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nimgoldman
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Recovering Silver from Silver Bromide
The method has been covered for silver by ChemPlayer and I am trying to adapt the method for silver bromide as well. The working method for silver iodide follows:
1. dissolve silver iodide in freshly prepared aqua regia (nitric acid + hydrochloric acid, 1:3) to precipitate silver chloride
2. dilute with water for more convenient filtration, vacuum filter via glass sinter
3. dissolve silver chloride in dilute aq. ammonia solution (forms various silver-halide-ammonia complexes)
4. reduce with sodium dithionite solution to precipitate metallic silver
5. filter, wash with water, followed by absolute ethanol, dry
However, I've read that silver bromide also forms complexes in ammonia solution, so we might go straight to step 3 for AgBr (I am not interested in bromine recovery).
Could there be any issues with silver bromide that silver chloride does not posses?
I would use the reference (Leden, I., Persson, G.; Persson; Sjöberg; Dam; Sjöberg; Toft (1961). "The Solubility of Silver Chloride and Silver
Bromide in Aqueous Ammonia and the Formation of Mixed Silver-Ammonia-Halide Complexes". Acta Chem. Scand. 15) to get more information, but it is
not accessible for me.
I then plan to make silver nitrate from the recovered silver, then silver nitrite further. Maybe there is even shorter way to go from silver bromide
to silver nitrite, but given the silver bromide is almost insoluble in water, this might be the current go to way.
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nimgoldman
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One problem might be low solubility of AgBr in dilute ammonia solution.
ChemPlayer used only 10-15% ammonia to dissolve AgCl, but this is not strong enough to dissolve AgBr. This solubility difference is actually used in silver silver halide test to distinguish different silver halide salts.
Furthermore, ChemPlayer warns against using conc. ammonia solution as any residual silver oxide or silver nitrate can form silver nitride, an explosive compound (also known as "fulminating silver").
However, silver nitride seems to be really dangerous if dry, hence washing the product with fresh water would remove it as well as reduce the
explosion hazard. The question remains whether the reducing agent, sodium dithionite might affect it. I never was into explosives and don't want to make any, not even unintentionallly  .
Indeed, a conversion to chloride salt might be safer route.
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DavidJR
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Can't you just heat it with a torch until it decomposes?
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nimgoldman
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It's possible though the decomposition temperature for AgBr is quite high (1 500 deg C). I don't have a torch, only an alcohol lamp.
UV light might work better though I am not sure about exposure times for full decomposition.
Okay I found different method, here the experimentator uses 25% ammonia soln. to resolve different silver halides, then silver is precipitated on a
piece of copper. Near quantitative yield is obtained.
The copper method looks attractive as no special reducing agent has to be prepared and the copper metal can be reused.
It seems the silver nitride is not a big concern (the ammonia method for silver halide identification is a common lab exercise) and it can be detected
easily since it forms black precipitate which can be removed by decanting the supernatant.
[Edited on 6-11-2018 by nimgoldman]
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DavidJR
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I don't think it requires temperatures anywhere near that high. I've certainly done it with my el-cheapo butane torch. Not sure if an alcohol lamp
will do but I'd try it.
Try vitamin C as a reducing agent - I know it can work because I've used it to develop silver halide photographic paper before.
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12thealchemist
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I'd attach the paper you reference, but I can't seem to be able to upload it for some reason. I will try again tomorrow on a different computer.
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nimgoldman
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Thanks.
Yes the whole area of designing historical B&W photographic developers circles around reducing silver halide (white) to silver metal (black). What
I read is that they are made to be somewhat selective to increase contrast of the photography and should not reduce all the halide.
On the other hand, I need to reduce all the halide to the metal.
Okay so far several methods seem viable:
1) suspension of silver halide + UV lamp (or sun) + stirring ... cheap but lenghty method, possibly incomplete conversion as light won't penetrate
lumps
2) heat decomposition ... open flame + liberates free bromine (yuck)
3) conc. ammonia + copper
4) conc. ammonia + some sort of reducing agent (sodium dithionite)
5) water + ascorbic acid + light (?)
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phlogiston
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With silver chloride, an easier method is:
1) add NaOH, which converts the silver chloride into silver oxide.
2) Then either:
A)add a reducing sugar (e.g. glucose) to convert the oxide to metallic silver
or
B)simply heat the silver oxide to decompose it. It's decomposition temperature is only 280 C.
Don't know if the first step works with silver bromide, but it would be easy to try on a small scale. There should be an immediate and obvious color
change (silver oxide is black)
Added advantages of this method are that you don't contaminate the metal with small amounts of copper, it is really cheap and uses readily available
materials and you don't run the risk of accidentally producing fulminating silver.
[Edited on 6-11-2018 by phlogiston]
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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unionised
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Heat with washing soda until the silver (formed by decomposition) melts.
Washing soda is cheaper and less nasty than NaOH
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nimgoldman
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Okay, so this is basically B&W photography.
The silver halide crystals are illuminated, forming seeds of metallic silver. Light alone would do the job but it would take a long time.
A suitable developer solution is poured over silver halide crystals (sodium carbonate, sodium hydroxide, hydroquinone, ascorbic acid etc. etc.) that
reduces the entire seeded crystals to metallic silver, silver hydroxide or silver oxide (depending on the developer).
Unseeded crystals (without imperfections) are not attacked by the developer initially, but given enought time, they will be too, especially if light
is still provided (no darkroom, we want black&black, not black&white).
At this point, undeveloped silver halide crystals would be dissolved and washed out with a fixer agent (e.g. sodium thiosulfate). We can still use it
to wash away traces of silver halide and bind any free bromine.
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So simply suspend the silver halide in water, provide developer solution, light, possibly also stir the solution while being exposed on sunlight or
under UV lamp.
Filter, wash with sodium thiosulfate, then water.
By drying, silver hydroxide will go to silver oxide.
By applying direct heat, silver oxide will go to silver metal.
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I hope I understood the principle.
But at this point I have no idea why ChemPlayer chose such a complex procedure (dissolving in ammonia + sodium dithionite), if just a simple washing
soda would do.
ChemPlayer wrote that a strong enough base must be used so "that it doesn't slowly form a silver mirror all over your glassware".
[Edited on 6-11-2018 by nimgoldman]
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DavidJR
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Re light exposure: normal room lighting will be more than sufficient to create development centres on the AgBr crystals. In fact, unless your AgBr was
made and stored in total darkness, there will already be development centres.
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chemplayer...
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We tried a few things but the aim was to find a way to get pure filterable and washable silver metal powder. We avoided using copper/zinc etc. metals
to do the reduction as we weren't sure if the metal particles would get coated in silver but ultimately not react completely, leaving an impure metal
product. We therefore wanted to find a way to precipitate the silver metal using a reducing agent, and we needed a reducing agent that would cause a
solid filterable precipitate to form rather than a mirror, and also ideally something cheap and easy to produce (thus ruling out exotic things like
hydrazine). After our 'filter paper art' experiments with copper (I) oxide we opted to try dithionite and it worked well. Not that there aren't
probably alternatives though!
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Boffis
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I did some work a few years back and intend to pick up this work again in the near future where the products are isolated as their silver salts. This
results in a lot of silver chloride residues. I work it up using the ammonia - copper plate precipitation technique mentioned above as I have found
that recovery is near quantitative and the silver can be removed from the plate using a wash bottle which reduces the risk of contamination of the
silver with copper if you have to scrape it. The resulting silver is soft but crystalline and easily filtered and washed. I the recovered silver
nitrate is contaminated by copper it is to a very small degree and it doesn't interfere with my preparations.
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nimgoldman
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Okay. Thanks, chemplayer, for providing a valuable input.
I think I will just leave it on light as AgBr is said to be particularly light sensitive.
The silver can be filtered out and washed with conc. ammonia. Unreacted AgBr can be crashed by simply diluting the ammonia with water and repeat the
process.
If the process would be inefficient, I would try the copper displacement or the dithionite, the latter being preferable as not producing copper waste.
Finally, the crude silver might be treated with flame to decompose any oxide formed, by I believe almost none would be formed this way.
Okay time to go do the work. I will report back w. the result.
[Edited on 7-11-2018 by nimgoldman]
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Sulaiman
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The most efficient method of converting silver bromide to silver metal is to
sell the silver bromide and with the proceeds buy Silver Maple coins.
Greater than 100% yield of 99.99% pure product.
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DavidJR
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Yes, the order of sensitivity is AgCl < AgBr < AgI.
Quote: Originally posted by Sulaiman  | The most efficient method of converting silver bromide to silver metal is to
sell the silver bromide and with the proceeds buy Silver Maple coins.
Greater than 100% yield of 99.99% pure product.
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That might not be a bad idea...
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phlogiston
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The problem with using photolysis is that the light can not penetrate deeply into the powder. Only a very thin surface layer is decomposed. You'll
need to stir it very often to expose fresh surface and it will take very long.
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nimgoldman
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| Quote: Originally posted by phlogiston | | The problem with using photolysis is that the light can not penetrate deeply into the powder. Only a very thin surface layer is decomposed. You'll
need to stir it very often to expose fresh surface and it will take very long. |
Yes this is exactly my concern and the main reason for wanting AgBr dissolved.
Currently the only usable solvents I know of are conc. ammonia and sodium thiosulfate.
I wonder if sodium dithionite is compatible with sodium thiosulfate - in that case conc. ammonia (potentially forming silver nitride) can be avoided.
(I am still learning redox reactions, so pardon my ignorance or stupid questions)
Such process (aq. thiosulfate + aq. dithionate) will be free of extra metals and the waste can be easier to manage.
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DavidJR
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Actually now that I think about it, I've recovered silver from spent photographic fixer before, so dissolving in a thiosulphate solution might not be
a bad idea. IIRC the method was to add NaOH until it all precipitated out, filter, then heat the filter cake in a crucible with a torch to decompose
the oxide back to metallic silver.
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nimgoldman
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I have seen some videos of people recovering silver from spent fixer, but they either used electrolysis or some mysterious reducing solution (some
said it was sodium hydroxide).
Well I will do test tube experiments and see.
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unionised
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| Quote: Originally posted by nimgoldman |
But at this point I have no idea why ChemPlayer chose such a complex procedure (dissolving in ammonia + sodium dithionite), if just a simple washing
soda would do.
[Edited on 6-11-2018 by nimgoldman] |
I was wondering that too.
You need to heat it to about 430C to get the decomposition to work (once the AgBr melts the reaction's going to go) and, obviously, to about 962C to
melt all the silver together.
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nimgoldman
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The case might be that ChemPlayer managed to precipitate Ag.
Sodium hydroxide might rather precipitate AgOH (AgO). But getting metalic silver from that, even if just in powder form, is easy. Powder is okay,
actually better, since it will be used right away to produce fresh silver nitrate.
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phlogiston
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Recovering silver from spent fixer does not require any fancy or mysterious reducing agent.
I've used steel wool with good success.
It's a well known and cheap method to recover silver from fixer.
[Edited on 8-11-2018 by phlogiston]
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clearly_not_atara
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If cobalt is available, the conversion might be done without reductions via hexamminecobalt (III) acetate:
(NH3)6Co(OAc)3 (aq) + 3 (NH3)2AgBr (aq) >> (NH3)6CoBr3 (s) + 3 (NH3)2AgOAc (aq)
IIRC the silver-ammine complex slowly decays via evaporation of NH3, which should leave you with (relatively!) pure AgOAc. This decomposes to silver
at about 220 C; notably, in this case Br2 is not released and might even be recovered. The below link discusses the (in)solubility of hexamminecobalt
salts:
https://books.google.com/books?id=walWQrTRNK4C&pg=PA220&...
| Quote: | | Sodium hydroxide might rather precipitate AgOH (AgO). |
I find this odd. I've heard a number of claims that AgO can be used to perform this reaction in reverse, producing NaOH from NaCl. Only one of these
reactions should be favorable, right?
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unionised
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| Quote: Originally posted by clearly_not_atara |
I find this odd. I've heard a number of claims that AgO can be used to perform this reaction in reverse, producing NaOH from NaCl. Only one of these
reactions should be favorable, right? |
The clever bit is to heat it.
It's true that the AgBr + NaOH <--> AgOH = +NaBr is an equilibrium reaction and I don't really know which side is more favoured.
However, if you heat it then any silver (hydr)oxide that gets formed will rapidly decompose.
Once the oxygen bubbles out, the reaction can't reverse so it goes to completion.
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