still and heat it in bright sunlight
2AgBr --> 2Ag + Br2

Bromine boils off and condenses, elemental silver left in the flask.

AgNO3 is recovered by adding HNO3 to the Ag (cant be bothered to work out a balanced symbol equation)

So the net reactants are KBr and HNO3, and the net products are KNO3, Br2, water and whatever nitrogen oxides the nitric acid makes dissolving the silver.

Or have I overcomplicated it? This would make high purity Br2, with no BrCl, but I don't know if the H2SO4+H2O2+KBr synthesis is quicker and easier....
entropy51
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pHzero, that's an innovative prep, if it works. Heat may decompose AgBr to bromine, with or without sunlight, but the H2SO4 + H2O2 synthesis is easier, as are some others. I wouldn't use an expensive reagent such as AgNO3 for this, even if you might be able to recover some of it.

If you want to work with bromine, first gain some experience with bromine in water solution. Its much safer than liquid bromine, which is about as dangerous as it gets. Many experienced chemists would not gladly distill bromine. Bromine water can be easily made from KBr, bleach and HCl (or even vinegar). See http://mattson.creighton.edu/BromineWater.html You could learn a lot about bromine from experiments with bromine water. Even bromine water can cause severe skin and eye irritation, and you should make and work with it outside your house. You need gloves and eye protection, OK?
woelen

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One way to easily obtain a few drops of fairly pure bromine (main impurity being BrCl) is the following:
- Dissolve as much as possible of KBr in water
- Add approximately the same volume of 30% HCl. A white precipitate is formed. This is mainly KCl. Decant the liquid from the precipitate.
- Add solid calcium hypochlorite to the liquid and swirl (no shaking needed) after each addition. After a few additions you'll see that the liquid get turbid and rust-like. This is due to formation of many tiny droplets of bromine which slowly collect at the bottom as a single blob of bromine.

With a pasteur pipette the blob of bromine can be drawn away form under the aqueous layer.

No distillation needed and the yield is somewhere around 70% if done carefully.

This is not really suitable for a larger scale preparation, but it makes a good demo experiment and the drop of bromine can be used for a nice experiment such as adding it to Al-foil in a fire-proof test tube.

ALL OF THE THINGS ABOVE SHOULD BE DONE OUTSIDE! Use gloves and goggles, there will be highly concentrated bromine around. Dense fumes of bromine are formed!

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not_important
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As AgBr can be melted and cast, and is listed as decomposing around 1500 C, it's going to be a bit difficult to do that at home.

Photo-decomposition of silver halides tends to be incomplete as the particle interiors are shielded from light to a lesser or greater degree. The silver halide particles would need to be very small to avoid this. Silver can also recombine with the elemental halides, so the reaction will be incomplete.

Hydragyrum
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pHzero, I'm not sure if your idea would be the best method for producing bromine, but it is a good idea and it would be good to try it for the purpose of "proof of principle" if for no other reason.

Can you get the reagents? I'd love to hear if it works, but as woelen says do this outside if you do try it.

Perhaps a safer experiment would be to try with the iodide - you should be able to see purple iodine fumes if it is working.

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pHzero
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 Quote: Originally posted by Hydragyrum pHzero, I'm not sure if your idea would be the best method for producing bromine, but it is a good idea and it would be good to try it for the purpose of "proof of principle" if for no other reason. Can you get the reagents? I'd love to hear if it works, but as woelen says do this outside if you do try it. Perhaps a safer experiment would be to try with the iodide - you should be able to see purple iodine fumes if it is working.

Yep, I can get sillver from ebay, or there's a stall in the town centre with a "We buy any gold" sign. I expect they'll have some dirt cheap scrap silver. And I've got HNO3 and KBr too

Edit: as for the iodine part, unfortunately I dont have any I-, there's only one ebay seller selling KI, and the price is ridiculous - over 50 quid a kilo

[Edited on 3-6-2009 by pHzero]
DJF90
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Aldrich sells 100g for £18.20, so 1kg for £50 is not a bad price. I know other lab supplies that sell KI at about £7/100g, so again this seller is being reasonable. Iodine is sold on ebay cheaper sometimes, reaching lows of about £8/100g.
UnintentionalChaos
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 Quote: Originally posted by pHzero Yep, I can get sillver from ebay, or there's a stall in the town centre with a "We buy any gold" sign. I expect they'll have some dirt cheap scrap silver.

Unlikely. Those places are in the business of ripping off anyone stupid enough to sell them jewelry, melting it all down, and shipping it in bulk to refiners who send them money less fees.

Try a coin dealer. They often have fine silver available just as 1ozt (31.1g) unmarked discs for not too much above spot price.

http://www.kitco.com/market/ Currently $15.75/ozt, which is quite high. it was around$9 a few months ago. I should have bought a couple hundred troy ounces and sat on it.

[Edited on 6-3-09 by UnintentionalChaos]

Department of Redundancy Department - Now with paperwork!

'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
pHzero
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Quote: Originally posted by UnintentionalChaos
 Quote: Originally posted by pHzero Yep, I can get sillver from ebay, or there's a stall in the town centre with a "We buy any gold" sign. I expect they'll have some dirt cheap scrap silver.

Unlikely. Those places are in the business of ripping off anyone stupid enough to sell them jewelry, melting it all down, and shipping it in bulk to refiners who send them money less fees.

Try a coin dealer. They often have fine silver available just as 1ozt (31.1g) unmarked discs for not too much above spot price.

http://www.kitco.com/market/ Currently $15.75/ozt, which is quite high. it was around$9 a few months ago. I should have bought a couple hundred troy ounces and sat on it.

[Edited on 6-3-09 by UnintentionalChaos]

I bought some silver off ebay a little while ago and it made some nice silver nitrate. £16/oz delivered though. But today was market day so i went to that stall and he said if i ever need scrap, he'll sell it to me for £9/ozt. Forgot to ask what purity though. My mate said it'll probably be sterling silver (99.5%) rather than fine silver (99.9%)

[Edited on 3-6-2009 by pHzero]
DJF90
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Sterling is 92.5% silver, 7.5% copper. Aka "925" silver (fine silver is "999")

[Edited on 3-6-2009 by DJF90]
pHzero
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 Quote: Originally posted by DJF90 Sterling is 92.5% silver, 7.5% copper.

Ah right, cheers for the info. I guess you could purify it by adding nitric acid to it to make AgNO3 and Cu(NO3)2, then add NaCl to make a pure AgCl precipitate (CuCl2's soluble). Then you could add Al to the AgCl to get AlCl3 gas and elemental Ag. Might be more hassle than its worth though.

Edit: and the thermite reaction might be too exothermic to do in lab glassware

[Edited on 3-6-2009 by pHzero]
woelen

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Do not hassle with the impure silver, unless you see it as an adventure in its own to purify AgNO3 from impure silver sources. If your goal is pure and clean AgNO3, then I simply would go for buying a fine 99.9% silver bullion from an eBay seller, dissolve that in HNO3 and slowly evaporate to dryness. One recrystallize from distilled water removes last traces of acid. With one troy ounce of silver from eBay you get appr. 50 grams of AgNO3 and the price of this will be $20 ...$25, which actually is a good price for AgNO3.

I myself have experimented with silver solder from a hardware store, but it indeed is quite a hassle. The hot Al/AlCl3 method is not really good for making pure silver. You must do it the wet way, making a precipitate of AgCl, rinsing with a lot of water and then dissolving this in ammonia. With a suitable strong reductor (e.g. ascorbic acid, hydroquinone or maybe even glucose) you can make silver from this and the precipitated metal then must be rinsed well and melted in a crucible. All organic crap and other impurities will separate from the molten silver. But as you can see, this also is not that easy.

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UnintentionalChaos
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 Quote: Originally posted by pHzero Edit: and the thermite reaction might be too exothermic to do in lab glassware

You shouldn't do any thermite reaction in any lab glassware you want to keep.

Also, http://www.bromicacid.com/mistakes.htm#5

Department of Redundancy Department - Now with paperwork!

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pHzero
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 Quote: Originally posted by woelen Do not hassle with the impure silver, unless you see it as an adventure in its own to purify AgNO3 from impure silver sources. If your goal is pure and clean AgNO3, then I simply would go for buying a fine 99.9% silver bullion from an eBay seller, dissolve that in HNO3 and slowly evaporate to dryness. One recrystallize from distilled water removes last traces of acid. With one troy ounce of silver from eBay you get appr. 50 grams of AgNO3 and the price of this will be $20 ...$25, which actually is a good price for AgNO3. I myself have experimented with silver solder from a hardware store, but it indeed is quite a hassle. The hot Al/AlCl3 method is not really good for making pure silver. You must do it the wet way, making a precipitate of AgCl, rinsing with a lot of water and then dissolving this in ammonia. With a suitable strong reductor (e.g. ascorbic acid, hydroquinone or maybe even glucose) you can make silver from this and the precipitated metal then must be rinsed well and melted in a crucible. All organic crap and other impurities will separate from the molten silver. But as you can see, this also is not that easy.

Ah right, i see. I might actually do that though, just for the fun of trying to purify it
So you add ammonia and that makes diaminesilver (i) chloride, ie tollen's reagent?

And unitnentionalchaos, I remember reading that a while ago, but forgot about it. Sounds nasty, so I don't think i'll be trying any thermite reactions xD

[Edited on 3-6-2009 by pHzero]
woelen

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Yes, you make a silver-ammine complex. You must do the reaction immediately after dissolving the AgCl in ammonia, never let such a solution stand for a longer time, because that may cause formation of explosive silver nitride compounds (fulminating silver).

If you don't want the risk of formation of silver fulminate, then you can use NaOH instead of ammonia and you get a precipitate of Ag2O, which then can be reduced. The disadvantage of this is that you have a reaction with a solid (less contact) and you introduce a non-volatile impurity through the sodium ions, which makes purification more difficult.

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pHzero
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 Quote: Originally posted by woelen Yes, you make a silver-ammine complex. You must do the reaction immediately after dissolving the AgCl in ammonia, never let such a solution stand for a longer time, because that may cause formation of explosive silver nitride compounds (fulminating silver). If you don't want the risk of formation of silver fulminate, then you can use NaOH instead of ammonia and you get a precipitate of Ag2O, which then can be reduced. The disadvantage of this is that you have a reaction with a solid (less contact) and you introduce a non-volatile impurity through the sodium ions, which makes purification more difficult.

Ah right. I'll add some NH3 to it and then chuck some methanol in it, that should work, right? The methanol should get oxidised to methanal or formic acid, which I can boil off, or I could even add liquid butane which I expect would be a stronger reducer.
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While starting with pure silver metal is the easiest route for getting to AgNO3, if you must start from Stirling or some other silver-copper alloy don't go through the bother of separation by making AgCl.

Instead slowly evaporate the solution of silver and copper nitrates to dryness, then heat it to around 300 C and hold it thee for several minutes. AgNO3 melts at 212 C and decomposes at around 440 C, copper nitrate decomposes to the oxide starting at around 180 C. Then let the melt cool, swirling it so as to form a thin solid layer as it cools. Dissolve in distilled/deionised water, decant/filter to separate the AgNO3 solution from the CuO. If the solution still has a blue tint, repeat the dry-fuse-dissolve cycle, otherwise just concentrate and allow the hot solution to cool to give AgNO3 crystals.

You need to keep dust out of solutions of AgNO3 as it can cause reduction to the metal and/or formation of the chloride and sulfate. Also remember that while evaporation to dryness recovers all of a compound in solution, it effects no purification - you must have some liquid left to hold impurities.
woelen

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Not_important's method also is a nice method. It even is more accessible than the method I presented, because you need less strong heating (melting silver metal in the final step of my process requires much higher temperature).
Still, it would be really nice if one of us could come up with a method, which does not require heating above 100 C at all. Many less equipped home chemists still will be hesitant to work with a melt of 300 C. Maintaining it at 300 C sounds more easy than it actually is. You need a high temperature range thermometer and you need to keep the liquid mixing all the time in order to prevent formation of hot spots.

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pHzero
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 Quote: Originally posted by woelen Not_important's method also is a nice method. It even is more accessible than the method I presented, because you need less strong heating (melting silver metal in the final step of my process requires much higher temperature). Still, it would be really nice if one of us could come up with a method, which does not require heating above 100 C at all. Many less equipped home chemists still will be hesitant to work with a melt of 300 C. Maintaining it at 300 C sounds more easy than it actually is. You need a high temperature range thermometer and you need to keep the liquid mixing all the time in order to prevent formation of hot spots.

Hmm I've got a small problem. I tried your method, to recover the Ag from some AgCl. I added NH3 to it to make diaminesilver chloride, which didnt work: there was still a grey solid (AgCl contaminated with elemental Ag) at the bottom. I decided to add methanol to that anyway. I got a creamy precipitate. I figured i should boil off the NH3 and MeOH before any fulminate forms, but then I get toxic MeOH and corrosive NH3 everywhere. Should I just filter off the precipitate and chuck the MeOH/NH3 away?
woelen

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Hmmm... you added methanol? That probably is not the best reductor in this case. I'm afraid you now need something stronger. I would not heat all the stuff and drive off the methanol and ammonia, that gives a lot of noxious fumes and there is the risk of fire/explosion due to buildup of methanol vapor.

One thing you could try is adding some NaOH (making Ag2O) and ascorbic acid. That certainly makes Ag from the precipitate, but workup now is somewhat more hassle, due to the presence of sodium ions, which lead to non-volatile impurities. Good rinsing with water might help though. The precipitate you obtain is compact and quite heavy and quickly settles at the bottom with virtually no loss. So, there still is hope of recovering your material.

This kind of trouble reminds me of the troubles I've had in the past with purifying things and isolating compounds. This kind of experiences really is a good way to learn things.

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pHzero
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 Quote: Originally posted by woelen Hmmm... you added methanol? That probably is not the best reductor in this case. I'm afraid you now need something stronger. I would not heat all the stuff and drive off the methanol and ammonia, that gives a lot of noxious fumes and there is the risk of fire/explosion due to buildup of methanol vapor. One thing you could try is adding some NaOH (making Ag2O) and ascorbic acid. That certainly makes Ag from the precipitate, but workup now is somewhat more hassle, due to the presence of sodium ions, which lead to non-volatile impurities. Good rinsing with water might help though. The precipitate you obtain is compact and quite heavy and quickly settles at the bottom with virtually no loss. So, there still is hope of recovering your material. This kind of trouble reminds me of the troubles I've had in the past with purifying things and isolating compounds. This kind of experiences really is a good way to learn things.

Right, I added the NaOH. Nothing happened for a second, then it suddenly boiled, presumably due to the exothermic dissolution of NaOH. Now the solution's turned from metallic grey (presumably a suspension of Ag) to black (http://img80.imageshack.us/img80/8717/1000349.jpg) which im guessing is my Ag2O

Then I was writing this post, and i though I might as well add some more and see if i could get it all convered to Ag2O. It was even more exothermic than the last time, and it boiled over :s
I've still got about three quarters of it left though...

[Edited on 5-6-2009 by pHzero]
entropy51
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That poor carpet!
pHzero
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 Quote: Originally posted by entropy51 That poor carpet!

I did it all outside
That's the garden table in the picture
Nicodem
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 Quote: Originally posted by pHzero I was playing around with some AgCl when I came up with an idea for a method of bromine synthesis. Start with silver nitrate and KBr (or NaBr) AgNO3+KBr --> AgBr+KNO3. Filter off the insoluble AgBr Place the AgBr into a still and heat it in bright sunlight 2AgBr --> 2Ag + Br2 ... Or have I overcomplicated it? This would make high purity Br2, with no BrCl, but I don't know if the H2SO4+H2O2+KBr synthesis is quicker and easier....

I see that the thread topic took another direction. Yet I just wanted to say that HNO3 oxidises bromides to bromine anyway. No need to overcomplicate with some funky multistep semi-catalysis ideas. Though, in practice nobody would use HNO3 as the oxidant for an preparative bromide oxidation since separating nitrogen oxides from bromine is not particularly easy. Oxidising bromides is much easier that you might think. There are plenty of simple oxidants that will do just fine. Check the thread on bromine preparation - there are numerous methods mentioned or described there (where you should have posted your idea as well, by the way).

…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)

pHzero
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Quote: Originally posted by Nicodem
 Quote: Originally posted by pHzero I was playing around with some AgCl when I came up with an idea for a method of bromine synthesis. Start with silver nitrate and KBr (or NaBr) AgNO3+KBr --> AgBr+KNO3. Filter off the insoluble AgBr Place the AgBr into a still and heat it in bright sunlight 2AgBr --> 2Ag + Br2 ... Or have I overcomplicated it? This would make high purity Br2, with no BrCl, but I don't know if the H2SO4+H2O2+KBr synthesis is quicker and easier....

I see that the thread topic took another direction. Yet I just wanted to say that HNO3 oxidises bromides to bromine anyway. No need to overcomplicate with some funky multistep semi-catalysis ideas. Though, in practice nobody would use HNO3 as the oxidant for an preparative bromide oxidation since separating nitrogen oxides from bromine is not particularly easy. Oxidising bromides is much easier that you might think. There are plenty of simple oxidants that will do just fine. Check the thread on bromine preparation - there are numerous methods mentioned or described there (where you should have posted your idea as well, by the way).

Wouldnt it oxidise it to nitrosyl bromide rather than bromine?

Anyway, yeah. Now I see that this process wouldnt be all that useful, but I still feel like trying it just, as Hg said, as a proof of concept.
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 Sciencemadness Discussion Board » Fundamentals » Chemistry in General » Possible semi-catalytic bromine synthesis Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues