Panache
International Hazard
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Registered: 18-10-2007
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Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein
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Solubility of silver halide salts in thiosulphate solutions
Does anyone have any tables of these, i have looked through all my usual streams and come up only with that they are soluble in them. The silver
halide forms a complex with the thiosulphate i believe which is the soluble entity but there would be significant salting out effects guess if your
thionate concentration was too high ( or low).
A second related question if any one knew, if one recovers the silver metal by letting the silverhalide/thiosulphate complex contact copper i assume a
Copper halide thiosulphate complex is the species remaining in solution. Would further application of silver halide to the solution displace some of
the copper halide thionate complex thus precipitating copper halide?
Finally silver halides appear to have some solubility in sulfite solutions, especially if NaCl is also present. Is the co-ordination of the silver
halide thionate complex through the sulphur or oxygen on the thiosulphate, i assume it's the oxygen which however would lead one to believe that
sulfite should co-ordinate with similar effect but appears to be much poorer.
[Edited on 5-5-2008 by Panache]
[Edited on 6-5-2008 by Panache]
[Edited on 6-5-2008 by Panache]
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woelen
Super Administrator
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Thionate? Which one? There are so many thionates, and they all have quite different properties.
Two common thionates are S2O6(2-) and S4O6(2-). The first is made by oxiziding a sulfite solution at low pH and low temperature with e.g. MnO2. The
latter can be made by oxidizing a thiosulfate solution with iodine.
Another thionate is S3O6(2-), which can be made by oxidizing ice-cold thiosulfate solution with ice-cold H2O2.
You have to be more specific with your question.
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Panache
International Hazard
Posts: 1290
Registered: 18-10-2007
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Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein
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i am a fool!!!!!
i meant to say thiosulphate every time i wrote thionite in that post, god i knew i was tired but that tired? When i first read your post woelen i
thought 'what's he talkigna bout thionite for?'.
the thread should make sense now
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woelen
Super Administrator
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Mood: interested
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I have no table for these, but the silver-thiosulfate complex is very soluble. Even at high concentration it remains in solution, also in the presence
of many other cations.
The complex is [Ag(S2O3)2](3-) and only if the concentration of the thiosulfate is too low, it will decompose again and cause precipitation of silver
ions with other cations in solution, such as chloride. A very high concentration of thiosulfate does not cause precipitation of a silver complex or a
silver salt.
Recovering the silver by using copper does not seem like a good thing to me. The silver/thiosulfate complex is quite stable and I'm quite sure that it
is not as strongly oxidizing as plain aqueous Ag(+) ion. Keep in mind that coordinated metal ions can have very different redox properties, e.g.
aqueous cobalt(II) is stable and aqueous cobalt(III) is an extremely strong oxidizing ion. However with ammonia or cyano ligands around, things become
totally different, in that case the cobalt(II) complex is strongly reducing and the cobalt(III) complex is stable. A similar thing most likely occurs
with your silver complex, I think that the thiosulfato complex is not oxidizing anymore.
Copper does form a complex with thiosulfate (some yellow complex), and at longer standing this decomposes giving CuS and other material.
If you want to recover silver from the solutions, then one option is to use zinc metal powder, which replaces the silver, even when it is trapped in
the thiosulfato complex. Another option is to add a solution of Na2S. The solubility of Ag2S is so extremely low, that it is able to precipitate from
a thiosulfate silver solution, because it drives the following equilibrium to the left:
Ag(+) + 2S2O3(2-) <---> [Ag(S2O3)2](3-)
Any Ag(+) formed in this equilibrium at once is trapped in Ag2S.
The precipitate of silversulfilde, or the metallic silver, obtained with zinc, must be strongly heated, such that the precipitate decomposes and
sulphur burns off and the metal melts. Any crap floats on the silver.
This working up of silver from your solutions only is economical on a large scale, or when there is a fairly high concentration of silver metal in the
solution. Working up e.g. 1 liter of a 0.1% silver solution makes no sense.
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