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Heptylene
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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)
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DavidJR
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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.
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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]
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nimgoldman
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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]
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nimgoldman
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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.
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nimgoldman
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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...
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nimgoldman
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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]
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phlogiston
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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]
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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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fusso
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Is there any nonmetallic reducing agents that can reduce Ag halides without NH3 & NaOH?
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nimgoldman
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| 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]
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fusso
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| 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.
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DavidJR
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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 407 times
[Edited on 14-11-2018 by DavidJR]
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phlogiston
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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.
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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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AJKOER
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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]
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unionised
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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.
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AJKOER
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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]
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nimgoldman
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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]
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nimgoldman
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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)
This file has been downloaded 305 times
Attachment: j.hydromet.2007.06.001.pdf (960kB)
This file has been downloaded 390 times
[Edited on 21-1-2020 by nimgoldman]
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clearly_not_atara
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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]
[Edited on 04-20-1969 by clearly_not_atara]
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nimgoldman
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| 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.
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beta4
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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
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