vmelkon
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silver
Simple question but I have wondered about this for a long time.
If you had pure silver and you heat it in air, does it form any compounds (with oxygen or anything else in air)?
I think that Ag2O is not stable and therefore does not form.
Also, I have been told that the brown stuff that forms on silverware is silver sulfide. Would heating a pure piece of silver decompose the sulfide and
burn the sulfur?
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AJKOER
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By itself, Silver is resistance to oxidation by air. However, in the presence of H2S and O2, Silver sulfide readily forms a sulfide:
4 Ag + O2 + 2 H2S → 2 Ag2S + 2 H2O
Similarly, Ag can be dissolved slowly by even weak Acetic acid in the presence of an oxygen source (like H2O2), forming Silver acetate, which is
soluble in acidic solutions (like Acetic acid).
Per Wiki (see http://en.wikipedia.org/wiki/Silver_oxide ):
"A slurry of Ag2O is readily attacked by acids:
Ag2O + 2 HX → 2 AgX + H2O
where HX = HF, HCl, HBr, or HI, HO2CCF3."
As such, I would say that while Ag2O is stable, it appears to readily react with acids even H2S.
On heating Silver, I know that Acetylene (C2H2) reacts with Silver salts, and I would speculate with Silver sulfide possibly as well:
C2H2 (g) + Ag2S (s) --> 2 Ag (s) + 2 C (s) + H2S (g)
or even possibly Silver acetylide, Ag2C2, which is greyish to white. Interestingly, per Wiki: "Silver acetylide can be formed on the surface of
silver or high-silver alloys, e.g. in pipes used for transport of acetylene, if silver brazing was used in their joints." In either reaction with
primarily Acetylene, probably not the best way to clean Silver. However, in a C2H2/O2 mixture, one of many possible reactions with Ag2S:
Ag2S (s) + C2H2 (g) + O2 (g) --> 2 Ag (s) + 2 CO (g) + H2S (g)
may prove more successful, but excess C2H2 may actually form the explosive Silver acetylide.
[Edited on 6-7-2012 by AJKOER]
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vmelkon
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I don't follow.
I was thinking in terms of heating. If you heat AgO, it decomposes to the elements, similar to HgO decomposes to the elements.
Therefore, if you heat silver in air (by heating I mean from 300 °C to 1500 °C), it doesn't get coated by a layer of AgO.
If you look on the wiki page, next to melting point it says 280 °C (decomposition).
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blogfast25
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Quote: Originally posted by vmelkon  |
I don't follow.
I was thinking in terms of heating. If you heat AgO, it decomposes to the elements, similar to HgO decomposes to the elements.
Therefore, if you heat silver in air (by heating I mean from 300 °C to 1500 °C), it doesn't get coated by a layer of AgO.
If you look on the wiki page, next to melting point it says 280 °C (decomposition). |
'Stability' isn't really a scientific term. Ag2O has a very low (as absolute value) Standard Enthalpy of formation
(ΔH<sub>298</sub><sup>0</sup> = - 31 kJ/mol, Wiki). Compare that to ‘more stable’ oxides and it’s peanuts. As a
result, Ag2O decomposes at relatively low temperatures, compared to more ‘stable’ oxides. But at RT it exists w/o a problem.
I’m having a bit of silver trouble right now with fairly freshly precipitated AgCl that seems resistant to dissolution in (> 25 w% ammonia. I would have thought it would dissolve in that solvent effortlessly but
part of it resists the ammonia. Any thoughts, anyone? I’m loathe to heat it because the ammonia stench would be unbearable…
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Endimion17
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Silver won't tarnish if heated in air without volatile sulphides. Its oxide decomposes readily, and by the time temperature reaches 280 °C, the
oxidation rate is still negligible. So it's not like mercury, which oxide can be made if the metal is being attacked by oxygen right below the
temperature of oxide's decomposition (Priestley, Lavoisier)
However, there's one problem with silver and oxygen. When molten, it dissolves it. When it solidifies, oxygen goes out and forms annoying little
craters on the metal's surface. I've read about it in one manual. It's possible that nitrogen contributes to it, too.
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AJKOER
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I agree with your points on thermal decomposition, but having worked with it, my opinion is that Ag2O is relatively a more chemically stable Silver
salt then say AgOH or AgClO3 (only stable in the presence of excess Ag2O) or Silver acetate (subject to hydrolysis as are other Silver salts) or any
of the AgX (X= Cl, Br or I ), which are all light sensitive.
[Edited on 6-7-2012 by AJKOER]
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kristofvagyok
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Not even Ag2O or Ag2S will decompose by simply heating in air. It it written in Gmelin, but it doesn't goes like this.... I have heated Ag2S and even
some Ag2O by an O2-propane torch till it was white hot, under some borax (prevents the reoxidation by air), but it didn't decomposed:
Same way like this:
If want to make metallic silver, then add some hydrazine-hydrate to silver oxide in water and boil it (but get rid of silver halides before, because
it can explode).
Or dissolve the silver oxide/sulfide in nitric acid, heat it until it's free from acids (dry it), place it in water and add some metallic copper,
easy.
[Edited on 6-7-2012 by kristofvagyok]
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watson.fawkes
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| Quote: Originally posted by kristofvagyok | | Not even Ag2O or Ag2S will decompose by simply heating in air. It it written in Gmelin, but it doesn't goes like this.... I have heated Ag2S and even
some Ag2O by an O2-propane torch till it was white hot, under some borax (prevents the reoxidation by air), but it didn't decomposed
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I don't think you should expect the same results when you heat under a flux as opposed to heating in open
air. You're cutting off the gas escape route, which means that Le Chatelier's principle will operate differently in these two circumstances. You can
add flux to the crucible after the oxide has decomposed to prevent oxidation in the temperature range between decomposition and solidification.
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AJKOER
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Blogfast25:
My experience is that even strong NaOH has trouble dissolving Ag2O.
However, I have subsequently read that adding sugar is required. Give it a try.
Found a reference (not my original), see http://www.sciencemadness.org/talk/viewthread.php?tid=18788
To quote Waffles SS:
"You could easily reduce then AgCl/Ag2O with hot NaOH/sugar solution. The mud containing non-hydrolised matter, Ag, AgCl and Ag2O is reduced by
NaOH/sugar because beyond it will hydrolise all the organic remnants from bleach step, in hot alkaline-reducing media, all AgCl is converted in Ag2O
which oxidizes sugar to CO2, formic and levulinic acid (you can see acid smell during this step, even though solution is alkaline) and gets reduced
back to elemental Ag."
[Edited on 6-7-2012 by AJKOER]
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blogfast25
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AJ:
I wanted to make a silver mirror, for that you really need Ag(NH<sub>3</sub><sub>2</sub><sup>+</sup> (aq). But I guess I'll now have to reduce with sucrose/NaOH just to recover the silver as
Ag (0) mud... sigh.
Kristof: what's the silver compound being calcinated there?
[Edited on 6-7-2012 by blogfast25]
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kristofvagyok
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On the picture some refined silver (from silver nitrate and a copper wire) is melted in the pot under borax powder
But with the same apparatus was the silver oxide and the even the sulfide heated. And I also tried to heat the sulfide with no borax on it... Nothing
happened, just the ceramic pot melted and cracked 
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-Pictures from chemistry, check it out(:
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Fleaker
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| Quote: | Blogfast25:
I’m having a bit of silver trouble right now with fairly freshly precipitated AgCl that seems resistant to dissolution in (> 25 w% ammonia. I would have thought it would dissolve in that solvent effortlessly but
part of it resists the ammonia. Any thoughts, anyone? I’m loathe to heat it because the ammonia stench would be unbearable…
| Quote: |
If it won't dissolve in ammonia, then it is lead (II) chloride. This can be removed by boiling in hot water, or else just smelting the silver with
sodium nitrate, sodium carbonate, and silica (2:1:0.5).
We run sterling electrolytically. Every 10,000 t oz there is between 4-5 t oz gold, 0.2 t oz Pt, 0.5-1 t oz Pd, usually some rhodium if it has been
plated, and always a hefty proportion of lead oxides that stay in the anode bag.
Silver metal doesn't really have a visible oxide in air nor one while melting it.
Its oxide is useful if you start from silver chloride and need to cast anodes, other than that, stay away from AgCl.
Much more convenient to do for high purity silver from sterling is to use a saturated solution of sodium formate at 60-70 Centigrade and pH 3 (it'll
raise the pH). This gives a heavy silver sand of high fineness. Before adding the formate, be sure to add a few mL sulfuric acid and then filter it
and the gold out. The residues should be retained until there is enough to merit processing.
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Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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blogfast25
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| Quote: Originally posted by Fleaker |
If it won't dissolve in ammonia, then it is lead (II) chloride. This can be removed by boiling in hot water, or else just smelting the silver with
sodium nitrate, sodium carbonate, and silica (2:1:0.5).
Much more convenient to do for high purity silver from sterling is to use a saturated solution of sodium formate at 60-70 Centigrade and pH 3 (it'll
raise the pH). This gives a heavy silver sand of high fineness. Before adding the formate, be sure to add a few mL sulfuric acid and then filter it
and the gold out. The residues should be retained until there is enough to merit processing.
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The quantity of ammonia insoluble residue just seems a bit too large for PbCl2 but I will try and boil it out as you suggest.
Thanks for the formate tip: I've been looking for a more suitable reducing agent than sucrose. What chemical form does the silver have to be in to be
reduced that way?
You guys process sterling scrap electrolytically?
Silver porn, here:
http://www.youtube.com/watch?v=umg3WSdPWHY
[Edited on 7-7-2012 by blogfast25]
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vmelkon
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| Quote: Originally posted by kristofvagyok | | But with the same apparatus was the silver oxide and the even the sulfide heated. And I also tried to heat the sulfide with no borax on it... Nothing
happened, just the ceramic pot melted and cracked |
So what he have so far is:
1. if you have a shiny silver block, it won't tarnish if you heat it in air
2. If you have Ag2O, heating it in air doesn't cause it to decompose although wikipedia says it decomposes at 280 °C
3. Ag2S, heating it in air doesn't cause it to decompose and burn off the sulfur.
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Fleaker
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It will not tarnish in air.
Ag2O will decompose to silver going from brown-black powder to gray powder that is silver. It will form a crust that cracks. We do this all of the
time.
Ag2S will oxidize to Ag metal in hot air. You can clean a set of sterling by putting it in the oven on broil. It will burn to SO2.
Blogfast25:
You need to have it as the nitrate which is basically the only form it should ever be in. If we do make AgCl, it's convenient to put in 15% w/v
sulfuric acid and toss in a bunch of nails and spin it for a few hours in a polypropylene cement mixer with a cover. Filter on a 36" buchner, rinse,
add carbonate, borax, melt, and then pour more anodes.
Next time we pour some 1000 t oz bars I'll make you some real silver porn. Most of our silver is sold as shot or crystal. Gets made into braze.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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blogfast25
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Re. AgCl, I keep having trouble with its light sensitivity. Even shielding it from light the top layer (of the filter cake) keeps getting discoloured
by fine Ag. Of course when I try to dissolve it in strong ammonia the solution ends up turbid.
[Edited on 8-7-2012 by blogfast25]
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AJKOER
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Blogfast25:
Note this comment from http://en.wikiversity.org/wiki/Reactions_of_ionic_halides
"We get the corresponding silver halide forming (AgCl, AgBr, AgI) which are insoluble (except for AgF) so they form a precipitate:
AgF is soluble; no precipitate formed
AgCl forms a white precipitate
AgBr forms a cream precipitate
AgI forms a yellow precipitate" and
"Aqueous halides with ammonia solutions
Distinguishing between white, cream and yellow can be difficult, so a second test can be used to confirm the results: The white precipitate formed
with chloride ions will dissolve in dilute ammonia, the cream precipitate formed with bromide ions will dissolve only in concentrated ammonia, and the
yellow precipitate formed with iodide ions will not dissolve in any concentration of ammonia."
So, per this source, AgCl should dissolve in even dilute ammonia.
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blogfast25
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Nothing new, AJ. All that's well known.
[Edited on 8-7-2012 by blogfast25]
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