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12AX7
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Might be able to do something like this...
Ag2O + 2NH4Cl = [Ag(NH3)2]Cl, plus some acid and oxidizer (H2O2?) to get it going.
Once you get the Ag in solution, you should be able to fumble it around. I don't know, not sure you can crystallize something like silver nitrate
without breaking the complex, causing chloride to precipitate it straightaway.
Tim
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not_important
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I think you will end up with an AgCl film on the metal. The mixed chloride/nitrate works a bit like aqua regia, great for gold and platinum but silver
will form the chloride. It might be complexed and in solution, but as soon as you dilute the solution the AgCl will drop out.
The fact that you got crud on the silver makes me wonder how pure the ammonium nitrate is; fertilizer grade compounds aren't famous for being pure.
You might try soaking that silver in strong ammonia solution and see what the crude does, pour the aqueous part into vinegar and see if any AgCl drops
out.
I know you can recover metallic silver from the chloride by fusing it with sodium carbonate. I don't know if you can covert AgCl to the carbonate by
boiling it with Na2CO3, or by bringing AgCl into solution with ammonia the adding ammonium carbonate. If that did work, then ammonium chloride +
nitrate to get AgCl, convert to carbonate, boil with aqueous NH4NO3. (on its own AgCl is less soluble that Ag2CO3)
------
A varient on the hydroxide+sugar reaction : AgNO3, NH4NO3, NaOH, honey
http://dwb.unl.edu/Chemistry/MicroScale/MScale42.html
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agent_entropy
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I tried the NaOH and sugar method for silver recovery, the one that garage chemist suggested, but I can't figure out how the NaOH reacts with the
AgCl. I did the gibbs free energy calculations for the reaction (net ionic):
AgCl + OH- -> Ag2O + H2O + Cl-
and it comes out to ~ +23 so it should not be spontaneous at 25C; the delta G doesn't even turn negative until about 205C (which of course is
ridiculous for an aqueous solution).
I'm thinking, maybe that isn't the reaction that is happening. Any idea what is going on here?
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not_important
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| Quote: | Originally posted by agent_entropy
I tried the NaOH and sugar method for silver recovery, the one that garage chemist suggested, but I can't figure out how the NaOH reacts with the
AgCl. I did the gibbs free energy calculations for the reaction (net ionic):
AgCl + OH- -> Ag2O + H2O + Cl-
and it comes out to ~ +23 so it should not be spontaneous at 25C; the delta G doesn't even turn negative until about 205C (which of course is
ridiculous for an aqueous solution).
I'm thinking, maybe that isn't the reaction that is happening. Any idea what is going on here? |
Likely it is this
2 AgCl + 2 OH(-) <=> Ag2O + H2O + 2 Cl(-)
which is much to the left but there will be a bit on the right
Ag2O + O=C(R)-H => O=C(R)-OH + 2 Ag(s)
where O=C(R)-H is an aldehyde group in the sugar.
Metallic silver is less soluble that AgCl, and the reduction reaction doesn't go backwards very well, acid reduced to aldehyde, so the 2nd step drives
things to completion.
In fact, the first step might be better as AgCl + OH(-) <=> Cl(-) + {AgOH}
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guy
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| Quote: | Originally posted by agent_entropy
I tried the NaOH and sugar method for silver recovery, the one that garage chemist suggested, but I can't figure out how the NaOH reacts with the
AgCl. I did the gibbs free energy calculations for the reaction (net ionic):
AgCl + OH- -> Ag2O + H2O + Cl-
and it comes out to ~ +23 so it should not be spontaneous at 25C; the delta G doesn't even turn negative until about 205C (which of course is
ridiculous for an aqueous solution).
I'm thinking, maybe that isn't the reaction that is happening. Any idea what is going on here? |
That reaction isn't even balanced so of course youre going to be off. And make sure when you calculate the values for the anions, make sure its not
the values for the GAS phase.
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agent_entropy
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Sorry, I typed the reaction in wrong, forgot the coefficients, but I did do the calculations with the coefficients (and they were for the correct
phases). This is what I meant:
2AgCl (s) + 2OH- (aq) -> Ag2O (s) + H2O (l) + 2Cl- (aq)
But not important says its probably an equilibrium... hmm...
2 AgCl + 2 OH(-) <=> Ag2O + H2O + 2 Cl(-)
Also, I looked into AgOH and I can't find any data on it (even in the Merck Index) and online I keep getting redirected to Ag2O. So I'm still
looking.
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12AX7
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AgOH is a transient product. Like CuOH, it isn't stable, at least at room temperature, so it disproportionates to M2O. Probably fair to use the
energy of Ag2O instead of AgOH (which would be followed by the energy of 2AgOH = Ag2O + H2O anyway).
Tim
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guy
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| Quote: | Originally posted by agent_entropy
Sorry, I typed the reaction in wrong, forgot the coefficients, but I did do the calculations with the coefficients (and they were for the correct
phases). This is what I meant:
2AgCl (s) + 2OH- (aq) -> Ag2O (s) + H2O (l) + 2Cl- (aq)
But not important says its probably an equilibrium... hmm...
2 AgCl + 2 OH(-) <=> Ag2O + H2O + 2 Cl(-)
Also, I looked into AgOH and I can't find any data on it (even in the Merck Index) and online I keep getting redirected to Ag2O. So I'm still
looking. |
Ah Ok. I see why. Ag2O is about 200 times more soluble than AgCl so it is non spontaneous at standard conditions (1 M NaOH).
You got to use G = Go - RT ln Q
when you use different concentrations than 1 M NaOH.
~~~~~~~~~~~~~~~~~~~
Also for the silver mirror reaction (sugar and Ag2O), the reaction is not 2AgCl + 2OH- ---> Ag2O + H2O + Cl-
Ammonia first complexes with Ag+ to make it soluble. Then the hydroxide is just needed to react with the aldehyde.
[Edited on 10/10/2006 by guy]
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agent_entropy
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@ guy: But I didn't use any ammonia.
My procedure is detailed below:
5 grams of AgNO3 were dissolved in 150ml of distilled water, this solution served as simulated silver containing wastewater solution. Since normally
the amount of silver in a given sample of wastewater is not known, the experiment was continued as though the amount of silver in solution was not
known.
Sodium chloride was added to the solution until it appeared that no further AgCl would precipitate, thus approximating an excess addition of chloride.
Ag+(aq) + Cl- (aq) → AgCl (s)
The solution was acidified with approximately 0.5 ml concentrated HCl, and was heated to boiling until the AgCl precipitate had aggregated and the
supernatant solution was clear. Another few drops of HCl were added to the supernatant solution to check for complete precipitation of AgCl, no
further precipitate formed upon the addition of HCl, thus it was concluded that the precipitation was complete. Precipitation of silver from the
solution as AgCl is not necessary but this step was included since silver waste often already contains insoluble salts of silver.
Heating was continued and NaOH was added slowly and carefully as each addition is accompanied by violent bubbling, even when the solution is not
boiling. As the NaOH was added the solution turned red, then brown and finally black as the white silver chloride precipitate dissolved and was
replaced by a small amount of fine, dusty, black precipitate which did not settle out quickly. When all of the silver chloride had disappeared
further addition of NaOH produced no bubbling and it was concluded that a sufficient excess of NaOH had been added.
Heating was continued and sucrose was added slowly (in case of violent boiling due to the sudden introduction of nucleation sites). As the sucrose
was added the solution turned opaque milky-green, as a gray-black precipitate formed (metallic silver). As further sucrose was added the solution
turned bright orange-red, and when no further precipitate formed it was concluded that sufficient sucrose had been added.
The supernatant solution was decanted and another 150ml of distilled water was added to the precipitate and the mixture was boiled in order to aid in
the removal of the supernatant solution that had been included in the precipitate. The precipitate was filtered and rinsed with several portions of
distilled water and methanol in order to allow quicker drying.
The theoretical yield for the initial 5 grams of silver nitrate is 3.24g of silver. The actual mass of silver recovered was 3.20g ; a recovery of
99%.
*The two things I'm still trying to figure out are:
- the AgCl and NaOH reaction
- why sucrose was able to reduce Ag+ to metallic silver (since sucrose is not a reducing sugar), I was thinking it got cleaved into glucose and
fructose but that hydrolysis reaction is described as being done under acidic conditions in my organic chem book
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