Sciencemadness Discussion Board

Recovering Silver from Silver Bromide

nimgoldman - 6-11-2018 at 09:59

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.

nimgoldman - 6-11-2018 at 12:19

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.

DavidJR - 6-11-2018 at 12:31

Can't you just heat it with a torch until it decomposes?

nimgoldman - 6-11-2018 at 12:47

Quote: Originally posted by DavidJR  
Can't you just heat it with a torch until it decomposes?


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]

DavidJR - 6-11-2018 at 12:53

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.

12thealchemist - 6-11-2018 at 12:54

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.

nimgoldman - 6-11-2018 at 13:22

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 (?)

phlogiston - 6-11-2018 at 13:24

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]

unionised - 6-11-2018 at 13:49

Heat with washing soda until the silver (formed by decomposition) melts.
Washing soda is cheaper and less nasty than NaOH

nimgoldman - 6-11-2018 at 14:26

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.

---

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.

---

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]

DavidJR - 6-11-2018 at 15:24

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.

chemplayer... - 6-11-2018 at 18:49

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!

Boffis - 6-11-2018 at 23:00

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.

nimgoldman - 7-11-2018 at 00:15

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]

Sulaiman - 7-11-2018 at 03:13

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.

DavidJR - 7-11-2018 at 03:24

Quote: Originally posted by nimgoldman  

I think I will just leave it on light as AgBr is said to be particularly light sensitive.

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.

That might not be a bad idea...

phlogiston - 7-11-2018 at 03:39

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.

nimgoldman - 7-11-2018 at 04:25

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.

DavidJR - 7-11-2018 at 04:29

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.

nimgoldman - 7-11-2018 at 13:08

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.

unionised - 7-11-2018 at 13:32

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.

nimgoldman - 8-11-2018 at 07:56

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.

phlogiston - 8-11-2018 at 11:20

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]

clearly_not_atara - 8-11-2018 at 11:28

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?

unionised - 8-11-2018 at 12:43

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.

Heptylene - 8-11-2018 at 14:13

I know this thread is about AgBr, but I think this is relevant nevertheless. I've been wondering for a while if UV light would be suitable to recover the silver from silver iodide.

I made some AgI by adding KI soln to AgNO3 soln, filtering, washing, drying. I tried shining a purple laser (405 nm) at the bulk powder for several seconds, which produced a slightly brownish discolored spot. But, surprisingly, when left for a few hours (exposed to light or not), the discolored spot disappeared completely. Even after weeks in a well-lit room, the powder looks exactly the same as when I first made it (pale yellow)

What I assume is happening is that iodine from the decomposition of AgI is trapped inside the crystal and simply reacts with the silver also formed to regenerate AgI.

Maybe electrolysis of the molten salt would be suitable to recover silver. The halides melt between 450 and 550 °C. There is an eutectic mixture of AgCl/AgI that melts at 258 °C (35% %m/m AgCl) according to Cornwell, K; Dyson, R. W. Brit. J. Appl. Phys. Vol. 2 1969


Attachment: Cornwell1969.pdf (238kB)
This file has been downloaded 502 times

DavidJR - 10-11-2018 at 16:25

Quote: Originally posted by Heptylene  
But, surprisingly, when left for a few hours (exposed to light or not), the discolored spot disappeared completely. Even after weeks in a well-lit room, the powder looks exactly the same as when I first made it (pale yellow)

What I assume is happening is that iodine from the decomposition of AgI is trapped inside the crystal and simply reacts with the silver also formed to regenerate AgI.


That is precisely what is happening. In silver halide photography this effect is referred to as solarisation (creates an apparent inversion of the image in areas of extreme exposure).

[Edited on 11-11-2018 by DavidJR]

nimgoldman - 12-11-2018 at 17:22

I found a brief description of the silver recovery method here.

The chemical method consists or adding sodium sulfide (Na2S) followed by sodium hydroxide.

In this 1937 thesis I've read that sodium sulfide causes this reaction:

2 Na3Ag(S2O3)2 + Na2S -> 4 Na2S2O3 + Ag2S

so this turn the silver complex back into sodium thiosulfate and precipitates silver sulfide. Large stoichiometric excess of sodium sulfide is needed for full precipitation.

I am not interested in recovering the thiosulfate, I just need the silver. So I wonder if simply adding NaOH would work.

On forums, people advice for using metals (zinc, copper, steel wool) instead. But this introduces new problem (metal waste).

The process of silver recovery from fixers seem to be over 100 years old and one of the major sources of recovered silver. Yet I can't find much info about it on the web.

So far I will have to blindly experiment ... lots of silver will be probably lost in experimentation :(

[Edited on 13-11-2018 by nimgoldman]

nimgoldman - 12-11-2018 at 17:52

Experiment is king. So here is a residue of AgBr on filter - it already darkened from exposure:



I poured little bit of warm 25% sodium thiosulfate solution and most dissolved immediately just as expected (the residue is probably silver and impurities):



Then I added copious amount of 50% NaOH. No reaction:



Hence sodium thiosulfate + hydroxide cannot be used to precipitate silver.

I can try ammonia + hydroxide, ammonia + dithionite, thiosulfate + dithionite.

If nothing of the above works, I am afraid I will have to resort to metals.

nimgoldman - 13-11-2018 at 18:45

Now I tried the "metal" route.

Sample of silver bromide added to the beaker:



25% ammonia poured over, AgBr dissolved completely:



Now we simply add copper to displace the silver. I don't have a piece of solid copper, only powder, so I added some. Reaction occured rather quickly and I observed silver precipitation:





I filtered out the silver precip. and washed it few times.



It's kind of wet, hard to get all off the filter, but this is a minor technical issue:



Here is the copper-ammonia complex solution. How to get the copper back? What will happen if the water is evaporated? How to precipitate it? How to discard it?



Anyway. I added the supposed silver precipitate in a clean beaker and poured 68% nitric acid over it:





The characteristic green colour indicates a copper contamination but there are also some weird white-ish solids. Unreacted AgBr? Pieces of paper from scraping maybe? How to remove the copper once dissolved?

This route seems very simple, no signs of the dreaded silver nitride are observed, the process is very quick, straightforward and convenient, no strict stoichiometry has to be followed, no exotic reagents required.

Hopefully the copper contamination can be reduced by using piece of solid copper instead of powder or using some other separation or purification technique later.

Maybe there is easier way to go from AgBr back AgNO3 (and then AgNO2). I have lots to learn, if only learning from textbooks would be as much fun as experimenting...

nimgoldman - 13-11-2018 at 20:24

After little more research over some precious metal recovery groups, there seems to be an easy way to get metallic silver from AgCl by the action of lye and reducing sugar (glucose), both being much "greener" then using metals. I am not sure about the contamination though.

I am also not sure if the lye-sugar method would work directly on AgBr or conversion to AgCl is needed first (either by adding HCl to the diammine silver or dissolving AgBr in aqua regia as per ChemPlayer's AgI method).

Ideally, the silver recovery should be convenient since the method will be repeated many times as silver nitrate is expensive, available in small amount and thus has to be reused many times. The waste should be either non-toxic or at least easily destroyed/managed.

[Edited on 14-11-2018 by nimgoldman]

phlogiston - 14-11-2018 at 06:30

So, you now rediscovered yourself what I posted 8 days ago.

This thread is quite interesting for the many different creative ways that were suggested you do this.

Fun for experiments, but if you just want to recover silver cheaply and easily:

1. Add lye to AgBr --> Ag2O
2. Heat, or add reducing sugar to Ag2O--> silver metal]

[Edited on 14-11-2018 by phlogiston]

fusso - 14-11-2018 at 06:56

Is there any nonmetallic reducing agents that can reduce Ag halides without NH3 & NaOH?

nimgoldman - 14-11-2018 at 07:21

Quote: Originally posted by phlogiston  
So, you now rediscovered yourself what I posted 8 days ago.

This thread is quite interesting for the many different creative ways that were suggested you do this.

Fun for experiments, but if you just want to recover silver cheaply and easily:

1. Add lye to AgBr --> Ag2O
2. Heat, or add reducing sugar to Ag2O--> silver metal]

[Edited on 14-11-2018 by phlogiston]


Can you back up this method with literature reference or explanation how would that work?

I Googled terms like: "silver bromide sodium hydroxide", "silver halide hydroxide precipitation" etc. and found nothing except school projects (halide test) and some metal recovery forum posts here and there mentioning lye briefly, but not giving any elaboration. There are some videos of people recovering Ag from AgCl but also details or explanation missing.

Furthermore, I added lye to dissolved AgBr with no results. It seems once it complexes, it cannot be precipitated by hydroxide, only displaced with metal.

Pouring lye over AgBr won't dissolve it either, right? It might convert AgBr to Ag2O on surface only, then reaction will stop - and that's the problem. Hence dissolution is needed. AgBr is known do dissolve as complex in conc. ammonia and Na-thiosulfate, but NaOH is not mentioned anywhere.

What I need is thorough and complete conversion of AgBr to Ag, of course. Even 5% loss means over 1/3 of silver being lost in 10 repetitions of the recovery.

The lye-sugar method is known in the metal recovery community, but they exclusively use silver chloride (AgCl), not AgBr. Could this method work with AgBr as well? Different halides have different reactivities and Cl is more reactive than Br suggesting the method might not work with the Br ion.

If heat has to be applied for thorough conversion, then how much heat? What temperature? When to add reducing agent and how much? How long to react?

These are all details I am looking for, but maybe looking at bad places. The metal recovery forums are also very vague in the process descriptions. A literature references would be helpful.

Another question: What people do with the liquid after metal displacement? They can't pour it down the drain right? Copper is pollutant and extremely toxic to aquatic life. Other metals are toxic too in water.

[Edited on 14-11-2018 by nimgoldman]

fusso - 14-11-2018 at 08:21

Quote: Originally posted by nimgoldman  

Another question: What people do with the liquid after metal displacement? They can't pour it down the drain right? Copper is pollutant and extremely toxic to aquatic life. Other metals are toxic too in water.

[Edited on 14-11-2018 by nimgoldman]
Just pour down drain if no metal used, otherwise use CO2 to ppt CuCO3 if NH3 is aslo present.

DavidJR - 14-11-2018 at 08:42

Quote: Originally posted by nimgoldman  


Hence sodium thiosulfate + hydroxide cannot be used to precipitate silver.


This is not true as I have done it before (in recovering silver from photographic fixer). Perhaps the conditions weren't right in some way.

Here's a reference: Galarpe V, Leopoldo GD (2017) Potential Recovery of Silver (Ag) from X-ray Fixer Waste by Alkaline Treatment. Engineering, Technology & Applied Science Research 7: 2094-2097
Attachment: 1526-4156-1-PB.pdf (588kB)
This file has been downloaded 501 times

[Edited on 14-11-2018 by DavidJR]

phlogiston - 14-11-2018 at 09:03

You added NaOH to AgBr in thiosulphate solution, yielding a solution containing silver as a complex ion [Ag(S2O3)2]3−. That reacts differently with lye than solid AgBr does.

You will probably not find much on this method if you search specifically for AgBr, but descriptions and movies of each step for AgCl are easily found.

I myself only have experience with AgCl as well. Adding lye to it seems to convert it quickly and completely into Ag2O without heating.
(the NaOH solution is usually a bit warm when I do this due to its high enthalpy of solution)

Admittedly, if AgCl is prepared by precipitation it is very finely divided, so if your AgBr is course, I understand your concerns. But perhaps grinding may be all that is needed.

When I dissolve silver recovered as above in nitric acid, I get a clear and colorless solution, which I interpret as an absence of significant amounts of residual AgCl.

AJKOER - 14-11-2018 at 19:47

Per Atomistry.com on Ag2O (see http://silver.atomistry.com/silver_monoxide.html ) to quote:

"Its heat of formation is about 6.4 Cal. It decomposes hydrogen peroxide, with liberation of metallic silver."

where Atomistry content is usually sourced from old chemistry journals.

When I get more silver, it would be interesting to verify the efficiency of the conversion of Ag2O to Ag without having to heat the Silver monoxide.
--------------------------------------------------

A possible understanding of the above mechanism employing some more recent science, starting with, for example, "Ag2O as a New Visible-Light Photocatalyst: Self-Stability and High Photocatalytic Activity", by Xuefei Wang, link: http://onlinelibrary.wiley.com/doi/10.1002/chem.201101032/ab... to quote from the abstract:

"Ag2O is unstable under visible-light irradiation and decomposes into metallic Ag during the photocatalytic decomposition of organic substances. However, after partial in situ formation of Ag on the surface of Ag2O, the Ag2O-Ag composite can work as a stable and efficient visible-light photocatalyst"

A photocatalyst in light is capable of producing electrons, e-, and electron holes, h+ (see, for example, http://www.imaging.org/site/PDFS/Papers/1999/PICS-0-42/1080.... ). Further photolysis can create solvated electrons which in the presence of dissolved oxygen (from say decomposing H2O2 acted upon by a metal oxide) can lead to superoxide:

O2 + e-(aq) = •O2- (aq) (reversible)

Also superoxide from the action of an electron hole on HO2- via:

H2O2 = H+ + HO2-

HO2- + h+ = •HO2 = H+ + •O2- (aq) (if pH > 4.88)

Or, possibly superoxide from the action of an electron hole on water (creating the hydroxyl radical) further acting on hydrogen peroxide:

(H+ + OH-) + h+ = H+ + •OH

•OH + H2O2 = •HO2 + H2O (see https://aip.scitation.org/doi/10.1063/1.2943315 )

•HO2 = H+ + •O2- (if pH > 4.88)

The above hydroxyl radical promoted path to superoxide may also be applicable if there is not a significant photocatalytic path, but instead say a transition metal impurity presence, which together with acidic H2O2, could lead to a fenton-like reaction with associated created •OH radicals.

So, in the presence superoxide and silver ions, possibly created by the action of acid (which can attack Ag2O) added to H2O2 to increase its stability:

•O2- (aq) --> e- (aq) + O2

Ag(+) + e- (aq) --> Ag

Net: Ag(+) + •O2- = Ag (s) + O2 (see, for example, https://pubs.acs.org/doi/abs/10.1021/es103757c?src=recsys&am... )

where the final product is highly reactive (and valuable) nano-silver. Note, with the creation of nano particles, they are known to be easily excited/charged (say by light) and reportedly produce reactive oxygen species (ROS) with oxygen. ROS with transition metals can further engage in associated REDOX reactions (a basis for health concerns, see https://en.wikipedia.org/wiki/Antibiotic_properties_of_nanop... ).

So the initial action of light acting on Ag2O-Ag with pH adjusted H2O2, may act on silver ions (from Ag2O) to liberate nano-silver, which can further foster superoxide creation with oxygen (some amazing chemistry afoot).

If I have surmised somewhat correctly the underlying starting mechanism, then some procedural points likely important to increasing the yield could include:

> Apply strong solar light or UV.

> Pre-treat just the Ag2O with light to form the Ag2O-Ag photocatalyst.

> Maintain a pH > 5 for the H2O2, or try a shifting strategy between low and ending at neutral pH (where silver metal is not attacked by the acid).

> Shaking of the mix to increase the dissolution of the silver monoxide in acid.

> Sealing the vessel to capture oxygen together with shaking increasing dissolved oxygen concentration.

where my anticipation of the yield is still much less than 100%, but the process does avoid some issues like heating Ag2O to a temperature of 490 C, and AgO, AgCO3,..., impurities at lower temperatures (see, for example, https://pubs.acs.org/doi/abs/10.1021/cm00046a022 ) with a final product of less reactive silver metal.

[Edited on 15-11-2018 by AJKOER]

unionised - 15-11-2018 at 12:35

Lots of potentially interesting, or potentially wrong, stuff there.

Or you could just melt the stuff with an alkaline flux.
Borax works, NaOH works, Na2CO3 works and is the cheap option.

AJKOER - 16-11-2018 at 05:49

Quote: Originally posted by unionised  
Lots of potentially interesting, or potentially wrong, stuff there.
..........


Per https://chemiday.com/en/reaction/3-1-0-2515 , to quote:

"Silver(I) oxide react with hydrogen peroxide

Ag2O + H2O2 → 2Ag + H2O + O2 "

Also per https://www.proprofs.com/discuss/q/115497/hydrogen-peroxide-... , to quote:

"What will hydrogen peroxide behave as in the following equation

Ag2O(s) + H2O2(l) ---> 2Ag(s) + H2O(l) + O2(g) "

Also in the same reference:

"The reaction is important in the synthesis of silver as the reaction can be used to produce the pure element that is not oxidized or combined with any other element."

Another source https://chemistry.stackexchange.com/questions/76042/what-hap... to quote:

"Probably, the silver (I) oxide will directly/indirectly catalyze the decomposition of hydrogen peroxide according to the reaction:

Ag2O+H2O2⟶2Ag+H2O+O2

Given that the majority of silver compounds/salts are photosensitive,.....Even if you have other substances/compounds as impurities in hydrogen peroxide solution which can react with silver oxide you still will probably end up with elemental silver (Ag)."

There appears to be a similar suspicion of a possible role for the action of light and the reference to H2O2 impurities that may react with Ag2O also parallels my proposed introduction of the Ag+ ion from acid stabilized hydrogen peroxide.

Lastly, the use of the word 'indirectly', agrees with my suspicion that the underlying mechanics is potentially complex. In defense, I quote parts of the prior source provided above "Superoxide-Mediated Formation and Charging of Silver Nanoparticles", by Adele M. Jones, et al, in Environ. Sci. Technol., 2011, 45 (4), pp 1428–1434, DOI: 10.1021/es103757c , link: https://pubs.acs.org/doi/abs/10.1021/es103757c?src=recsys&am... :

"While there have been suggestions that superoxide is able to reduce silver(I) ions with resultant production of AgNPs, .....we present definitive experimental evidence for the reduction of silver(I) by superoxide.... The overall rate constant, however, increases by at least 4 orders of magnitude in the presence of AgNPs. A model based on electron charging and discharging of AgNPs satisfactorily describes the kinetics of this process. The ability for AgNPs to undergo catalytic cycling provides a pathway for the continual generation of ROS and the regeneration of AgNPs following oxidation."

where nano particles based charged transfers is not a commonly considered factor in a hydrogen peroxide REDOX.

So, in my proposed 'possible understanding' of what Unionised described as 'potentially wrong', which certainly could be true, it seems I may not be entirely alone.
-----------------------------------------------------------------------

[EDIT] Did find a source (see 'Large-scale synthesis of polyhedral Ag nanoparticles for printed electronics' by Shlomi Polani, et al, in RSC Adv., 2017, 7, 54326, DOI: 10.1039/c7ra11370fVegab , link: https://pubs.rsc.org/en/content/articlepdf/2017/ra/c7ra11370... ) discussing the mechanism of the reaction in some detail. To quote:

"Ag2O(s) + H2O2(aq) --> 2Ag(s) + H2O(l) + O2(g) (formally but with a large excess of H2O2) (3)"

So the much quoted reaction above (but, not by me) appears dubious in practice. Also, some favorable comments as to the process:

"A rather less known and very original synthetic route to produce metallic silver involves a particle-mediated pathway in which silver(I) oxide (Ag2O) usually serves as a sacrificial template, and NaBH4 or H2O2 act as the reducing agent."

Of note:

"formation of Ag2O, which is then reduced by H2O2 in the presence of a stabilizing polymer."

And finally:

"The Ag2O solid phase mediates the formation of silver in a particle-mediated mechanism. The Ag2O particles are reduced by H2O2, and the TEM analysis of a sub-stoichiometric reaction (Fig. 3B) shows the formation of a biphasic Ag2O/Ag Janus particle as an intermediate.
The reaction follows a surface driven mechanism where H2O2 chemisorbs and reacts on the surface of Ag2O particles followed by reduction and growth of the pure Ag phase, like a reaction with a reducing gas...."

And, as this source employs a stabilizing polymer, suggests a large excess of H2O2, and also neglects the photo active properties of Ag2O/Ag, it may not be a precise rendition of the raw action of H2O2 on Ag2O in light either.

[Edited on 16-11-2018 by AJKOER]

[Edited on 16-11-2018 by AJKOER]

nimgoldman - 7-12-2018 at 03:24

Okay after several test tube experiments, I found that AgBr dissolves in nitric acid!

I thought it won't as all the "silver halide" tests talked about cream precipitate. After careful reading, these tests use dilute nitric acid.

So I simply dissolved AgBr in nitric acid. Actually all the expected silver contaminants will dissolve (nitrate, nitrite, bromide, oxide).

Recovering silver from silver nitrate is then easy and described on many places over the internet and literature.

Silver nitrate can then purified by recrystallization from water and then I would go straight to silver nitrite since I am interested in recycling silver nitrite for the Victor Meyer reaction. There was no need to go all the way back to silver metal actually...

As for the silver metal recovery, adding sodium chloride solution or hydrochloric acid will precipitate silver chloride (HCl[aq.] might be slightly better as table salt has limited solubility in water). The chloride solution is added until no more precipitation is observed.

This silver chloride can be purified by repeated washings with water, boiling it for a while in water won't hurt and will remove more impurities.

There are then two common ways to get silver metal:

- dissolve in dilute ammonia, add more reactive metal to displace the silver (e.g. copper, zinc, steel)
- add sodium hydroxide with little water, heat and stirr until all silver chloride becomes silver oxide, then reduce with either formaldehyde or sugar (glucose, fructose, honey...)

Some people advised to avoid the reduction and just heat silver oxide to obtain silver. I've read on metal recovery groups that this method produces silver of poor quality - it's better to precipitate metal powder first and after it is pure enough, then melt it.

Okay so finally I reinvented the wheel, but me who suffer little chemistry knowledge, have to just google more ...

One interesting note: After making silver nitrate crystals, the poured off water will still contain considerable amount of nitrate. This can be precipitated as chloride and saved for next recycling run.

[Edited on 7-12-2018 by nimgoldman]

nimgoldman - 21-1-2020 at 12:07

This thread popped up for me in Google search when I researched more about this, so I will just add new findings to help a random lurker.

AgBr and AgI also dissolve in sodium thiosulfate (ammonium thiosulfate might be better though). It is possible to precipitate silver using sodium ditionite, but the reaction is slower for bromide (it is almost instant for chloride). See the referenced paper below.

Here is the precipitation of silver from AgBr/Na-thiosulfate with Na-dithionite:



The "sugar and lye" method should also work for bromide and iodide given the sodium bonds to halogens stronger than silver, but I have not tried yet.

It might be even possible to reclaim bromine and iodine by simply bubbling excess chlorine through the soln. (after filtering out the silver of course) and collecting the elemental halide. Maybe excess TCCA will also work though I have no idea how all that would interact with the residual thiosulfate and dithionite ions.
Quote:
Rivera, I., et al. "Study of silver precipitation in thiosulfate solutions using sodium dithionite. Application to an industrial effluent." Hydrometallurgy 89.1-2 (2007): 89-98.


Quote:
Willbanks, Otto L. "Reclaiming silver from silver chloride residues." Journal of Chemical Education 30.7 (1953): 347.


Attachment: RECLAIMING SILVER FROM SILVER CHLORIDE RESIDUES.pdf (706kB)
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[Edited on 21-1-2020 by nimgoldman]

clearly_not_atara - 21-1-2020 at 12:29

Dithionite huh? Nice that someone finally found a use for it! :p

Luckily both dithionite and thiosulfate can be prepared from bisulfite (by rxn with zinc and sulfur respectively).

[Edited on 21-1-2020 by clearly_not_atara]

nimgoldman - 21-1-2020 at 17:29

Quote: Originally posted by clearly_not_atara  
Dithionite huh? Nice that someone finally found a use for it! :p

Luckily both dithionite and thiosulfate can be prepared from bisulfite (by rxn with zinc and sulfur respectively).

[Edited on 21-1-2020 by clearly_not_atara]


This approach first came from ChemPlayer's video on the recovery of silver and iodine from AgI. He needed a reducing agent that nicely precipitates silver powder instead of creating a silver mirror.

He also provided the procedure for the synthesis of sodium dithionite (by zinc reduction as you said).

I was lucky enough to find this chemical OTC at local chem supplier so I just bought it as powder.

beta4 - 11-4-2021 at 14:23

Since I had the need to purify some silver nitrate, I followed the path of precipitating silver chloride to separate it from impurities.
I would like to report that ascorbic acid works as a reducing agent to form silver metal powder from Ag2O, just in case someone happens to have some lying around.

I also made a video of the reaction: https://youtu.be/tHSdZri2xrU