Mixell
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Tin transformation from lumps to crystals
I kept a solution of SnCl2 in dilute HCl for a few months untouched. In the vessel I've placed some tin pieces to keep the tin in oxidation state +2
(let's assume some oxygen gets in and oxidizes then Sn+2 to Sn+4, the Sn+4 cation will in turn oxidize the metallic tin to Sn+2, and reduce itself to
Sn+2 as well).
So I looked at the solution today, and to my surprise, the tin lumps were gone and instead the bottom of the bottle was covered with beautiful
tree-like metallic crystals.
Has anyone noticed other metals behaving similarly?
I've also discovered that my bismuth nitrate solution froze entirely, probably due to the temperature drop (from ~26 C to ~16 C).
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DraconicAcid
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I haven't observed it, but I can see it happening. One tin atom will decide to leave its electrons and head out into the solution; a tin ion in
solution will pick up those electrons from the surface of the tin, and crystallize as solid tin. It's similar to the mechanism of plating zinc onto
copper by putting it into a solution of zinc chloride and in contact with metallic zinc.
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barley81
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That sounds really interesting. When winter break comes in a few days, I'll put a lump of tin into some dilute HCl (excess tin) and see if I can
replicate your results. I have not seen this effect before.
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Agecer
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Quote: Originally posted by Mixell  | I kept a solution of SnCl2 in dilute HCl for a few months untouched. In the vessel I've placed some tin pieces to keep the tin in oxidation state +2
(let's assume some oxygen gets in and oxidizes then Sn+2 to Sn+4, the Sn+4 cation car dvd players will in turn oxidize the metallic tin to Sn+2, and reduce itself to Sn+2 as well).
So I looked at the solution today, and to my surprise, the tin lumps were gone and instead the bottom of the bottle was covered with beautiful
tree-like metallic crystals.
Has anyone noticed other metals behaving similarly?
I've also discovered that my bismuth nitrate solution froze entirely, probably due to the temperature drop (from ~26 C to ~16 C).
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I am not sure other metal would have same property. But I will try to work in my lab and find it out
[Edited on 13-2-2013 by Agecer]
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kavu
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It might be that by slow redox reaction and metal growth the original lump ha slowly dissolved and deposited back onto the surface. This would lead to
nice crystalline mass.
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blogfast25
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The key here is indeed the presence of oxygen which constantly but very slowly oxidises the Sn(II) to Sn(IV), which in turm gets reduced back to
Sn(II) by the Sn(O).
This constant equilibrium ensures that the amount of Sn(O) is always the same but tin metal constantly goes into solution, then drops out of it again
(somewhere else), hence the shape changing metal.
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DraconicAcid
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| Quote: Originally posted by blogfast25 | The key here is indeed the presence of oxygen which constantly but very slowly oxidises the Sn(II) to Sn(IV), which in turm gets reduced back to
Sn(II) by the Sn(O).
This constant equilibrium ensures that the amount of Sn(O) is always the same but tin metal constantly goes into solution, then drops out of it again
(somewhere else), hence the shape changing metal. |
I don't think so. If oxygen was oxidizing the tin, you'd be losing metallic tin. I suspect it's simply:
Sn2+(aq) + Sn(lump) -> Sn(xtal) + Sn2+(aq)
No overall reaction, but atoms of tin constantly entering solution and reforming as metal.
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MrHomeScientist
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I agree with the general idea of metallic tin going into solution then being redeposited elsewhere - that would definitely lead to crystals. Do you
have any pictures of these? Metal crystals always look beautiful to me. Any more details on the concentration of the solution (of SnCl2 and HCl)? I'd
like to try this out myself.
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Mixell
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Metallic tin is probably lost, albeit at an extremely slow rate (the cap is sealed tightly).
I too think that tin atoms are slowly going into solution due to a change in the reduction potential of the immediate surroundings of a specific tin
atom. This allows that atom to go into solution and another one to be deposited (probably nearby).
And charge distribution over a surface is proportional to the curvature of the surface (or something of that sort) and because existing crystals are
more pointy than the surface of the lump itself (more curved so to say) there is a higher probability that the atoms will be deposited on them than on
a less pointy (or curved) surface.
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DraconicAcid
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Now I want to dig up that microscope and see if a small piece of tin will grow crystals at a visible rate. Copper in silver nitrate grows crystals
rapidly at 40x or so, and is a great demo. So how about tin....?
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blogfast25
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| Quote: Originally posted by DraconicAcid | | Now I want to dig up that microscope and see if a small piece of tin will grow crystals at a visible rate. Copper in silver nitrate grows crystals
rapidly at 40x or so, and is a great demo. So how about tin....? |
Trying is believing! Magnification also magnifies the growth rate, so you're probably in luck...
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watson.fawkes
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| Quote: Originally posted by DraconicAcid | Sn2+(aq) + Sn(lump) -> Sn(xtal) + Sn2+(aq)
No overall reaction, but atoms of tin constantly entering solution and reforming as metal. |
This.
It's thermodynamically favorable for a crystalline structure to have fewer crystal domains rather than more. When tin solidifies from melt, the
crystal domains are very small. Having the metallic tin in contact with a solution that allows ions to form greatly accelerates the pace at which the
recrystallization reaction takes place. You have, in essence, a room-temperature hydrothermal process in action.
This recrystallization process happens without solvent, but much more slowly. In electronics, it can be a problem with whisker formation, which can
cause shorts over time. Copper does it, as do some infelicitous solder alloys. It's definitely an issue with high-reliability gear such as
communication satellites, which have some rather strict material requirements as a result.
What's the concentration of SnCl2? You seem to have lucked upon a good range by accident. I'd imagine this same recrystallization effect would work
with copper metal and CuCl, with Cu(1+) ions in acidic solution, but I have little idea a priori what the effect on concentration would be. I
do know, however that a long thin tube with a bit of thermal gradient, with a seed in the cool section and lump metal in the warm one, would make the
whole process faster. It would make a good demonstrator for hydrothermal processes, since it's both visible and within a safe temperature and pressure
realm.
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Mixell
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I can't recall the exact concentration, but the solution was fairly concentrated.
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Endimion17
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This behaviour is similar to the longterm behaviour of volatile solids like iodine or ammonium nitrate. If left undisturbed for decades, they reshape
and crawl inside the closed container. The system is slowly but gradually finding its thermodynamically stable shape. If someone was taking a photo of
it every hour during 40 years and then made a highspeed video, one could see how the crystal mass "dances" like it's a living organism. The more
disturbed the system is (daily and seasonal temperature changes), the more futile this creeping is - it just goes on.
This is often the reason why sometimes you hear about people trying to open a container filled with some explosive solid that has been sitting in the
cabinet since the 60s, and then the shit detonates. The crystals climb towards the cap and creep between the bottle-cap screw, so when someone tries
to open the container, they get crushed and detonate.
Or someone sees an ancient lump of potassium chlorate and tries to poke it with a glass rod... pop, goes the weasel.
In liquid-solid heterogenous systems, this behaviour is a lot more prominent (saturated solution of salts with solid on the bottom), especially if the
substances inside are chemically different (metal, metal salt solution), so there is not just a physical transformation, but also a chemical one. All
that contributes to the speed of the movement, so you don't need to wait for decades to see the show.
Lump of metal surrounded by its ions is in an equilibrium. Some atoms dissolve and get ionized, some ions deposit and become atoms. It will happen as
long as there's a thermodynamically unfavorable setup inside the vessel. If the temperature is constant, you have only entropy working on the
reshaping of the lump, but there are always some minute temperature changes.
There's also one often ignored, but important fact - impurities in metals aren't equally distributed, and even if they were, they have different speed
of dissolving from the matrix they're in. That will make them concentrate in layers and spots, and that changes the EMF of local surfaces,
contributing to the reshaping of the solid.
So, yeah... I think that sums it up.
[Edited on 14-2-2013 by Endimion17]
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woelen
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With iodine this creeping around goes quite fast. I have a sample of very pure iodine, ampouled in a sphere and really beautiful crystals move around
in this sphere and they disappear at one place, leaving not a single trace of dirt and settle somewhere else. This process takes weeks, not years!
Iodine is quite volatile, so it does not surprise me that this process only takes weeks.
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Endimion17
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It depends on the temperature, but yes, iodine is much, much faster. I have iodine stored underground under cool conditions and it's been few years
without appreciable movement. If the vessel was stored under slightly warmer than normal temperature, there'd be some action after few weeks.
Ammonium salts require decades at room temperature. I think picric acid does this, too.
One class of compounds I don't remember ever seeing such behaviour are water complexed transition metal salts. They just stay where they are. And it's
expected, being that stabilized with crystal water.
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AndersHoveland
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You could probably grow crystals of gold also. Gold is actually soluble in highly concentrated mixtures of nitric and sulfuric acids.
There is apparently an equilibrium with gold in its oxidized state, along with nitrosylsufuric acid. If the acids are extremely concentrated, the gold
will gradually react, but if the solution becomes diluted with water the gold will separate back out again.
3Au + 3NO3- + 18H+ ⇌ 3Au+3 + 3NO+ + 6H3O+
(To get the reaction to go in reverse it is best to add the concentrated acid to excess water, otherwise
oxides of nitrogen will escape and not all the gold will be reduced back to its elemental form, so gold oxide will precipitate out as well. These
oxides of nitrogen are soluble in both concentrated sulfuric acid and in excess water, but not in moderately acidic solution. Apparently when the
concentrated acids containing the dissolved gold is diluted, the nitrosylsulfuric acid hydrolyzes to nitrous acid, which can reduce the oxidized
gold)
So if a piece of gold foil was left in such a concentrated acid mixture, but just under the required concentration to react with the gold, it would
probably gradually form crystals of gold after several days, as the gold was slowly dissolved and then redeposited. This might be interesting to try.
I am not sure exactly what concentration of acids would be best for growing crystals. But just to give some idea of the probable range here, perhaps
roughly 3 parts 98% conc. H2SO4 and 1 part 70% HNO3. Essentially a nitration mixture, if one wants to think of it that way.
Gold can form octohedral crystals. To form nice big crystals, it is important that the crystallization process be very slow.
[Edited on 16-2-2013 by AndersHoveland]
I'm not saying let's go kill all the stupid people...I'm just saying lets remove all the warning labels and let the problem sort itself out.
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AndersHoveland
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An easier way to grow crystals of gold might just be to leave them in an enclosed beaker with a little crystal of elemental iodine. There appears to
be an equilibrium here also.
| Quote: |
Action of Iodine on Gold.
In the presence of water, gold and iodine react in a closed vessel to form aurous iodide, but the reaction is limited, and, at normal temperatures, if
the iodine can escape, the iodide is entirely decomposed.
At ordinary temperatures pure dry iodine is without action on gold ; between 50° C. and the melting point of iodine combination takes place with the
formation of amorphous iodide; above that temperature crystalline aurous iodide is formed. The direct reaction is always limited by the inverse
decomposition of the iodide formed, but in the presence of excess of iodide pure aurous iodide may be obtained; this in excess is then best removed by
subliming the mixture at a temperature of 30° .
— F. Meyer (Comptes rend., 1904, 139, 733).
Pharmaceutical journal; A weekly record of pharmacy and allied sciences, Volume LXXIV, Great Britain, 1905
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I'm not saying let's go kill all the stupid people...I'm just saying lets remove all the warning labels and let the problem sort itself out.
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