clearly_not_atara
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Gallium amalgams II: Zn-Ga and the Al-Zn-Ga "metatectic"
Al-Ga eutectic has a composition of 99.1% Ga and 0.9% Al, with a melting point of about 28 C. Making things worse, Ga has about 20% solubility in
solid aluminum. As a result, combining aluminium and gallium results in a solid alloy which is not a very good reducing agent.
By contrast, Zn-Ga eutectic has four times as much zinc, 96.4% Ga to 3.6% Zn, melts at 23 C, and Ga has very low solubility in solid zinc. So zinc,
unlike aluminium, will form a liquid amalgam with gallium. This might be useful in reductions which call for amalgamated zinc, particularly the
Clemmensen reduction. Zn-Ga doesn't ever seem to get attention; it might also be effective in the formation of organozinc compounds.
Above 123 C, the behavior of the Al-Zn-Ga system changes. On a phase diagram that mixes a 4:1 Zn:Al alloy with gallium, the system above 123 C has a
solid phase which contains almost no gallium, and a liquid phase containing approximately 4% aluminium. So aluminium can be amalgamated with gallium
and zinc above 123 C. This metatectic also appears when a 3:2 Zn:Al alloy is used, although larger amounts of gallium is required.
Such an amalgam might be formed by adding pieces of Al to a hot liquid mixture of Ga and Zn. Alternatively, if it is available, high-aluminium (10-20%
is "high") zinc-aluminum cast alloy may be used. The availability of Zn-Al casting alloys certainly seems promising for people who wish to liquefy Al.
An analysis of the Al-Zn-Ga system is attached.
[Edited on 26-5-2017 by clearly_not_atara]
Attachment: aragon1998.pdf (631kB)
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Melgar
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I've been experimenting with such a system, although you can get a much lower melting point by including indium and tin in the alloy. At the very
least, you should start with a gallium/indium eutectic alloy, since they're both group 13 metals, and the eutectic alloy melts at 15˚C. You can make
it with a 3:1 ratio of gallium to indium by mass.
Zinc is reduced slightly more easily than gallium, and much more readily than aluminum, so in an aqueous solution, the aluminum will reduce any
ambient zinc or gallium salts back to their metals. However, because zinc doesn't dissolve very well in gallium-based alloys, often what will happen
is that the zinc will be reduced to dust, which will then drift away in suspension. The suspended zinc dust can then reduce gallium out of solution,
preventing gallium salts from being recycled back to metal by the aluminum. Since the gallium is necessary for giving the alloy its low melting
point, this effect can eventually stop the reduction entirely.
However, zinc has an unusually high hydrogen overpotential, which is good for applications like organic reductions, so it makes sense to try and find
a way to use it. Using catalytic amounts of zinc seems to be a workable solution, since the zinc will then dissolve in the gallium alloy to its limit
(which isn't very high) and any zinc that dissolves will then be reduced back into the alloy by aluminum.
I posted some related information in prepublication if you're interested.
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clearly_not_atara
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I'm aware of the use of indium, but the ability to achieve similar results with zinc was new to me. Zinc is after all much cheaper and more available
than indium. Also, the use of Ga/Zn alone is of interest, since zinc amalgam will do some things that Al won't, and is quite useful as a reductant in
its own right.
| Quote: | | Zinc is reduced slightly more easily than gallium, and much more readily than aluminum, so in an aqueous solution, the aluminum will reduce any
ambient zinc or gallium salts back to their metals. However, because zinc doesn't dissolve very well in gallium-based alloys, often what will happen
is that the zinc will be reduced to dust, which will then drift away in suspension. The suspended zinc dust can then reduce gallium out of solution,
preventing gallium salts from being recycled back to metal by the aluminum. Since the gallium is necessary for giving the alloy its low melting point,
this effect can eventually stop the reduction entirely. |
Have you tried this? What were the conditions?
[Edited on 27-5-2017 by clearly_not_atara]
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Melgar
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Quote: Originally posted by clearly_not_atara  | | I'm aware of the use of indium, but the ability to achieve similar results with zinc was new to me. Zinc is after all much cheaper and more available
than indium. Also, the use of Ga/Zn alone is of interest, since zinc amalgam will do some things that Al won't, and is quite useful as a reductant in
its own right. |
The idea is, you're not substituting indium for zinc, you're substituting it for gallium. And gallium and indium are roughly the same price per gram.
Ie, eutectic gallium/indium is roughly the same price per gram as pure gallium, and has a melting point that's about 15˚C lower. With tin added,
the price and melting point both go down.
Zinc solubility is low in both gallium and gallium/indium eutectic though, and gallium-based alloys don't adhere to zinc like they do aluminum. Even
adding gallium chloride and indium chloride solutions to a piece of solid zinc at an elevated temperature (60˚C or so) doesn't work very well to
activate its surface.
I haven't written this yet, but it works quite well to activate aluminum's surface, and I'd planned to add this to my thread in prepublication.
Basically, it's for people more familiar with Al/Hg reduction; you add gallium chloride to water with bits of aluminum in it, at about 50˚C, then
when you can see it bubbling, add one third that mass of indium chloride or so. The bubbling becomes much more vigorous, at which point you quickly
filter off the liquid and add the aluminum pieces to your solution that you want to reduce. You can allow everything to cool before adding the indium
chloride if you want to give yourself more time. Note that this doesn't work at all if you do it at room temperature. My theory is that you need to
do it above gallium's melting point, so that the gallium can form discrete liquid sites on the surface of the aluminum, which reduce additional
gallium (and later indium) out of solution MUCH more readily than ordinary aluminum does.
| Quote: | | Quote: | | Zinc is reduced slightly more easily than gallium, and much more readily than aluminum, so in an aqueous solution, the aluminum will reduce any
ambient zinc or gallium salts back to their metals. However, because zinc doesn't dissolve very well in gallium-based alloys, often what will happen
is that the zinc will be reduced to dust, which will then drift away in suspension. The suspended zinc dust can then reduce gallium out of solution,
preventing gallium salts from being recycled back to metal by the aluminum. Since the gallium is necessary for giving the alloy its low melting point,
this effect can eventually stop the reduction entirely. |
Have you tried this? What were the conditions? |
Yes, and that's what happens. What I wrote is actually my interpretation of what I observed, and is open to debate. Aluminum will reduce gallium,
indium, and tin out of solution fairly easily, but when zinc becomes part of that equation, a suspension of zinc metal particles forms, and the
reaction slows down, and in some cases stops. Adding zinc chloride to dissolving Al/Ga/In/St does the same thing. A similar effect happens with
transition metals too, including iron, copper, etc. to varying degrees, but those metals only slow down the reaction, rather than stop it. My theory
is that these metals don't reduce gallium out of solution, as opposed to zinc, which would.
[Edited on 5/27/17 by Melgar]
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Melgar
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Apparently I can't edit my post, but I'd like to also state that the main difference between zinc and other metal salts being reduced, is that the
liquid metal eventually solidifies if enough zinc is present. That doesn't happen with transition metals. With copper, for instance, the reaction
will slow down to the point where it doesn't look like it's doing anything, but the metal will remain liquid. With zinc, the liquid metal will
eventually solidify, and seems to also become smaller, although I can't remember for sure now.
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