phlogiston
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silver from x-ray film
While we have several other threads going on recovering silver from various sources, I could also use a little help with a silver recovery puzzle:
What I wanted to do:
I had two kg's of old x-ray film and I was trying to recover the silver (for fun, not profit) from them
What I did:
washed with water a couple of times to remove remaining fixative, and dumped it into a few liters of bleach.
The gelatin started to dissolve quickly, giving a suspension of white particles, which slowly turned grey. However, halfway through, the processed
slowed to a halt. I ran out of bleach and decided to add solid NaOH, hoping that the gelatine would hydrolyze in the alkaline environment.
Then, to my surprise, the solution immediately turned rust-brown. However, it remainined clear. No brown particles in suspension. I can't see any
white particles either anymore, but I may be missing them due to the intense color. The solution is exactly the color of very strong black tea.
If I take a bit of this brown solution and add a excess of HCl, I get an immediate foggy precipitate, and the solution becomes colorless slowly over
the course of an hour or so. If take a bit of the brown solution, and add a piece of copper wire it gets plated with silver.
So clearly, I have somehow dissolved (part of) the silver.
What puzzles me:
What state is the silver in? What is the brown color?
A complex of some kind? Perhaps involving amino acids from the gelatin? Or were they destroyed in the bleach? Does the gelatin play any role?
Ag(III)+ seems unlikely to me, and I have no idea what color it is.
Argentate? I can't find much about it.
It has been sitting outside for a day or three, and it still looks the same.
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Pyridinium
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It definitely sounds like you have some interesting complex silver ion there.
There is a good chance you are getting some mixed aldehydes and NH3 from the amino acids with that treatment.
Alkaline conditions can also de-aminate amino acids to some extent, but someone else here might know better what temperature that requires.
There are definitely colored complexes of Ag(III) with carbon-nitrogen compounds. I looked this up in ACS Monograph # 131 (Coordination Chemistry)
and there is a colored complex with ethylenedibiguanide. I could see this general type of complex easily forming with some di-, tri-, or oligopeptide
fragment of gelatin.
Just curious, can you repeat this experiment on a micro scale and see what happens when you add NH4OH instead of NaOH? I know silver is incompat.
with ammonia, but if you don't let it sit around long, it should be ok to do a micro color test.
EDIT: No, wait. Don't add the NH4OH. In fact, you already have some NH3 generated from the oxidation, which is not good with bleach. Then again,
the solution is probably too dilute to give you a serious problem, but just make sure you don't see a yellow oily liquid collecting at the bottom. (
I also think that very high pH will prevent the formation of NCl3 in the first place, but no guarantees there).
A teensy little micro test could tell us the difference between the effect of NaOH and NH4OH. Very curious now.
[Edited on 15-4-2012 by Pyridinium]
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AJKOER
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Quote: Originally posted by phlogiston  |
What I did:
washed with water a couple of times to remove remaining fixative, and dumped it into a few liters of bleach.
The gelatin started to dissolve quickly, giving a suspension of white particles, which slowly turned grey. However, halfway through, the processed
slowed to a halt. I ran out of bleach and decided to add solid NaOH, hoping that the gelatine would hydrolyze in the alkaline environment.
Then, to my surprise, the solution immediately turned rust-brown. However, it remainined clear. No brown particles in suspension. I can't see any
white particles either anymore, but I may be missing them due to the intense color. The solution is exactly the color of very strong black tea.
If I take a bit of this brown solution and add a excess of HCl, I get an immediate foggy precipitate, and the solution becomes colorless slowly over
the course of an hour or so. If take a bit of the brown solution, and add a piece of copper wire it gets plated with silver. |
OK, I suspect the white particles were a Silver salt (AgCl and the colorless AgClO3), created by the NaClO. Apparently, the salts were decomposed
liberating Ag, which is grey possibly from strong light exposure, or exposure to an organic compound (impurities). The NaOH created Ag2O
(brown/black). Adding HCl formed the white cloud of AgCl.
[Edited on 21-4-2012 by AJKOER]
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mycotheologist
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I've seen silver oxide turn water to amber/rust brown colour. When its concentrated enough, there'll be black AgO2 precipitated at the bottom of the
beaker but at lower concentrations, it looks more like a solution. I'm not sure if it actually is a solution or a colloid or what. Don't know if that
explains your observation though.
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Pyridinium
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| Quote: Originally posted by AJKOER |
OK, I suspect the white particles were a Silver salt (AgCl and the colorless AgClO3), created by the NaClO. Apparently, the salts were decomposed
liberating Ag, which is grey possibly from strong light exposure, or exposure to an organic compound (impurities).
[Edited on 21-4-2012 by AJKOER] |
I concur with our resident bleach expert on this point; however I still think there are complexes forming.
If they are AgCl particles (which they probably are) you may notice a tendency for them to turn slightly purplish, not pure gray, on exposure to
light. I don't know why that happens. Then again when I've seen that happen it's been in precipitates formed out of HNO3 / Cu(NO3)2 solutions.
| Quote: Originally posted by mycotheologist | | I've seen silver oxide turn water to amber/rust brown colour. When its concentrated enough, there'll be black AgO2 precipitated at the bottom of the
beaker but at lower concentrations, it looks more like a solution. I'm not sure if it actually is a solution or a colloid or what. Don't know if that
explains your observation though. |
Could be some silver oxide formation for sure, but the fact that the brown solution will plate a piece of copper sounds definitely like a complex
(these are often used in plating baths).
@phlogiston: just curious, how smooth is the silver plating (or is it really mossy / crystalline)? Not that it has much bearing on anything, just
wondering.
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phlogiston
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@AJKOER, the formation of AgCl and Ag makes sense. However, two observations argue against Ag2O as the source of the brown color. Firstly, the brown
color really appears to be a dissolved ion, not a precipitate. There are no visible brown particles, and it does not settle, even after 3 weeks of
standing now. If I shine a laser pointer sideways through the solution I don't see the beam (tyndall effect). Secondly, a copper wire got plated
evenly with silver, which I don't think would happen with an Ag2O suspension.
@Pyridinium, I'll try your experiment, substituting NH4OH for the NaOH sometime this week. If I have a bit of time, I'll see if I can do an experiment
with various combinations of pure silver salts with and without added gelatin, to see if anything from the gelatin (aldehydes, ethylene guanidine, ?)
has anything to do with it.
The silver plating actually was a very beautifull shiny, even coating on the copper. I could wipe it off pretty easily, though.
Also, thanks for the warning regarding NCl3 formation. I was already a bit worried about the formation of fulminates, but I had not considered the
NCl3 yet. No visible yellow oily droplets, but the color might obscure them if they were there... Is there any way to safely destroy NCl3? I am
tempted to add an excess of HCl to the solution to (hopefully) precipitate any silver as AgCl. Bad idea?
I once visited a christmass ball factory in czech republic years ago, where they had some interesting stories to tell about accidents with their
silver plating bath...
[Edited on 23-4-2012 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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fledarmus
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Is it possible that the dark brown color is a decomposition product from the gelatin rather than anything to do with dissolved ions? The remnants of
your organic materials?
EDIT: just noticed you mentioned this possibility in your last post
[Edited on 24-4-2012 by fledarmus]
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